A36.1      Noise certification test and measurement conditions.

A36.3      Measurement of aircraft noise received on the ground.

A36.5      Reporting and correcting measured data.

A36.7      Symbols and units.

A36.9      Atmospheric attenuation of sound.

A36.11      Detailed correction procedures.

Section A36.1      Noise certification test and measurement conditions.

(a) General. This section prescribes the conditions under which aircraft noise certification tests must be conducted and the measurement procedures that must be used to measure aircraft noise during each test conducted on or after April 3, 1978.

(b) Test site requirements. (1) Tests to show compliance with established aircraft noise certification levels must consist of a series of takeoffs and approaches (or stabilized flight path segments thereof) during which measurements must be taken at noise measuring stations located at the measuring points prescribed in section C36.3 of appendix C of this part. Each recorded segment must include measurements throughout the entire time period in which the recorded signal is within 10 dB of PNLTM.

(2) During each test takeoff, simultaneous measurements should be made at the sideline noise measuring stations on each side of the runway and also at the takeoff noise measuring station. However, if test site conditions make it impractical to simultaneously measure takeoff and sideline noise, and if each of the other sideline measurement requirements is met, independent measurements may be made of the sideline noise under simulated flight path techniques. If the reference flight path includes a power cutback before the maximum possible sideline noise level is developed, the reduced sideline noise level which is the maximum value developed by the simulated flight path technique must be the certificated sideline noise value.

(3) If the height of the ground at a noise measuring station differs from that of the nearest point on the runway by more than 20 feet, corrections must be made as prescribed in section A36.5(d) of this appendix.

(4) The location of each noise measuring station must be surrounded by relatively flat terrain having no excessive sound absorption characteristics, such as might be caused by thick, matted, or tall grass, shrubs, or wooded areas.

(5) An airport tower, or other facility, used to obtain required measurements of meteorological conditions at the test site must be approved in accordance with section A36.9(b)(1) of this appendix.

(6) During the period when the flyover noise/time record indicates the noise measurement is within 10 dB of PNLTM, no obstruction that significantly influences the sound field from the aircraft may exist --

(i) For a takeoff, approach, or sideline measuring station, within a conical space above the measuring position (the point on the ground vertically below the microphone), the cone being defined by an axis normal to the ground and by a half-angle 80 degrees from this axis; and

(ii) For a sideline noise measuring station, above the line of sight between the microphone and the aircraft.

(7) A minimum of two noise measuring stations, symmetrically positioned about the test flight track, must be used to define the maximum sideline noise with respect to location and level as required by section C36.3 of appendix C of this part. For turbojet powered aircraft, when approved by the FAA, the maximum sideline noise at takeoff thrust may be assumed to occur at the point (or its approved equivalent) along the extended centerline of the runway where the aircraft reaches 1000 feet (305 meters) altitude above ground level. A height of 1440 feet (439 meters) may be assumed for Stage 1 or Stage 2 four engine airplanes. The altitude of the aircraft as it passes the microphone stations must be within +500 to −0 feet (+150 to −0 meters) of the target altitude. For aircraft powered by other than turbojet engines, the altitude for maximum sideline noise must be determined experimentally.

(c) Weather restrictions. The tests must be conducted under the following atmospheric conditions:

(1) No rain or other precipitation.

(2) Ambient air temperature between 36 degrees F and 95 degrees F (2.2 degrees C and 35 degrees C), inclusively, over that portion of the sound propagation path between the aircraft and a point 10 meters above the ground at the noise measuring station.

(3) Relative humidity and ambient temperature over that portion of the sound propagation path between the aircraft and a point 10 meters above the ground at the noise measuring station is such that the sound attenuation in the one-third octave band centered a 8 kHz is not greater than 12 dB/100 meters and the relative humidity is between 20 and 95 percent, inclusively. However, if the dew point and dry bulb temperature used for obtaining relative humidity are measured with a device which is accurate to within ±0.5 °C, the sound attenuation rate shall not exceed 14 dB/100 meters in the one-third octave band centered at 8kHz.

(4) Average wind velocity 10 meters above ground is not to exceed 12 knots and the crosswind velocity for the airplane is not to exceed 7 knots. The average wind velocity shall be determined using a thirty-second averaging period spanning the 10 dB down time interval. Maximum wind velocity 10 meters above ground is not to exceed 15 knots and the crosswind velocity is not to exceed 10 knots during the 10 dB down time interval.

(5) No anomalous wind conditions (including turbulence) which will significantly affect the noise level of the aircraft when the noise is recorded at each noise measuring station.

(d) Aircraft testing procedures. -- (1) The aircraft testing procedures and noise measurements must be conducted and processed in an approved manner which yields the noise evaluation measure designated as Effective Perceived Noise Level (EPNL) in units of EPNdB, as prescribed in appendix B of this part.

(2) The aircraft height and lateral position relative to the extended centerline of the runway must be determined by an FAA approved method which is independent of normal flight instrumentation, such as radar tracking, theodolite triangulation, laser trajectography, or photographic scaling techniques.

(3) The aircraft position along the flight path must be related to the noise recorded at the noise measuring stations by means of synchronizing signals at an approved sampling rate. The position of the aircraft must be recorded relative to the runway during the entire time period in which the recorded signal is within 10 dB of PNLTM. Measuring and sampling equipment must be approved by the FAA.

(4) Each takeoff test must meet the conditions of section C36.7 of appendix C of this part.

(5) If a takeoff test series is conducted at weights other than the maximum takeoff weight for which noise certification is requested, the following additional requirements apply:

(i) At least one takeoff test must be conducted at a weight at, or above, the maximum certification weight.

(ii) Each test weight must be within +5 percent or −10 percent of the maximum certification weight.

(6) Each approach test must be conducted with the aircraft stabilized and following a 3.0 degree ±0.5 degree approach angle and must meet the requirements of section C36.9 of appendix C of this part.

(7) If an approach test series is conducted at weights other than the maximum landing weight for which certification is requested, the following additional requirements apply:

(i) At least one approach test must be conducted at a weight at, or above, the maximum landing weight.

(ii) Each test weight must exceed 90 percent of the maximum landing weight.

(8) Aircraft performance data sufficient to make the correction required under section A36.5 of this appendix must be recorded at an approved sampling rate using FAA approved equipment.

Section A36.3      Measurement of aircraft noise received on the ground.

(a) General. (1) The measurements prescribed in this section provide the data for determining the one-third octave band noise produced by aircraft during testing at specific noise measuring stations, as a function of time.

(2) Sound pressure level data for aircraft noise certification purposes must be obtained with approved acoustical equipment and measurement practices.

(3) Paragraphs (b), (c), and (d) of this section prescribe the required equipment specifications. Paragraphs (e) and (f) prescribe the calibration and measurement procedures required for each certification test series.

(b) Measurement system. The acoustical measurement system must consist of approved equipment equivalent to the following:

(1) A microphone system with frequency response and directivity which are compatible with the measurement and analysis system accuracy prescribed in paragraph (c) of this section.

(2) Tripods or similar microphone mountings that minimize interference with the sound energy being measured.

(3) Recording and reproducing equipment whose characteristics, frequency response, and dynamic range are compatible with the response and accuracy requirements of paragraph (c) of this section.

(4) Calibrators using sine wave, or pink noise, of known levels. When pink noise (defined in paragraph (e)(1) of this section) is used, the signal must be described in terms of its root-mean-square (rms) value.

(5) Analysis equipment with the response and accuracy which meets or exceeds the requirements of paragraph (d) of this section.

(6) Attenuators used for range changing in sensing, recording, reproducing, or analyzing aircraft sound must be capable of being operated in equal-interval decibel steps with no error between any two settings which exceeds 0.2 dB.

(c) Sensing, recording, and reproducing equipment. (1) The sound produced by the aircraft must be recorded in such a way that the complete information, including time history, is retained. A magnetic tape recorder is acceptable.

(2) The microphone must be a pressure sensitive capacitive type, or its approved equivalent, such as free field type with incidence corrector.

(i) After an adequate "warm-up" period, at least as long as that specified by the equipment manufacturer, the system output for constant acoustical input shall change by not more than 0.3 dB within any one hour nor by more than 0.4 dB within 5 hours.

(ii) The variation of microphone and preamplifier system sensitivity within an angle of ±30 degrees of grazing (60-120 degrees from the normal to the diaphragm) must not exceed the following values:

 
------------------------------------------------------------------------
             Frequency (HZ)                 Change in sensitivity (dB)
------------------------------------------------------------------------
45 to 1,120............................  1.0
1,120 to 2,240.........................  1.5
2,240 to 4,500.........................  2.5
4,500 to 7,100.........................  4.0
7,100 to 11,200........................  5.0
------------------------------------------------------------------------

With the wind screen in place, the variation in sensitivity in the plane of the diaphragm of the microphone system shall not exceed 1.0 dB over the frequency range 45 to 11,200 Hz.

(iii) The free-field frequency response of the microphone system at the reference incidence direction shall lie within an envelope having the following values:

 
------------------------------------------------------------------------
             Frequency (HZ)                  Change in Tolerance (dB)
------------------------------------------------------------------------
45 to 4,500............................  <plus-minus>1.0
4,500 to 5,600.........................  <plus-minus>1.5
5,600 to 7,100.........................  +1.5 to -2.0
7,100 to 9,000.........................  +1.5 to -3.0
9,000 to 11,200........................  +2.0 to -4.0
------------------------------------------------------------------------

(iv) Specifications concerning sensitivity to environmental factors such as temperature, relative humidity, and vibration must in conformity with the recommendations of International Electrotechnical Commission (IEC) Publication No. 179, entitled "Precision Sound Level Meters" (as incorporated by reference under §36.6 of this part).

(v) If the wind speed exceeds 6 knots, a windscreen must be employed with the microphone during each measurement of aircraft noise. Correction for any insertion loss produced by the windscreen as a function of frequency, must be applied to the measured data and any correction applied must be reported.

(3) If a magnetic tape recorder is used to store data for subsequent analysis, the record/replay system (including tape) must conform to the following:

(i) The electric background noise produced by the system in each one-third octave must be at least 35 dB below the standand recording level, which is defined as that level which is either 10 dB below the 3 pecent harmonic distortion level for direct recording or ±40 percent deviation for frequency modulation (FM) recording.

(ii) At the standard recording level, the corrected frequency response in each selected one-third octave band between 44 Hz and 180 Hz must be flat within ±0.75 dB, and in each band between 180 Hz and 11,200 Hz must be flat within ±0.25 dB.

(iii) If the overall system satisfies the requirements of paragraph (c)(2)(ii) of this section, and if the limitations of the dynamic range of the equipment are insufficient to obtain adequate spectral information, high frequency pre-emphasis may be added to the recording channel with the converse de-emphasis on playback. If pre-emphasis is added, the instantaneously recorded sound pressure level between 800 Hz and 11,200 Hz of the maximum measured noise signal must not vary more than 20 dB between the levels of the maximum and minimum one-third octave bands.

(d) Analysis equipment. (1) A frequency analysis of the acoustic signal must be performed using one-third octave filters which conform to the recommendations of International Electrotechnical Commission (IEC) Publication No. 225, entitled "Octave, Half-Octave, and Third-Octave Band Filters Intended for Analysis of Sounds and Vibrations" (as incorporated by reference under §36.6 of this part).

(2) A set of 24 consecutive one-third octave filters must be used. The first filter of the set must be centered at a geometric mean frequency of 50 Hz and the last filter at 10,000 Hz.

(i) The output of each filter must contain less than 0.5 dB ripple.

(ii) The correction for effective bandwidth relative to the response at the center frequency response for each one-third octave band filter must be determined by measuring the filter response to sinusoidal signals at a minimum of 20 frequencies equally spaced between the two adjacent preferred one-third octave frequencies or by using an approved equivalent procedure.

(3) The analyzer indicating device may be either analog or digital, or a combination of both. The preferred sequence of signal processing is:

(i) Squaring the one-third octave filter outputs;

(ii) Averaging or intergrating; and

(iii) Coverting linear formulation to logarithmic.

(4) Each detector must operate over a minimum dynamic range of 60 dB and perform as a true-mean-square device for sinusoidal tone bursts having crest factors of at least 3 over the following dynamic range:

(i) Up to 30 dB below full-scale reading must be accurate within ±0.5 dB;

(ii) Between 30 dB and 40 dB below full-scale reading must be accurate within ±1.0 dB; and

(iii) In excess of 40 dB below full-scale reading must be accurate within ±2.5 dB.

(5) The averaging properties of the integrator must be tested as follows:

(i) White noise must be passed through the 200 Hz one-third octave band filter and the output fed in turn to each detector/integrator. The standard deviation of the measured levels must then be determined from a large number of samples of the filtered white noise taken at intervals of not less than 5 seconds. The value of the standard deviation must be within the interval 0.48±0.06 dB for a probability limit of 95 percent. (An approved equivalent method may be substituted for this test on those analyzers where the test signal cannot readily be fed directly to each detector/integrator.)

(ii) For each detector/integrator, the response to a sudden onset or interruption of a constant amplitude sinusoidal signal at the respective one-third octave band center, frequency must be measured at sampling times 0.5, 1.0, 1.5, and 2.0 seconds after the onset or interruption. The rising responses must be the following amounts before the steady-state level: 0.5 seconds 4.0±1.0 dB 1.0 seconds 1.75±0.75 dB 1.5 seconds 1.0±0.5 dB 2.0 seconds 0.6±0.5 dB

(iii) The falling response must be such that the sum of the decibel readings (below the initial steady-state level) and the corresponding rising response reading are 6.5±1.0 dB, at each sampling time.

(iv) Analyzers using true integration cannot meet the requirements of paragraphs (d)(5)(i), (ii), and (iii) of this section directly, because their overall average time is greater than the sampling interval. For these analyzers, compliance must be demonstrated in terms of the equivalent output of the data processor. Further, in cases where readout and resetting require a dead-time during acquisition, the percentage loss of the total data must not exceed one percent.

(6) The sampling interval between successive readouts shall not exceed 500 milliseconds and its precise value must be known to within ± one (1) percent. The instant in time by which a readout is characterized, shall be the midpoint of the average period. (The averaging period is defined as twice the effective time constant of the analyzer.)

(7) The amplitude resolution of the analyzer must be at least 0.25 dB.

(8) After all systematic errors have been eliminated, each output level from the analyzer must be accurate within ±1.0 dB of the level of the input signal. The total systematic errors for each of the output levels must not exceed ±3.0 dB. For contiguous filter systems, the systematic correction between adjacent one-third octave channels must not exceed 4.0 dB.

(9) The dynamic range capability of the analyzer for display of a single aircraft noise event (in terms of the difference between full-scale output level and the maximum noise level of the analyzer equipment) must be at least 60 dB.

(e) Calibrations. (1) Within the five days before the beginning of each test series, the complete electronic system (as installed in the field, including cables) must be electronically calibrated for frequency and amplitude by the use of a pink noise signal of known amplitudes covering the range of signal levels furnished by the microphone. For purposes of this section, a "pink noise" means a noise whose noise-power/unit-frequency is inversely proportional to frequency at frequencies within the range of 44 Hz to 11,200 Hz. The signal used must be described in terms of its average root-mean-square (rms) values for a nonoverload signal level. This system calibration must be repeated within five days of the end of each test series, or as required by the FAA.

(2) Immediately before and after each day's testing, a recorded acoustic calibration of the system must be made in the field with an acoustic calibrator to check the system sensitivity and provide an acoustic reference level for the analysis of the sound level data. The performance of equipment in the system will be considered satisfactory if, during each day's testing, the variation does not exceed 0.5 dB.

(3) A normal incidence pressure calibration of the combined microphone/preamplifier must be performed with pure tones at each preferred one-third octave frequency from 50 Hz to 10,000 Hz. This calibration must be completed within the 90 days before the beginning of each test series.

(4) Each reel of magnetic tape must:

(i) Be pistonphone calibrated; and

(ii) At its beginning and end, carry a calibration signal consisting of at least a 15 second burst of pink noise, as defined in paragraph (e)(1) of this section.

(5) Data obtained from tape recorded signals are not considered reliable if the difference between the pink noise signal levels, before and after the tests in each one-third octave band, exceeds 0.75 dB.

(6) The one-third octave filters must have been demonstrated to be in conformity with the recommendations of IEC Publication 225 (as incorporated by reference under §36.6 of this part) during the six calendar months preceding the beginning of each test series. However, the correction for effective bandwidth relative to the center frequency response may be determined for each filter --

(i) By measuring the filter response to sinusoidal signals at a minimum of twenty frequencies equally spaced between the two adjacent preferred one-third octave frequencies; or

(ii) By using an approved alternative technique.

(7) A performance calibration analysis of each piece of calibration equipment, including piston phones, reference microphones, and voltage insert devices, must have been made during the six calendar months preceding the beginning of each day's test series. Each calibration must be traceable to the National Bureau of Standards.

(f) Noise measurement procedures. (1) Each microphone must be oriented so that the diaphragm is substantially in the plane defined by the flight path of the aircraft and the measuring station. The microphone located at each noise measuring station must be placed so that its sensing element is approximately 4 feet above ground.

(2) Immediately before and immediately after each series of test runs and each day's testing, a recorded acoustic calibration of the system prescribed in section A36.3(e)(2) of this appendix must be made in the field to check the acoustic reference level for the analysis of the sound level data. Ambient noise must be recorded for at least 10 seconds and be representative of the acoustical background, including systemic noise, that exists during the flyover test run. During that recorded period, each component of the system must be set at the gain-levels used for aircraft noise measurement.

(3) The mean background noise spectrum must contain the sound pressure levels, which, in each preferred third octave band in the range of 50 Hz to 10,000 Hz, are the averages of the energy of the sound pressure levels in every preferred third octave. When analyzed in PNL, the resulting mean background noise level must be at least 20 PNdB below the maximum PNL of the aircraft.

(4) Corrections for recorded levels of background noise are allowed, within the limits prescribed in §A36.5(d)(3) of this appendix.

Section A36.5 Reporting and correcting measured data.

(a) General. Data representing physical measurements, or corrections to measured data, including corrections to measurements for equipment response deviations, must be recorded in permanent form and appended to the record. Each correction must be reported and is subject to FAA approval. An estimate must be made of each individual error inherent in each of the operations employed in obtaining the final data.

(b) Data reporting. (1) Measured and corrected sound pressure levels must be presented in one-third octave band levels obtained with equipment conforming to the standards prescribed in section A36.3 of this appendix.

(2) The type of equipment used for measurement and analysis of all acoustics, aircraft performance, and meteorological data must be reported.

(3) The atmospheric environmental data required to demonstrate compliance with section A36.1(c) of this appendix, measured throughout the test period under section A36.9(b)(3) of this appendix, must be reported.

(4) Conditions of local topography, ground cover, or events which may interfere with sound recording must be reported.

(5) The following aircraft information must be reported:

(i) Type, model, and serial numbers (if any) of aircraft engines.

(ii) Gross dimensions of aircraft and location of engines.

(iii) Aircraft gross weight for each test run.

(iv) Aircraft configuration, including flap and landing gear positions.

(v) Airspeed in knots.

(vi) Engine performance parameters relevant to noise generation, such as net thrust, engine pressure ratio, exhaust temperatures, and fan or compressor rotational speeds.

(vii) Aircraft flight path (above ground level in feet) determined by an FAA approved method which is independent of normal flight instrumentation, such as radar tracking, theodolite triangulation, laser trajectography, or photographic scaling techniques.

(6) Aircraft speed and position, and engine performance parameters must be recorded at an approved sampling rate sufficient to correct to the noise certification reference conditions prescribed in paragraph (c) of this section. Lateral position relative to the extended centerline of the runway, configuration, and gross weight must be reported.

(c) Noise certification reference conditions. (1) Meteorological conditions. Aircraft position and performance data and the noise measurements must be corrected to the following homogeneous noise certification reference atmospheric conditions:

(i) Sea level pressure of 2116 psf (76 cm mercury).

(ii) Ambient temperature of 77 degrees F (25 degrees C).

(iii) Relative humidity of 70 percent.

(iv) Zero wind.

(2) Aircraft conditions. The reference condition for takeoff is the maximum weight, except as provided in §36.1581(b) of this part. The reference conditions for approach tests consist of --

(i) Maximum landing weight, except as provided in §36.1581(d) of this part;

(ii) Approach angle of 3 degrees; and

(iii) Aircraft height of 394 feet above the ground at the noise measuring station.

(d) Data corrections. (1) Aircraft position and performance data and the noise measurement must be corrected to the noise certification reference conditions as prescribed in paragraph (c) of this section. The measured atmospheric conditions must be those obtained in accordance with section A36.1(c) of this appendix and paragraph (b)(3) of this section. Atmospheric attenuation sound corrections must be made under section A36.9 of this appendix.

(2) The measured flight path must be corrected by an amount equal to the difference between the applicants predicted flight path for the certification reference conditions and the measured flight path at the test conditions. Necessary corrections relating to aircraft flight path or performance may be derived from approved data other than certification test data. The source noise must be corrected from approved data for the difference between measured and reference engine conditions, together with appropriate allowances for sound attenuation with distance. The Effective Perceived Noise Level (EPNL) correction must be less than 2.0 EPNdB for any combination of the following:

(i) The aircraft's not passing vertically above the measuring station.

(ii) Any difference between 394 feet and the actual minimum distance of the aircraft's ILS antenna from the approach measuring station.

(iii) Any difference between the actual approach angle and the noise certification reference approach flight path.

(iv) Any correction of the measured noise levels which accounts for any difference between the test engine thrust or power and the reference engine thrust or power.

Detailed correction requirements are prescribed in section A36.11 of this appendix.

(3) Aircraft sound pressure levels within the 10 dB-down points (described in section B36.9 of appendix B) must exceed the mean background sound pressure levels determined under section A36.3(f)(3) by at least 3 dB in each one-third octave band (or be corrected under an FAA approved method) to be included in the computation of the overall noise level of the aircraft. An EPNL may not be computed or reported from data from which more than four one-third octave bands in any spectrum within the 10 dB-down points have been excluded under this paragraph.

(4) Where more than seven one-third octaves are within 3 dB of the ambient noise levels, a time/frequency interpolation of the noise data shall be performed using an approved procedure.

(5) If equivalent test procedures, different from the reference procedures, are used, the test procedures and all methods for adjusting the results to the reference procedures must be approved by the FAA. The amounts of adjustments must not exceed 16 EPNdB on takeoff and 8 EPNdB on approach, and if the adjustments are more than 8 EPNdB and 4 EPNdB respectively, the resulting numbers must not be within 2 EPNdB of the appropriate appendix C noise levels including tradeoffs.

(e) Validity of results. (1) The test results must produce three mean EPNL values within the 90 percent confidence limits, each value consisting of the arithmetic mean of the corrected noise measurements for all valid test runs at the takeoff, approach, and sideline measuring stations, respectively. If more than one noise measurement system is used at any single measuring station, the resulting data for each test run (after correction) must be averaged as a single measurement. If more than one test site or noise measuring station location is used, each valid test run must be included in the computation of the mean EPNL values and their confidence limits.

(2) The minimum sample size acceptable for each of the three certification measurements (takeoff, approaches, and sideline) is six. The number of samples must be large enough to establish statistically for each of the three mean noise certification levels a 90 percent confidence limit which does not exceed ±1.5 EPNdB. No test result may be omitted from the averaging process, unless otherwise specified by the FAA.

(3) The mean EPNL values and their 90 percent confidence limits obtained by the procedure described in this paragraph must be those by which the noise emission of the aircraft is assessed against the noise certification criteria, and must be reported.

(4) If equivalent procedures are to be used to certificate several airplane configurations of the same type from noise tests of a single airplane, the test procedures and analysis methods must be approved by the FAA. The request for approval must identify the noise measurement test procedures and data base, the airplane configurations, procedures and analysis methods, the method for establishing the 90 percent confidence limit for each noise certification level, and the proposed equivalent procedures.

Section A36.7      Symbols and units.

(a) General. The symbols used in appendixes A and B of this part have the following meanings.

 
------------------------------------------------------------------------
        Symbol                   Unit                    Meaning
------------------------------------------------------------------------
ant...................  ......................  Antilogarithm to the
                                                 Base 10.
C(k)..................  dB....................  Tone Correction. The
                                                 factor to be added to
                                                 PLN(k) to account for
                                                 the presence of
                                                 spectral irregularities
                                                 such as tones at the k-
                                                 th increment of time.
d.....................  Sec...................  Duration Time. The
                                                 length of the
                                                 significant noise time
                                                 history being the time
                                                 interval between the
                                                 limits of t(1) and t(2)
                                                 to the nearest second.
D.....................  dB....................  Duration Correction. The
                                                 factor to be added to
                                                 PNLM to account for the
                                                 duration of the noise.
EPNL..................  EPNdB.................  Effective Perceived
                                                 Noise Level. The value
                                                 of PNL adjusted for
                                                 both the presence or
                                                 discrete frequencies
                                                 and the time history.
                                                 (The unit EPNdB is used
                                                 instead of the unit
                                                 dB.)
f(i) or fi............  Hz....................  Frequency. The
                                                 geometrical mean
                                                 frequency for the i-th
                                                 one-third octave band.
F(i,k)................  dB....................  Delta-dB. The difference
                                                 between the original
                                                 and background sound
                                                 pressure levels in the
                                                 i-th one-third octave
                                                 band at the k-th
                                                 interval of time.
h.....................  dB....................  dB-Down. The level to be
                                                 subtracted from PNLTM
                                                 that defines the
                                                 duration of the noise.
H.....................  %.....................  Relative Humidity. The
                                                 ambient atmospheric
                                                 relative humidity.
(i) or i..............  ......................  Frequency Band Index.
                                                 The numerical indicator
                                                 that denotes any one of
                                                 the 24 one-third octave
                                                 bands with geometrical
                                                 mean frequencies from
                                                 50 to 10,000 Hz.
(k)...................  ......................  Time Increment Index.
                                                 The numerical indicator
                                                 that denotes the number
                                                 of equal time
                                                 increments that have
                                                 elapsed from a
                                                 reference zero.
log...................  ......................  Logarithm to the Base
                                                 10.
log n (a).............  ......................  Noy discontinuity
                                                 Coordinate. The log n
                                                 value of the
                                                 intersection point of
                                                 the straight lines
                                                 representing the
                                                 variation of SPL with
                                                 log n.
M(b), M(c)............  ......................  Noy Inverse Slope. The
                                                 reciprocals of the
                                                 slopes of the straight
                                                 lines representing the
                                                 variation of SPL with
                                                 log n.
n.....................  noy...................  Perceived Noisiness. The
                                                 perceived noisiness at
                                                 any instant of time
                                                 that occurs in a
                                                 specified frequency
                                                 range.
n(i, k)...............  noy...................  Perceived Noisiness. The
                                                 perceived noisiness at
                                                 the k-th instant of
                                                 time that occurs in the
                                                 i-th one-third octave
                                                 band.
n(k)..................  noy...................  Maximum Perceived
                                                 Noisiness. The maximum
                                                 value of all of the 24
                                                 values of n(i) that
                                                 occurs at the k-th
                                                 instant of time.
N(k)..................  noy...................  Total Perceived
                                                 Noisiness. The total
                                                 perceived noisiness at
                                                 the k-th instant of
                                                 time calculated from
                                                 the 24-instantaneous
                                                 values of n(i, k).
p(b), p(c)............  ......................  Noy Slope. The slopes of
                                                 the straight lines
                                                 representing the
                                                 variation of SPL with
                                                 log n.
PNL...................  PNdB..................  Perceived Noise Level.
                                                 The perceived noise
                                                 level at any instant of
                                                 time (the unit PNdB is
                                                 used instead of the
                                                 unit dB).
PNL(k)................  PNdB..................  Perceived Noise Level.
                                                 The perceived noise
                                                 level calculated from
                                                 the 24 values of SPL
                                                 (i, k) at the k-th
                                                 increment of time. (The
                                                 unit PNdB is used
                                                 instead of the unit
                                                 dB.)
PNLM..................  PNdB..................  Maximum Perceived Noise
                                                 Level. The maximum
                                                 value of PNL(k) that
                                                 occurs during the
                                                 aircraft flyover. (The
                                                 unit PNdB is used
                                                 instead of the unit
                                                 dB.)
PNLT..................  PNdB..................  Tone Corrected Perceived
                                                 Noise Level. The value
                                                 of PNL adjusted for the
                                                 presence of spectral
                                                 irregularities
                                                 (discrete frequencies)
                                                 at any instant of time.
                                                 (The unit PNdB is used
                                                 instead of the unit
                                                 dB.)
PNLT(k)...............  PNdB..................  Tone Corrected Perceived
                                                 Noise Level. The value
                                                 of PNL(k) adjusted for
                                                 the presence of
                                                 discrete frequencies
                                                 that occurs at the k-th
                                                 increment of time. (The
                                                 unit PNdB is used
                                                 instead of the unit
                                                 dB.)
PNLTM.................  PNdB..................  Maximum tone Corrected
                                                 Perceived Noise Level.
                                                 The maximum value of
                                                 PNLT(k) that occurs
                                                 during the aircraft
                                                 flyover. (The unit PNdB
                                                 is used instead of the
                                                 unit dB.)
s(i, k)...............  dB....................  Slope of Sound Pressure
                                                 Level. The change in
                                                 level between adjacent
                                                 one-third octave band
                                                 sound pressure levels
                                                 at the i-th band for
                                                 the k-th instant of
                                                 time.
Δ s(i, k).......  dB....................  Change in Slope of Sound
                                                 Pressure Level.
s'(i, k)..............  dB....................  Adjusted Slope of Sound
                                                 Pressure Level. The
                                                 change in level between
                                                 adjacent adjusted one-
                                                 third octave band sound
                                                 pressure levels at the
                                                 i-th band for the k-th
                                                 instant of time.
s(i, k)...............  dB....................  Average Slope of Sound
                                                 Pressure Level.
SPL...................  dB re 0.0002 microbar.  Sound Pressure Level.
                                                 The sound pressure
                                                 level at any instant of
                                                 time that occurs in a
                                                 specified frequency
                                                 range.
SPL(a)................  dB re 0.002 microbar..  Noy Discontinuity
                                                 Coordinate. The SPL
                                                 value of the
                                                 intersection point of
                                                 the straight lines
                                                 representing the
                                                 variation of SPL with
                                                 log n.
SPL(b)................  dB re 0.002 microbar..  Noy Intercept. The
SPL(c)................                           intercepts on the SPL-
                                                 axis of the straight
                                                 lines representing the
                                                 variation of SPL with
                                                 log n.
SPL(l, k).............  dB re 0.002 microbar..  Sound Pressure Level.
                                                 The sound pressure
                                                 level at the k-th
                                                 instant of time that
                                                 occurs in the i-th one-
                                                 third octave band.
SPL'(l, k)............  dB re 0.002 microbar..  Adjusted Sound Pressure
                                                 Level. The first
                                                 approximation to
                                                 background level in the
                                                 i-th one-third octave
                                                 band for the k-th
                                                 instant of time.
SPL'(l, k)............  dB re 0.002 microbar..  Background Sound
                                                 Pressure Level. The
                                                 final approximation to
                                                 background level in the
                                                 i-th one-third octave
                                                 band for the k-th
                                                 instant of time.
SPLi..................  dB re 0.002 microbar..  Maximum Sound Pressure
                                                 Level. The sound
                                                 pressure level that
                                                 occurs in the i-th one-
                                                 third octave band of
                                                 the spectrum for PNL-
                                                 TM.
SPLic.................  dB re 0.002 microbar..  Corrected Maximum Sound
                                                 Pressure Level. The
                                                 sound pressure level
                                                 that occurs in the i-th
                                                 one-third octave band
                                                 of the spectrum for
                                                 PNLTM corrected for
                                                 atmospheric sound
                                                 absorption.
t.....................  Sec...................  Elapsed Time. The length
                                                 of time measured from a
                                                 reference zero.
t(1), t(2)............  Sec...................  Time Limit. The
                                                 beginning and end of
                                                 the significant noise
                                                 time history defined by
                                                 h.
Δ t.............  Sec...................  Time Increment. The
                                                 equal increments of
                                                 time for which PNL(k)
                                                 and PNLT (k) are
                                                 calculated.
T.....................  Sec...................  Normalizing Time
                                                 Constant. The length of
                                                 time used as a
                                                 reference in the
                                                 integration method for
                                                 computing duration
                                                 corrections.
T.....................   deg.F................  Temperature. The ambient
                                                 atmospheric temperature
α i.............  dB/ft.................  Test Atmospheric
α i'............  dB/1000 ft............   Absorption. The
                                                 atmospheric attenuation
                                                 of sound that occurs in
                                                 the i-th one-third
                                                 octave band for the
                                                 measured atmospheric
                                                 temperature and
                                                 relative humidity.
α io............  dB/ft.................  Reference Atmospheric
α io'...........  dB/1000 ft............   Absorption. The
                                                 atmospheric attenuation
                                                 of sound that occurs in
                                                 the i-th one-third
                                                 octave band for the
                                                 reference atmospheric
                                                 temperature and
                                                 relative humidity.
β................  Degrees...............  First Constant Climb
                                                 Angle.
ψ.................  Degrees...............  Second Constant Climb
                                                 Angle.
δ...............  Degrees...............  Thrust Cutback Angles.
ε.............  Degrees...............  The angles defining the
                                                 points on the takeoff
                                                 flight path at which
                                                 thrust reduction is
                                                 started and ended
                                                 respectively.
η.................  Degrees...............  Approach Angle.
θ...............  Degrees...............  Takeoff Noise Angle. The
                                                 angle between the
                                                 flight path and noise
                                                 path for takeoff
                                                 operation. It is
                                                 identical for both
                                                 measured and corrected
                                                 flight paths.
μ..................  Degrees...............  Approach Noise Angle.
                                                 The angle between the
                                                 flight path and the
                                                 noise path for approach
                                                 operation. It is
                                                 identical for both
                                                 measured and corrected
                                                 flight paths.
Δ l.............  EPNdB.................  PNLT Correction. The
                                                 correction to be added
                                                 to the EPNL calculated
                                                 from measured data to
                                                 account for noise level
                                                 changes due to
                                                 differences in
                                                 atmospheric absorption
                                                 and noise path length
                                                 between reference and
                                                 test conditions.
Δ2..............  EPNdB.................  Noise Path Duration
                                                 Correction. The
                                                 correction to be added
                                                 to the EPNL calculated
                                                 from measured data to
                                                 account for noise level
                                                 changes due to the
                                                 noise duration because
                                                 of differences in
                                                 flyover altitude
                                                 between reference and
                                                 test condition.
Δ3..............  EPNdB.................  Weight Correction. The
                                                 correction to be added
                                                 to the EPNL calculated
                                                 from measured data to
                                                 account for noise level
                                                 changes due to
                                                 differences between
                                                 maximum and test
                                                 aircraft weights.
Δ4..............  EPNdB.................  Approach Angle
                                                 Correction. The
                                                 correction to be added
                                                 to the EPNL calculated
                                                 from measured data to
                                                 account for noise level
                                                 changes due to
                                                 differences between 3
                                                 deg. and the test
                                                 approach angle.
Δ AB............  Feet..................  (\1\)
Δβ.........  Degrees...............  (\1\)
                        Degrees...............  (\1\)
Δγ........
Δδ........  Degrees...............  (\1\)
Δα........  Degrees...............  (\1\)
Δε......  Degrees...............  (\1\)
------------------------------------------------------------------------
\1\Takeoff Profile Changes. The changes in the basic parameters defining
  the takeoff profile due to differences between reference and test
  conditions.

                 Flight Profile Identification Positions
------------------------------------------------------------------------
             Position                            Description
------------------------------------------------------------------------
A.................................  Start of takeoff roll.
B.................................  Liftoff.
C.................................  Start of first constant climb.
D.................................  Start of thrust reduction.
E.................................  Start of second constant climb.
Ec................................  Start of second constant climb on
                                     corrected flight path.
F.................................  End of noise certification takeoff
                                     flight path.
Fc................................  End of second constant climb on
                                     corrected flight path.
G.................................  Start of noise certification
                                     approach flight path.
Gr................................  Start of noise certification
                                     approach on reference flight path.
H.................................  Position on approach path directly
                                     above noise measuring station.
I.................................  Start of level off.
Ir................................  Start of level off on reference
                                     approach flight path.
J.................................  Touchdown.
K.................................  Takeoff noise measuring station.
L.................................  Sideline noise measuring station
                                     (not on flight track).
M.................................  End of noise type certification
                                     takeoff flight track.
N.................................  Approach noise measuring station.
O.................................  Threshold of approach end of runway.
P.................................  Start of noise type certification
                                     approach flight track.
Q.................................  Position on measured takeoff flight
                                     path corresponding to PNLTM at
                                     station K.
Qc................................  Position on corrected takeoff flight
                                     path corresponding to PNLTM at
                                     station K.
R.................................  Position on measured takeoff flight
                                     path nearest to station K.
Rc................................  Position on corrected takeoff flight
                                     path nearest to station K.
S.................................  Position on measured approach flight
                                     path corresponding to PNLTM at
                                     station N.
Sr................................  Position on reference approach
                                     flight path corresponding to PNLTM
                                     at station N.
T.................................  Position on measured approach flight
                                     path nearest to station N.
Tr................................  Position on reference approach
                                     flight path nearest to station N.
X.................................  Position on measured takeoff flight
                                     path corresponding to PNLTM at
                                     station L.
Xc................................  Position on corrected takeoff flight
                                     path corresponding to PNLTM at
                                     station L.
------------------------------------------------------------------------

                        Flight Profile Distances
------------------------------------------------------------------------
        Distance                 Unit                   Meaning
------------------------------------------------------------------------
AB.....................  feet................  Length of Takeoff Roll.
                                                The distance along the
                                                runway between the start
                                                of takeoff roll and lift
                                                off.
AK.....................  feet................  Takeoff Measurement
                                                Distance. The distance
                                                from the start of roll
                                                to the takeoff noise
                                                measurement station
                                                along the extended
                                                centerline of the
                                                runway.
AM.....................  feet................  Takeoff Flight Track
                                                Distance. The distance
                                                from the start of roll
                                                to the takeoff flight
                                                track position along the
                                                extended centerline of
                                                the runway for which the
                                                position of the aircraft
                                                need no longer be
                                                recorded.
KQ.....................  feet................  Measured Takeoff Noise
                                                Path. The distance from
                                                station K to the
                                                measured aircraft
                                                position Q.
KQc....................  feet................  Corrected Takeoff Noise
                                                Path. The distance from
                                                station K to the
                                                corrected aircraft
                                                position Qc.
KR.....................  feet................  Measured Takeoff Minimum
                                                Distance. The distance
                                                from station K to point
                                                R on the measured flight
                                                path.
KRc....................  feet................  Corrected Takeoff Minimum
                                                Distance. The distance
                                                from station K to point
                                                Rc on the corrected
                                                flight path.
LX.....................  feet................  Measured Sideline Noise
                                                Path. The distance from
                                                station L to the
                                                measured aircraft
                                                position X.
LXc....................  feet................  Corrected Sideline Noise
                                                Path. The distance from
                                                station L to the
                                                corrected aircraft
                                                position Xc.
NH.....................  feet................  Aircraft Approach Height.
                                                The vertical distance
                                                between the aircraft and
                                                the approach measuring
                                                station.
NS.....................  feet................  Measured Approach Noise
                                                Path. The distance from
                                                station N to the
                                                measured aircraft
                                                position S.
NSr....................  feet................  Reference Approach Noise
                                                Path. The distance from
                                                station N to the
                                                reference aircraft
                                                position Sr.
NT.....................  feet................  Measured Approach Minimum
                                                Distance. The distance
                                                from station N to point
                                                T on the measured flight
                                                path.
NTr....................  feet................  Reference Approach
                                                Minimum Distance. The
                                                distance from station N
                                                to point Tr on the
                                                corrected flight path;
                                                it equals 393 feet.
ON.....................  feet................  Approach Measurement
                                                Distance. The distance
                                                from the runway
                                                threshold to the
                                                approach measurement
                                                station along the
                                                extended centerline of
                                                the runway.
OP.....................  feet................  Approach Flight Track
                                                Distance. The distance
                                                from the runway
                                                threshold to the
                                                approach flight track
                                                position along the
                                                extended centerline of
                                                the runway for which the
                                                position of the aircraft
                                                need no longer be
                                                recorded.
------------------------------------------------------------------------

Section A36.9      Atmospheric attenuation of sound.

(a) General. The measured values of the one-third octave band spectra must conform, or be corrected, to the reference-day conditions listed in section A36.5(c) of this appendix. Each correction must account for any differences in the atmospheric attenuation of sound between the test-day conditions and the reference-day conditions along the sound propagation path between the aircraft and the microphone. Unless the meteorological conditions conform to those prescribed in section A36.1(c) of this appendix, the test data are not acceptable.

(b) Meteorological measurements. (1) The wind velocity, temperature and relative humidity measurements required under this part must be measured in the vicinity of the noise measuring stations. The location of the meteorological measurements must be approved by the FAA as representative of those atmospheric conditions existing near the surface over the geographical area in which aircraft noise measurements are made. In some cases, a fixed meteorological station (such as those found at airports or other facilities) may meet this requirement.

(2) The temperature and relative humidity must be measured from a point 10 meters above the surface at the measuring stations to the altitude of the aircraft, using previously approved equipment and methods.

(3) Meteorological measurements must be obtained within 25 minutes of each noise test measurement. Meteorological data must be interpolated to actual times of each noise measurement.

(c) Attenuation rates. The atmospheric attenuation rates of sound with distance for each one-third octave band from 50Hz to 10,000 Hz must be determined in accordance with the formulations and tabulations of SAE ARP 866A, entitled "Standard Values of Atmospheric Absorption as a Function of Temperatures and Humidity for Use in Evaluating Aircraft Flyover Noise" (as incorporated by reference under §36.6 of this part).

(d) Correction for atmospheric attenuation. (1) EPNL values calculated for measured data must be corrected by the methods prescribed in section A36.11(d) of this appendix whenever --

(i) The ambient atmospheric conditions of temperature and relative humidity do not conform to the reference conditions (77 degrees F. and 70 percent, respectively), or

(ii) The measured takeoff and approach flight paths do not conform to the reference flight paths.

(2) If the atmospheric absorption coefficients do not vary over the PNLTM sound propagation path by more than ± 1.6 dB/1000 ft (± 0.5 dB/100 meters) in the 3150 Hz one-third octave band from the value of the absorption coefficient derived from the meteorological measurement obtained at 10 meters above the surface, the mean of the values of the atmospheric absorption coefficients at 10 meters above the surface and at the altitude of the aircraft at PNLTM may be used to determine the atmospheric attenuation rates for each one-third octave band. The resulting atmospheric attenuation rate may be used to compute the PNLTM correction under section A36.11(d) of this appendix.

(3) If the conditions do not conform to those prescribed in paragraph (d)(2) of this section, the corrections for atmospheric attenuation must be determined by the following layered-atmosphere procedure:

(i) The sound propagation path must be divided into increments no greater than 100 feet in altitude, and the average temperature and relative humidity that exists within each increment at the time of the test must be calculated from the meteorological data required under paragraph (b) of this section.

(ii) Atmospheric attenuation rates must be determined under paragraph (c) of this section for each one-third octave band in each altitude increment.

(iii) The mean attenuation rate over the complete sound propagation path from the aircraft to the microphone must be computed for each one-third octave band from 50 Hz to 10,000 Hz. These rates must be used in computing the corrections required in section A36.11(d) of this appendix.

Section A36.11      Detailed correction procedures.

(a) General. If the test conditions do not conform to those prescribed as noise certification reference conditions under section A36.5 of this appendix, the following correction procedure and requirements apply:

(1) If a positive value results from any difference between reference and test conditions, and appropriate positive correction must be made to the EPNL calculated from the measured data. Conditions which can result in a positive value include:

(i) Atmospheric absorption of sound under test conditions which is greater than the reference;

(ii) Test flight path at an altitude which is higher than the reference; or

(iii) Test weight which is less than maximum certification weight.

(2) If a negative value results from any difference between reference and test conditions, no correction may be made to the EPNL calculated from the measured data, unless the difference results from:

(i) An atmospheric absorption of sound under test conditions which is less than the reference; or

(ii) A test flight path at an altitude which is lower than the reference.

(3) The following correction procedures may produce one or more possible correction values which must be added algebraically to the EPNL calculated as if the tests were conducted completely under the noise certification reference conditions:

(i) The flight profiles must be determined for both takeoff and approach, and for both reference and test conditions. The procedures require noise and flight path recording with a synchronized time signal from which the test profile can be delineated, including the aircraft position for which PNLTM is observed at the noise measuring station. For takeoff, the flight profile corrected to reference conditions may be derived from FAA approved manufacturer's data; however, for approach, the reference profile is prescribed under paragraph (c)(2) of this section.

(ii) The sound propagation paths to the microphone from the aircraft position corresponding to PNLTM are determined for both the test and reference profiles. The SPL values in the spectrum of PNLTM must then be corrected for the effects of --

(A) Change in atmospheric sound absorption;

(B) Atmospheric sound absorption on the change in sound propagation path length; and

(C) Inverse square law on the change in sound propagation path length. The corrected values of SPL are then converted to PNLT from which must be subtracted PNLTM. The resulting difference represents the correction which must be added algebraically to the EPNL calculated from the measured data.

(iii) The minimum distances from both the test and reference profiles to the noise measuring station must be calculated and used to determine a noise duration correction due to any change in the altitude of aircraft flyover. The duration correction must be added algebraically to the EPNL calculated from the measured data.

(iv) From approved data in the form of curves or tables giving the variation of EPNL with engine thrust or test speed, corrections are determined and must be added to the EPNL (which is calculated from the measured data) to account for noise level changes due to differences between test conditions and reference conditions.

(v) From approved data corrections are determined and must be added algebraically to the EPNL (which is calculated from measured data) to account for noise level changes due to differences between 3 degrees and the test approach angle.

(b) Takeoff profiles. (1) Figure A1 illustrates a typical takeoff profile.

(i) The aircraft begins the takeoff roll at point A, lifts off at point B, and initiates the first constant climb at point C at an angle β. The noise abatement thrust cutback is started at point D and completed at point E where the second constant climb is defined by the angle γ (usually expressed in terms of the gradient in percent). The end of the noise certification takeoff flight path is represented by aircraft position F whose vertical projection on the flight track (extended centerline of the runway) is point M. The position of the aircraft must be recorded for the entire interval during which the measured aircraft noise level is within 10 dB of PNLTM. Position K is the takeoff noise measuring station whose distance AK is specified as 21,325 feet (6,500 meters). However, if it is necessary to reduce AK to less than 21,325 feet, the procedures prescribed in paragraph (f) of this section must be followed. Position L is the sideline noise measuring station located on a line parallel to, and the prescribed distance from, the runway centerline where the noise level during takeoff is greatest.

(ii) The takeoff profile is defined by five parameters -- (A) AB, the length of takeoff roll; (B) β the first constant climb angle; (C) γ, the second constant climb angle; and (D) δ, and e, the thrust cutback angles. These five parameters are functions of the aircraft performance and weight, and the atmospheric conditions of temperature, pressure, and wind velocity and direction.

(2) If the test conditions do not conform to those prescribed as reference conditions under section A36.5 of this appendix, the corresponding test and reference profile parameters will be different, as shown in Figure A2. The profile parameter changes, identifies as Δ AB, Δβ, Δγ, Δδ, and Δα may be derived from the manufacturer's data (if approved by the FAA) and may be used to define the fight profile corrected to the reference conditions. The relationships between the measured and corrected takeoff flight profiles may then be used to determine the corrections, which, if positive, must be applied to the EPNL calculated from the measured data.

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Note: Under reference atmospheric conditions and with maximum takeoff weight, the gradient of the second constant climb angle (γ) may not be less than 4 percent. However, the actual gradient will depend upon the test atmospheric conditions, assuming maximum takeoff weight and the parameters characterizing engine performance are constant (rpm, epr, or any other parameter used by the pilot).

(3) Figure A3 illustrates portions of the measured and corrected takeoff flight paths including the significant geometrical relationships influencing sound propagation. EF represents the measured second constant flight path with climb angle γ, and EcFc represents the corrected second constant flight path at reduced climb angle γ−Δ. Position Q represents the aircraft location on the measured takeoff flight path for which PNLTM is observed at the noise measuring station K, and Qc is the corresponding position on the corrected flight path. The measured and corrected sound propagation paths are KQ and KQc, respectively, which form the same angle α with their flight paths. Position R represents the point on the measured takeoff flight path nearest the noise measuring station K, and Rc is the corresponding position on the corrected flight path. The minimum distance to the measured and corrected flight paths are indicated by the lines KR and KRc, respectively, which are normal to their flight paths.

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(c) Approach profiles. (1) Figure A4 illustrates a typical approach profile.

(i) The beginning of the noise certification approach profile is represented by aircraft position G whose vertical projection on the flight track (extended centerline of the runway) is point P. The position of the aircraft should be recorded for a distance OP from the runway threshold O to ensure recording of the entire interval during which the measured aircraft noise is within 10 dB of PNLTM.

(ii) The aircraft approaches at an angle passes vertically over the noise measuring station N at a height of NH, begins the level off at position I, and touches down at position J. The distance ON is prescribed as 6,562 feet (2,000 meters).

(iii) The approach profile is defined by the approach angle        and the height NH which are functions of the aircraft operating conditions controlled by the pilot. If the measured approach profile parameters do not conform to the corresponding reference approach parameters (3 degrees and 394 feet, respectively, as shown in Figure A5), corrections, if positive, must be applied to the EPNL calculated from the measured data.

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(2) Figure A6 illustrates portions of the measured and reference approach flight paths, including the significant geometrical relationships influencing sound propagation. GI represents the measured approach path with approach angle -- , and GrIr represents the reference approach flight path at lower altitude and approach angle of 3 degrees. Position S represents the aircraft location on the measured approach flight path for which PNLTM is observed at the noise measuring station N, and Sr is the corresponding position on the reference approach flight path. The measured and corrected sound propagation paths are NS and NSr, respectively, which form the same angle λ with their flight paths. Position T represents the point on the measured approach flight path nearest the noise measuring station N, and Tr is the corresponding point on the reference approach flight path. The minimum distances to the measured and reference flight paths are indicated by the lines NT and NTr, respectively, which are normal to their flight paths. NOTE: The reference approach flight path is defined by -- =3 degrees and NH=394 feet. Consequently NTr can also be defined; NTr=393 feet to the nearest foot and is, therefore, considered to be one of the reference parameters.

(d) PNLT corrections. If the ambient atmospheric conditions of temperature and relative humidity are not those prescribed as reference conditions under §A36.5(c) of this appendix (77 degrees F and 70 percent, respectively), corrections to the EPNL values must be calculated from the measured data under paragraph (a) of this section as follows:

(1) Takeoff flight path. For the takeoff flight path shown in Figure A3, the spectrum of PNLTM observed at station K for the aircraft at position Q is decomposed into its individual SPLi values.

(i) Step 1. A set of corrected values are then computed as follows:

SPLic=SPLi

      

      

+(αi−αio) KQ

+αio (KQ−KQ