Note: Care should be taken to ensure that the noise measuring, airplane flight path tracking, and meteorological instrumentation are also operated within their specific environmental limitations.
a. Explanation
This subsection prescribes the ranges of ambient air temperature and relative humidity that are permitted during noise certification measurements. For each noise measurement, atmospheric temperature and relative humidity must be determined over the whole noise path between a location in the vicinity of the noise measurement points (at 10 meters above the ground) and the airplane. See section 6.4 of the appended ICAO TM.
b. Supplemental Information
(1) Assumed Ground Atmospheric Conditions: The temperature and relative humidity near the earth’s surface can be affected by numerous factors, including solar heating, surface winds, local heating or cooling, increased or decreased local humidity, etc. To avoid localized anomalous conditions that often occur near the ground, meteorological measurements are to be made 10 m above the surface. During processing of acoustical data, the meteorological conditions measured at 10 m are assumed to be constant from that height down to the ground surface.
(2) Criteria for Measuring Atmospheric Conditions: Experience has shown that proper measurement of non-reference meteorological conditions, and the associated adjustment of noise data for those conditions, are crucial to obtaining accurate, consistent, and repeatable test results. Thus, meteorological observations of the temperature and relative humidity are required over the whole sound propagation path from the aircraft to the vicinity of the noise measurement points.
c. Procedures
(1) Atmospheric Measurements: Several methods have been approved for the measurement of atmospheric conditions from 10 m above the ground to the altitude of the test airplane. See section 6.4.2 of the appended ICAO TM. Some applicants have used instrumented unmanned model airplanes and balloons. The most common method consists of a manned meteorological airplane flown in a spiral flight path in the vicinity of the noise measurement points to measure the dry bulb temperature and dew point along the sound propagation path.
(2) Applicant’s Responsibility: The applicant is responsible for providing the required meteorological measurement system that was approved in the noise compliance demonstration plan.
113. Section A36.2.2.2(c)
Relative humidity and ambient temperature over the whole noise path between a point 33 ft (10 m) above the ground and the airplane such that the sound attenuation in the one-third octave band centered on 8 kHz will not be more than 12 dB/100 m unless:
(1) The dew point and dry bulb temperatures are measured with a device which is accurate to +0.9oF (+0.5oC) and used to obtain relative humidity; in addition layered sections of the atmosphere are used as described in section A36.2.2.3 to compute equivalent weighted sound attenuations in each one-third octave band; or
(2) The peak noy values at the time of PNLT, after adjustment to reference conditions, occur at frequencies less than or equal to 400 Hz.
a. Explanation
This subsection specifies limits on the atmospheric attenuation of sound at any point between 10 m above ground level and the altitude of the test airplane. These limits are applicable to each noise measurement.
b. Supplemental Information
None
c. Procedures
None
114. Section A36.2.2.2(d)
If the atmospheric sound attenuation coefficients vary over the PNLTM sound propagation path by more than +1.6dB/1000ft (+0.5dB/100m) in the 3150Hz one-third octave band from the value of the atmospheric sound attenuation coefficient derived from the meteorological measurement obtained at 33 ft (10 m) above the surface, "layered" sections of the atmosphere must be used as described in section A36.2.2.3 to compute equivalent weighted sound attenuations in each one-third octave band;
the FAA will determine whether a sufficient number of layered sections have been used. For each measurement, where multiple layering is not required, equivalent sound attenuations in each one-third octave band must be determined by averaging the atmospheric sound attenuation coefficients for each such band at 33 ft (10 m) above ground level, and at the flight level of the airplane at the time of PNLTM, for each measurement;
a. Explanation
This subsection specifies the use of layered sections of the atmosphere to adjust one-third-octave band sound pressure levels to reference conditions.
b. Supplemental Information
(1) When the use of multiple layered sections is required, the method to be used in defining the depth of the layered sections is specified in section A36.2.2.3 of this AC.
c. Procedures
(1) Section A36.2.2.3 provides examples that demonstrate the procedures for deriving atmospheric sound attenuation coefficients for the effects of atmospheric absorption when multiple layered sections are required.
(2) Example: The following example demonstrates the procedure for deriving the atmospheric sound attenuation coefficients when multiple layering is not required. The method for calculating values of atmospheric sound attenuation coefficients from meteorological data is specified in section A36.7.2 of this Advisory Circular. During an approach noise certification flight test for a subsonic transport jet airplane, the following atmospheric measurements were recorded (interpolated from flight data and adjusted to PNLTM flyover time):
Table 1: Example – Atmospheric Measurements
In this simplified example, the atmospheric sound attenuation coefficient (α3,150) over the sound propagation path varies less than the regulatory limit of 1.6 dB/1000 feet and therefore does not require use of multiple layers. The atmospheric sound attenuation coefficient for each one-third-octave band will be the average of the sound attenuation coefficients determined from the temperature and relative humidity data measured at 10m above the ground surface and at the airplane height, for maximum PNLT. Assuming an airplane test height of 400 feet, the average coefficient in the 3,150 Hertz band will be 5.67 dB/1,000 feet (i.e., the average of 5.54 and 5.79). The coefficients for the other one-third-octave bands are determined in a similar manner. These average coefficients are used to correct the measured data along the propagation path to the reference conditions for approach. The same general procedure would be used for determining atmospheric sound attenuation coefficients for take-off.
115. Section A36.2.2.2(e)
Average wind velocity 33 ft (10 m) above ground may not exceed 12 knots and the crosswind velocity for the airplane may not exceed 7 knots. The average wind velocity must be determined using a 30-second averaging period spanning the 10 dB-down time interval. Maximum wind velocity 33 ft (10
m) 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;
a. Explanation
This subsection specifies average and maximum wind velocity limits for noise certification testing.
b. Supplemental Information
None
c. Procedures
(1) Wind Limitations: The wind velocity limits throughout each noise measurement (PNLTM - 10 dB) taken at 10 m above the ground in the vicinity of the noise measurement points are as follows:
(i) Maximum wind velocity in any direction is not to exceed 15 knots
(ii) Maximum crosswind velocity is not to exceed 10 knots
(iii) Thirty-second average wind velocity in any direction is not to exceed 12 knots and
(iv) Thirty-second average crosswind velocity is not to exceed 7 knots.
Height, 10m (33ft) Temperature, °F RH, % a(3150), dB/1000 ft 33 ft 100 ft 200 ft 300 ft 400 ft 500 ft
71.0
70.5
68.4
68.8
67.5
65.0
65.5
66.0
63.0
60.0
56.0
54.0
5.54
5.51
5.48
5.57
5.79
5.98
The FAA ground observer and test witness should monitor these critical values. If at any time during a noise measurement these limits are exceeded, the measurement is to be declared invalid and will have to be repeated. The FAA has not approved any method for making data corrections for wind velocity or direction.
(2) Wind Velocity Measurement: The average wind velocity must be determined on the basis of a 30 second average based on 1 second samples throughout the 10 dB-down period. The maximum wind velocity must be determined on the basis of a 1 second sample throughout the 10 dB-down period. (See section A36.2.2.4 for more on wind measurement.)
(3) Real-time Crosswind Measurements: Applicants are advised to provide approved real-time crosswind component measurement systems such that the crosswind speeds can be verified after each run.
When the applicant uses a wind measurement system that is remotely located and not readily accessible, such as chart recorders that simultaneously and independently measure and record wind speed and direction, it may not be practical to determine the real-time crosswind component for each test run. If the applicant does not provide an acceptable real-time crosswind component measurement system, the 10- and 7-knot limitations specified under Wind Limitations for crosswind maximum and average limits become the maximum wind limitations regardless of wind direction.
116. Section A36.2.2.2(f)
No anomalous meteorological or wind conditions that would significantly affect the measured noise levels when the noise is recorded at the measuring points specified by the FAA; and
a. Explanation
This subsection prohibits noise certification testing when anomalous wind conditions exist.
b. Supplemental Information
(1) Anomalous Winds: Compliance of measured wind velocities with the requirements of section A36.2.2.2(e) may not be sufficient to ensure that the wind velocities at the airplane altitude or along the sound propagation path are not excessive. Such conditions may exist as a steady head, tail or cross wind or as a wind from varying directions with increasing altitude. Anomalous winds may affect the handling characteristics of an airplane during a noise measurement period (also see section B36.8(g)). They also may affect the transmitted noise. Anomalous winds include not only gusts and turbulent winds, but also wind shear, strong vertical winds, and high crosswinds at the aircraft altitude and along the sound propagation path. An applicant may be required to measure winds aloft and provide the FAA with the information. Acceptability of the wind conditions over the propagation path will be determined by the FAA.
(2) Winds Aloft Measurement: Modern Inertial Navigation Systems (INS) and Differential Global Positioning Systems (DGPS) can provide on-board aircraft data that can be used to quantify winds aloft. The measurement of winds aloft can further be processed to provide a permanent record of wind velocity and direction.
(3) Effects of Wind on Airplane Control: The FAA permits a ± 20 percent tolerance in overhead test altitude and a ±10° lateral tolerance relative to the extended runway centerline. If the flight crew cannot fly within the pretest-approved flight path tolerance limits, or experiences major variations in airspeed (see section B36.8(g)), or the airplane crabs or yaws significantly during the flight, adverse or anomalous wind conditions aloft are often the cause.
c. Procedures
(1) Flight Path: The flight crew must observe and record any occurrence where conditions aloft cause difficulty in maintaining the flight path or airspeeds, or when rough air in general makes the flight
unacceptable. When the flight crew determines that such conditions are present, noise measurement should be terminated.
(2) Applicant’s Responsibility: When proposing a test site an applicant must consider that certain geographical areas are more susceptible to anomalous wind conditions than others. The applicant may only conduct certification testing when approved by the FAA.
117. Section A36.2.2.2(g)
Meteorological measurements must be obtained within 30 minutes of each noise test measurement; meteorological data must be interpolated to actual times of each noise measurement.
a. Explanation
This subsection specifies a requirement for measurement of atmospheric conditions within 30 minutes of each noise measurement.
b. Supplemental Information
(1) Upper Atmospheric Condition Measurements: Atmospheric conditions affect sound propagation; therefore, within 30 minutes of any noise measurement, temperature and relative humidity measurements from 10 m above the ground surface to the airplane test altitude at time of PNLTM must be made using an approved method. These measurements must be obtained and validated throughout the test period to ensure acceptable meteorological data for the noise data evaluation process.
c. Procedures
(1) Atmospheric Measurements: Applicants must consider the maximum altitude that will be attained within the next 60 minutes (or less) of testing to ensure that adequate upper atmospheric measurements are acquired. Interpolations for all noise measurements are made to airplane altitude at the time of PNLTM. To have sufficient meteorological data to perform the interpolation to the actual time of each noise measurement, the first meteorological measurement flight of the day should be made within 30 minutes before the first noise measurement; the last meteorological measurement flight of the day should be made within 30 minutes after the last noise measurement flight of the day.
(2) Atmospheric Data Interpolation: Either the time of airplane flight over the centerline noise measurement point or the time of attaining PNLTM is to be used as the interpolation time for each noise measurement.
118. Section A36.2.2.3
When a multiple layering calculation is required by section A36.2.2.2(c) or A36.2.2.2(d) the atmosphere between the airplane and 33 ft (10 m) above the ground must be divided into layers of equal depth. The depth of the layers must be set to not more than the depth of the narrowest layer across which the variation in the atmospheric sound attenuation coefficient of the 3150 Hz one-third octave band is not greater than +1.6 dB/1000 ft (+0.5 dB/100m), with a minimum layer depth of 100 ft (30 m). This requirement must be met for the propagation path at PNLTM. The mean of the values of the atmospheric sound attenuation coefficients at the top and bottom of each layer may be used to characterize the attenuation properties of each layer.
a. Explanation
This section specifies criteria for defining the depth of layers and a method for determining the mean atmospheric sound attenuation coefficients when a multiple layering calculation is required for compliance with subsection A36.2.2.2(d).
b. Supplemental Information
None
c. Procedures
(1) Example: The following example demonstrates the procedure for deriving atmospheric sound attenuation coefficients when multiple layered sections are required. During an approach noise certification flight test for a subsonic transport jet airplane, the following atmospheric measurements were recorded (temperature and humidity interpolated to time of PNLTM flyover time):
Table 2: Example – Atmospheric Measurements
Height, 10m (33ft) Temperature, °F RH, % a(3150), dB/1000 ft 33 ft 100 ft 200 ft 300 ft 400 ft 500 ft
40.0
39.0
38.4
38.0
37.6
37.0
20.0
20.0
20.0
19.0
18.0
18.0
17.80
17.30
17.00
16.26
15.36
14.93
In this example, the atmospheric sound attenuation coefficient (α3,150) over the sound propagation path varies by more than the limit of 1.6 dB/1,000 feet, requiring use of multiple layered sections. The example is based on the use of equal-depth layers, each being 100 feet, from the surface to the over-flight height, inclusive. The mean value of the atmospheric sound attenuation coefficients at the top and bottom of each layer is used to represent the atmospheric sound attenuation coefficient of the layer as follows:
Table 3: Example – Atmospheric Sound Attenuation Coefficients
Layer Height, ft a(3,150), dB/1,000 ft Ground to 100 100 to 200 200 to 300 300 to 400 400 to 500
17.55
17.15
16.63
15.81
15.15
The sound attenuation coefficient listed above for each layer would be used to obtain an average atmospheric sound attenuation coefficient to be applied in adjusting the measured one-third-octave band data (3,150 Hz) over the propagation path to reference conditions for approach. The coefficients for the other onethird-octave bands would be determined similarly. Procedure (2) under section A36.9.3.2.1 of this AC provides an example for obtaining the average atmospheric sound attenuation coefficient. This simplified example demonstrates the procedure used when multiple layers are required for determining the atmospheric sound attenuation coefficients used for data correction during approach testing. The procedures for determining atmospheric sound attenuation coefficients during take-off testing would be the same, although airplane test
heights would be greater and the effects of atmospheric sound attenuation could be more significant. See explanation, supplemental information, and procedures under section A36.7 and A36.9.3.2.1 of this Advisory Circular for information on determining and applying atmospheric sound attenuation adjustments.
119. Section A36.2.2.4
The airport control tower or another facility must be approved by the FAA for use as the central location at which measurements of atmospheric parameters are representative of those conditions existing over the geographical area in which noise measurements are made.
a. Explanation
This section specifies the requirement for approval of the location at which meteorological measurements are obtained.
b. Supplemental Information
(1) Meteorological Measurements: Wind velocity, wind direction, crosswind velocity component, ambient temperature, and ambient relative humidity must be determined throughout the test period by an approved method. These data should be measured at a height of 10 m in the vicinity of the noise measurement points.
(2) Airport Measurement Systems: Airport (or other facility) meteorological facilities (located within a 1 mile distance from the noise measurement points) may be used for noise certification testing only if approved by the FAA. Experience has indicated that there may be problems in approving these facilities because they normally do not include:
(i) Recent and acceptable calibrations of meteorological measurement systems.
(ii) Real-time recording systems that meet required sampling rates. The sampling rate should be at least one sample per second for wind velocity and wind direction and at least one sample per 10 seconds for temperature, relative humidity, and barometric pressure.
(iii) Meteorological measurement systems with adequate accuracy and response. The measurement systems used should meet the following minimum measurement tolerances:
(A) Wind Velocity: ±1.1 knots (±2.0 km/h) above 2.0 knots (3.7 km/h)
(B) Wind Direction: ±5 degrees
(C) Temperature: ±1.0 degree F (±0.5 degree C)
(D) Relative Humidity: ±3 percent
(E) Barometric Pressure: ±104 psf (5 kPa).
In addition, the wind velocity and wind direction sensors should have a minimum operating threshold of 2.0 knots. The average wind velocity and wind direction data must be determined on the basis of a 30 second average (or averaging filter). Maximum wind speed values must be determined from 1-second samples of the instantaneous wind sensor output. The temperature, humidity, and barometric pressure sensors must have a response time of no greater than 10 seconds.
(iv) Meteorological measurement system components at the required height of 10 m above the ground.
c. Procedures
(1) Meteorological Measurements: Meteorological data are to be continuously measured and recorded at a height of 10 m above the ground in the vicinity of the noise measurement points throughout the 10 dB-down period.
(2) Applicant’s Responsibility: Applicants must include a description of proposed meteorological measurement equipment for noise certification testing in a noise compliance demonstration plan.
120. Section A36.2.3 Flight path measurement
a. Explanation
This section specifies requirements for tracking the airplane position, synchronizing the airplane position with noise measurements, and ensuring that position and performance data are sufficient for adjustments of measured noise data to reference conditions.
b. Supplemental Information
None
c. Procedures
None
121. Section A36.2.3.1
The airplane height and lateral position relative to the flight track must be determined by a method independent of normal flight instrumentation such as radar tracking, theodolite triangulation, or photographic scaling techniques, to be approved by the FAA.
a. Explanation
This section specifies a requirement that the airplane's position in space be determined by an FAA- approved method.
b. Supplemental Information
(1) Airplane Position Measurement: The FAA has approved several methods for measurement of airplane position. Additional guidance is also provided in section 6.5 of the appended ICAO TM. The FAA has generally permitted any system that the applicant can use to accurately determine the airplane position relative to the extended runway centerline for each half-second throughout the 10 dB-down period noise measurement.
Photographic scaling has been approved when the applicant can produce coordinated time sequencing airplane positions at three or more locations during the overflight. These can then be interpolated to identify the airplane position throughout the 10 dB-down period and can be used as evidence to show that the airplane was within the maximum lateral and height deviations during each half-second of the noise test. Guidance for obtaining FAA approval of differential global positioning systems (DGPS) is available in VNTSC Letter Report DTS-75-FA753-LR3 (Reference 4 of this AC). FAA has also approved video camera systems in addition to the photo scaling methods. FAA approval of airplane position measurement systems requires demonstrated accuracy and system documentation.
(2) Independent Airplane Position Determination: The FAA will approve only those airplane position and altitude indicating and recording systems that are independent from the direct airplane flight path indicating systems. This restriction does not prohibit use of real time flight guidance systems (CDI/GDI) on board
the aircraft to assist the flight crew during noise certification tests. These systems such as microwave space position systems, INS, Precision DMU and DGPS can provide guidance to the flight crew by providing the direct, real-time airplane position relative to the extended runway centerline. The data from such independent systems can be recorded to produce a time coordinated permanent record of each test.
c. Procedures
None
122. Section A36.2.3.2
The airplane position along the flight path must be related to the noise recorded at the noise measurement locations by means of synchronizing signals over a distance sufficient to assure adequate data during the period that the noise is within 10 dB of the maximum value of PNLT.
a. Explanation
This section specifies a requirement that airplane position data be synchronized with the noise measurements.
b. Supplemental Information
(1) Measurement System Synchronization: FAA-approved airplane position and altitude indicating and measurement systems must be time synchronized with the noise measurement systems and meteorological measurement systems. The time synchronization between noise measurements and airplane position should be precise. A common time base must be used to synchronize noise, aircraft tracking, and meteorological measurements. Time-space-position information (TSPI) must be determined at half-second intervals throughout the sound-measuring period (within 10 dB of PNLTM) by an FAA-approved method that is independent from systems installed aboard, and normally used to control, the airplane. During processing, measured TSPI data must be interpolated over time to the time of sound emission of each half-second acoustic data record within the 10 dB-down period. The time associated with each half-second record is 0.75 seconds before the end of each 2-second exponential averaging period (As defined in section A36.3.7.6). Although the simplified procedure requires adjustment of only the (PNLT) maximum record to the reference track, emission coordinates must be determined for each half-second record for use in background noise adjustment procedures and/or for determination of incidence-dependent free-field microphone and windscreen corrections.
(2) Measurement System Component Approval: Some off-the-shelf TSPI equipment may require software enhancement to accommodate the specific installation. Each applicant must submit information to the US Department of Transportation’s Volpe National Transportation Systems Center (VNTSC) about the software used. VNTSC will determine whether the software satisfies part 36 requirements. All TSPI equipment and software must be demonstrated to and approved by the FAA to ensure the system's operational accuracy.
(3) Methods of Time Synchronization: Special care must be taken to properly synchronize acoustic data recordings with aircraft time-space-position information (TSPI) data. Specific methods, presented in order of preference, include:
(i) Continuous Time-Code Recording: This method uses a time-code signal, such as IRIG B, which is a modulated, audio-frequency signal used for encoding time-base data, developed by the Inter-Range Instrumentation Group (IRIG). In this method, the time-code signals from individual generators that have been synchronized to a common time-base are continuously recorded by both the noise data recorder(s) and by the TSPI system during measurement test runs. Synchronization of multiple generators can be performed physically (by interconnecting via cable) or by means of radio transmission. The transmitted continuous time-code signal can be recorded directly, or used either continuously or in bursts to maintain synchronization of an independent time-code generator which is being recorded directly. This method allows for high-quality continuous time-code recording when there are intermittent reception problems.
(A) Synchronization must be accomplished at the start of each measurement day and checked at the end of each measurement day to minimize the effects of generator time drift. Any such drift must be documented and accounted for in processing.
(B) The use of Global Positioning System (GPS)-based measurement systems for acquisition of TSPI data is becoming commonplace. GPS receivers are capable of providing the user with precise time-base information (broadcast from the GPS satellite system), in some cases, eliminating the need for a separate time keeping device in the TSPI system. For noise data recording or for non-GPS-based-TSPI systems, dedicated IRIG B time-code generators are available that uses the GPS signal to constantly update and maintain time synchronization. Use of such a universal broadcast time-base can greatly simplify the logistics of time synchronization between measurement systems. It should be noted that there are two available time-bases for GPS-based systems, GPS Time and Coordinated Universal Time (UTC), whose values differ by more than 10 seconds at any given instant. Although the GPS signal includes both time bases, not all GPS receivers give the user access to both, therefore, the user should exercise caution in receiver selection.
(C) Many acoustical data recorders provide separate annotation channels in addition to the normal data channels. These channels are often not suitable for recording a modulated time-code signal because of limitations on dynamic range or bandwidth. In such cases, a normal data channel of the recorder must be dedicated to recording the time-code signal.
(D) When continuous time-code recording is used, analysis of the recorded acoustic data can be initiated by routing the time-code channel output into a time-code reader and triggering the analyzer based on readout time.
(ii) Recording of Single Time Marker: This method involves transmittal and recording of a radio "hack", or tone, usually used to indicate the "recorders on" or "overhead" time instant. This method typically requires a dedicated channel on both the noise and the TSPI recording systems. When such a system is used, analysis can be triggered manually by an operator listening for the hack, or by a detector circuit responding to the tone. When the operator wishes to start analysis at a time other than that of the time marker, a stopwatch or delay circuit can be used to delay triggering of the analyzer. When manual triggering is employed, the operator must use extreme care to perform the triggering as accurately as possible. Accuracy to within one-tenth of a second can be expected from a conscientious human operator.
(iii) Measurement of Interval between Recorder Start and Overhead: This method of synchronization involves use of a stopwatch or elapsed-time indicator to measure the interval between start-up of the noise data recorder and the instant that the aircraft position is overhead of the centerline noise measurement point. This method can be employed successfully as long as (1) the operator exercises care in timing, (2) the determination of the overhead instant is performed accurately, and 3) the start-up characteristics of the recorder (in both record and playback modes) are known and repeatable. Some recorders have variable startup times that cannot be predicted. Such recorders are not suitable for this method of synchronization.
(iv) Setting of Internal Time-Stamp Clock: Many digital recorders maintain a continuous internal time-of-day function by encoding time data in the recorded data stream. This method uses a digital recorder's sub-code time, synchronized to the time-base used for the TSPI data. Unfortunately, the time-setting function on many recorders does not provide for the necessary precision. The "second" digits cannot be made to "tick" in synchrony with an external clock. Such recorders are unsuitable for this method of synchronization. As with the continuous time-code recording method, synchronization by this method must be checked at the beginning and end of each measurement day, and any drift accounted for in processing.
(4) Additional Time-Synchronization Considerations: Regardless of the synchronization method used, all elements affecting time synchronization (such as analyzer start-up delay, head displacement between normal and annotation data channels on analog recorders, delays in automated triggering circuits, etc.) must be identified, quantified, and accounted for in analysis and processing. Whenever human response to a timing event is required, errors cannot be accurately predicted, and conscientious operation is required to minimize such
errors. The use of automated methods is preferred. Other methods, or variants of the listed methods may be appropriate, but the use of all methods and instrumentation is subject to prior approval by FAA.
(5) 10 dB-down Period: This period is the portion of the airplane flyover in which the measured noise level is within 10 dB of PNLTM (i.e., the period to be used for the calculation of EPNL). Care should be taken during use of the flight path intercept method so that noise levels are outside the 10 dB-down period before flight path go-around procedures are initiated.
c. Procedures
None
123. Section A36.2.3.3
Position and performance data required to make the adjustments referred to in section A36.9 of this appendix must be automatically recorded at an approved sampling rate. Measuring equipment must be approved by the FAA.
a. Explanation
This section specifies requirements for measuring sufficient airplane position and performance data to permit adjustments from test to reference conditions. See section 6.6 of the appended ICAO TM. These data must be measured and recorded in permanent form, and the measurement systems used to record data must be FAA approved.
b. Supplemental Information
(1) Airplane and Engine Performance Parameters: Examples of parameters needed for measurement of airplane and engine performance include: airplane altitude, climb angle, airspeed and gross weight, flap position, landing gear position, engine thrust (power) setting parameters (e.g., compressor rotor speed, engine pressure ratio, exhaust gas temperature), and airplane accessory condition (e.g., A/C and APU "on" or "off"). Any other parameters that may affect measurement or adjustment of noise data and/or airplane or engine performance must also be recorded throughout the 10 dB-down period (e.g., SBV position, CG position).
c. Procedures
(1) Applicant's Responsibility: Aircraft performance parameters to be recorded and their range and tolerance should be proposed by an applicant in a noise compliance demonstration plan.
(2) Airplane Performance Measurements: Adequate airplane and engine parameters are to be recorded during all certification testing to ensure that airplane performance can be accurately determined. For example, for transport airplanes this may necessitate measurement and recording of airplane flap position, landing gear position, speed brake position, APU operation, and normal engine thrust (power) setting and associated airplane flight parameters. Determination and recording of adequate information enables validation of the test airplane configuration and correction of airplane performance and engine performance from test conditions to part 36 acoustic reference day conditions.
(3) Recorder Sampling Rate: The measurements of airplane position, airplane airspeed, airplane performance and engine performance parameters are to be recorded at an approved sampling rate sufficient to permit adjustments from test to reference conditions throughout the 10 dB-down period. An acceptable recording sampling rate for transport category airplanes is two to five samples per second.
124. Section A36.3 Measurement of Airplane Noise Received on the Ground
125. Section A36.3.1 Definitions
For the purposes of section A36.3 the following definitions apply:
126. Section A36.3.1.1
Measurement system means the combination of instruments used for the measurement of sound pressure levels, including a sound calibrator, windscreen, microphone system, signal recording and conditioning devices, and one-third octave band analysis system.
Note: Practical installations may include a number of microphone systems, the outputs from which are recorded simultaneously by a multi-channel recording/analysis device via signal conditioners, as appropriate. For the purpose of this section, each complete measurement channel is considered to be a measurement system to which the requirements apply accordingly.
127. Section A36.3.1.2
Microphone system means the components of the measurement system which produce an electrical output signal in response to a sound pressure input signal, and which generally include a microphone, a preamplifier, extension cables, and other devices as necessary.
128. Section A36.3.1.3
Sound incidence angle means in degrees, an angle between the principal axis of the microphone, as defined in IEC 61094-3 and IEC 61094-4, as amended and a line from the sound source to the center of the diaphragm of the microphone.
Note: When the sound incidence angle is 0°, the sound is said to be received at the microphone at "normal (perpendicular) incidence;" when the sound incidence angle is 90°, the sound is said to be received at "grazing incidence."
129. Section A36.3.1.4
Reference direction means, in degrees, the direction of sound incidence specified by the manufacturer of the microphone, relative to a sound incidence angle of 0°, for which the free-field sensitivity level of the microphone system is within specified tolerance limits.
130. Section A36.3.1.5
Free-field sensitivity of a microphone system means, in volts per Pascal, for a sinusoidal plane progressive sound wave of specified frequency, at a specified sound incidence angle, the quotient of the root mean square voltage at the output of a microphone system and the root mean square sound pressure that would exist at the position of the microphone in its absence.
131. Section A36.3.1.6
Free-field sensitivity level of a microphone system means, in decibels, twenty times the logarithm to the base ten of the ratio of the free-field sensitivity of a microphone system and the reference sensitivity of one volt per Pascal.
Note: The free-field sensitivity level of a microphone system may be determined by subtracting the sound pressure level (in decibels re 20 µPa) of the sound incident on the microphone from the voltage level (in decibels re 1 V) at the output of the microphone system, and adding 93.98 dB to the result.