107. Section A36.2.1.1
This section prescribes the conditions under which noise certification must be conducted and the measurement procedures that must be used.
Note: Many noise certifications involve only minor changes to the airplane type design. The resulting changes in noise can often be established reliably without resorting to a complete test as outlined in this appendix. For this reason, the FAA permits the use of approved "equivalent procedures". There are also equivalent procedures that may be used in full certification tests, in the interest of reducing costs and providing reliable results. Guidance material on the use of equivalent procedures in the noise certification of subsonic jet and propeller-driven large airplanes is provided in the current advisory circular for this part.
a. Explanation
b. Supplemental Information
(1) Test Conditions: Appendix A of part 36 specifies test site requirements, noise measurement procedures and data evaluation procedures for airplane noise certification testing of subsonic transport category large airplanes and jet airplanes. The applicant may use equivalent procedures, subject to FAA review and approval.
(2) Flight Path Intercept: The “flight path intercept” test procedure has been approved as an equivalent procedure wherein the airplane does not start from a static position on the airport runway for each flyover and lateral noise measurement (or land after each approach noise measurement), but remains in flight throughout the test series. See sections 2.1.1 and 3.1.1 of the appended ICAO TM. Stabilized airplane configuration and thrust (power) are maintained until it is determined that the measured noise level falls below PNLTM -10 dB. An A-weighted noise level is often used as a metric for determining this; noise measurement for a duration delimited by levels 15 dB (A) below the maximum A-weighted noise level is usually sufficient. The flight path intercept test procedure may be applied to all noise testing, including: flyover, lateral and approach noise measurements. This procedure saves valuable time by eliminating the takeoff, landing and turn-around for each measurement. This procedure also provides wider flexibility in the choice of test sites. The target airplane attitude, engine thrust (power), altitude over the microphones, constant configuration, etc., should be the same using the flight path intercept test procedure as it would be if the test were conducted from a standing start on the runway. See sections 2.1.1 and 3.1.1 of the appended ICAO TM for guidance on the use of the flight path intercept procedure.
(3) Equivalent Procedures: Equivalent procedures are discussed in sections 2 and 3 of the appended ICAO Technical Manual (TM). The procedure for obtaining FAA approval of an equivalent procedure is discussed under section 36.101 of this AC. Equivalent procedures are also discussed in sections 36.1501, A36.5.3.1, A36.5.4.3, A36.9.1.2, B36.4 and B36.7(a) of this AC. Information on equivalent procedures is also provided in paragraph 6-3(e) of reference 7.
c. Procedures
(1) Applicant’s Responsibility: An applicant must prepare a noise compliance demonstration plan that specifies a proposed certification process, including equivalencies. This plan is to be submitted to the appropriate ACO noise specialist allowing sufficient calendar time (usually 2 months prior to the scheduled test) to permit adequate FAA review and possible revisions prior to the start of any noise certification testing.
108. Section A36.2.2 Test environment
109. Section A36.2.2.1
Locations for measuring noise from an airplane in flight 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. No obstructions that significantly influence the sound field from the airplane must exist within a conical space above 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° from this axis.
Note: Those people carrying out the measurements could themselves constitute such obstruction.
a. Explanation
This section specifies the requirements for airplane noise certification test site characteristics to help ensure uniformity in the noise measurement process. It also describes the clear zone above and around the microphone station that must be free from obstructions that significantly influence the sound field from the airplane.
b. Supplemental Information
(1) Test Site Locations: When the flight path intercept test procedure is used, it may not be necessary for the test site to be located at an airport. Proposed noise certification test site locations must be submitted to the FAA for review and approval. Some test site criteria that could support selection of a non-airport test site include: level terrain, reduced air traffic, reduced ambient noise, improved weather conditions (temperature, humidity and wind), improved microphone placement, availability of field surveys, improved locations for aircraft position monitoring, and improved pilot sight and handling.
(2) Noise Measurement Points: For the flyover, lateral and approach noise measurement points, 25-foot radius circles of mowed grass (not exceeding 3” in height) are acceptable. For the lateral measurement point, the grass may be mowed in a semicircle (25 foot-radius) facing the line of flight.
(3) Snow: Snow in the area surrounding the noise measurement points may provide excessive absorption of airplane sound reflected from the ground. FAA has approved noise measurements during winter conditions when snow within a 50-foot radius of the noise measurement points has been removed. However, snow must not be piled at the borders facing the line of flight.
(4) Plowed Fields: Earthen or sandy surfaces within a 25 foot radius of the noise measurement points must be reasonably tamped down. Plowed furrows, silt, or soft powdered surfaces are unacceptable.
(5) Obstructions: Obstructions in the vicinity of the noise measurement points such as buildings, walls, trees, vehicles, and test personnel (if close enough) may be unacceptable because of reflections that influence measured noise levels.
c. Procedures
(1) Applicant’s Responsibility: An applicant must identify the proposed test site characteristics in a proposed noise certification compliance demonstration plan.
110. Section A36.2.2.2
The tests must be carried out under the following atmospheric conditions.
a. Explanation
This section specifies the atmospheric conditions that are acceptable for airplane noise certification testing.
b. Supplemental Information
(1) Unacceptable Test Data: When testing is conducted outside the limits of atmospheric conditions specified in this section, valid adjustments from test to reference conditions may not be possible, and the data may not be acceptable.
c. Procedures
(1) Applicant’s Responsibility: The applicant is responsible for the following:
(i) Specifying test atmospheric condition limits, meteorological measurement systems, calibration procedures, and data adjustment procedures in a noise compliance demonstration plan. See section A36.2.2.2(c) of this AC for more on atmospheric sound attenuation limits.
(ii) Instrumentation for the conduct of meteorological measurements.
(iii) Obtaining FAA approval for deletion of any noise measurements that were obtained outside the approved limits of atmospheric conditions.
111. Section A36.2.2.2(a)
No precipitation.
a. Explanation
This subsection prohibits noise certification testing during conditions of rain or other precipitation.
Rain and other precipitation (including fog, drizzle, snow) can affect the measurement system and sound propagation during noise measurements.
b. Supplemental Information
(1) Effects of Moisture on Microphones: Most microphones that are used during noise certification testing are susceptible to moisture. Precipitation, including snow, drizzle and fog, or excessive humidity may induce electrical arcing of the microphone sensors, making measured noise data unacceptable. However, some pre-polarized microphones are less susceptible to electrical arcing during high-moisture conditions (consult the equipment manufacturer's specifications). Special care must be taken to ensure that any windscreens exposed to precipitation be thoroughly dry, inside and out, before use. Foam windscreens can trap water; wet foam windscreens must be avoided.
(2) Microphone Internal Heaters: When internal heaters are provided, microphones are less likely to be affected by moisture in wet, humid, cold, or freezing atmospheric conditions.
c. Procedures
(1) Precautions: Special precautions must be taken by the applicant to protect microphones when a test shutdown is caused by wet, humid, or near freezing atmospheric conditions. Measurement System Components must be thoroughly dry before testing is resumed, to prevent arcing.
112. Section A36.2.2.2(b)
Ambient air temperature not above 95°F (35°C) and not below 14°F (-10°C), and relative humidity not above 95% and not below 20% over the whole noise path between a point 33 ft (10 m) above the ground and the airplane;
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.