a. Explanation
This section specifies equations for calculating the atmospheric attenuation of sound.
b. Procedures
(1) Data Adjustment Procedures for Atmospheric Sound Attenuation Effects: Procedure (1) under section A36.2.2.3 and Procedure (2) under section A36.9.3.2.1 of this AC provide example procedures (using the equations given in section A36.7.2) to adjust noise certification data from test to reference conditions accounting for differences in atmospheric sound attenuation.
c. Procedures
None
236. Section A36.8. [Reserved]
237. Section A36.9. Adjustment of Airplane Flight Test Results
238. Section A36.9.1
When certification test conditions are not identical to reference conditions, appropriate adjustments must be made to the measured noise data using the methods described in this section.
a. Explanation
This section specifies methods for adjustment of noise data when test conditions differ from reference conditions.
b. Supplemental Information
(1) Adjustments to Reference Conditions: Most noise certification tests are conducted during conditions other than the reference conditions. During these tests, the airplane may be at a different altitude (height over the microphone) or deviate laterally from the intended flight path. The engine thrust (power), atmospheric conditions, airplane altitude, and/or gross weight might also differ from reference conditions.
Therefore, measured noise data must be adjusted to reference conditions to determine whether compliance with part 36, Appendix B, certification noise limits may be achieved. Reference 7 requires that adjustment procedures and analysis methods be reviewed and approved by FAA AEE. FAA requires an audit procedure conducted by VNTSC to ensure that data adjustment and analysis methods that are proposed by applicants satisfy requirements of current regulations and approved procedures ( See Appendix 2 of this AC). Any changes, including software revisions, firmware upgrades, or instrumentation changes are subject to VNTSC review before they can be used for noise certification evaluations. Program validation should be planned and the required information submitted to the FAA very early in the certification cycle, since VNTSC evaluation and FAA approval may take longer than 18 months. FAA Order 8110.4B (reference 7) specifies that future amendments to part 36 will be evaluated by VNTSC to determine whether approved data adjustment methods and analysis methods will be subject to re-evaluation. Applicants will be notified if re-evaluation is required. An appropriate Directorate NCS will provide guidelines for such re-evaluations.
(2) Reference Conditions: Reference conditions are specified under section B36.7(a)(5).
(3) Ambient Noise Adjustments: See section A36.3.10 of this AC.
(4) Band-Dropping: FAA policy does not allow the implementation of band-dropping or band-elimination techniques. One-third octave band sound pressure levels are not to be set to zero, except as
might normally occur as the result of calculations during the noise data evaluation process (for example, during frequency extrapolation of masked high-frequency band levels).
(5) Minimum Approach Distance: At the reference approach parameters of 394 feet altitude and –3 degree glide slope, the reference minimum distance equals 394 feet times the cosine of –3, or 393.46 feet between the aircraft ILS antenna and the microphone measuring station.
(6) High Altitude Test Sites: For test sites at or above 1,200 feet (366 meters), data must be adjusted to account for jet noise suppression due to the difference in the engine jet velocity and jet velocity shear effects resulting from the change in air density. This adjustment is described in Appendix 6 of the appended ICAO TM.
c. Procedures
(1) Data Adjustments: The applicant is to adjust all measured data to certification reference conditions by FAA-approved procedures. The applicant must submit all proposed data adjustment procedures to the FAA ACO for review and approval. The FAA may require submittal of extensive detailed information about the data adjustment process and analysis methods for evaluation and acceptance. The Volpe Center Acoustics Facility of the U.S. Department of Transportation’s Research and Special Programs Administration (VNTSC) assists the FAA in the evaluation and approval of applicants' data adjustment and analysis methods.
(2) VNTSC Evaluations: Appendix 2 of this AC provides the information and forms that an applicant may use to submit noise, airplane tracking, and meteorological measurement data to the FAA for evaluation and approval of the applicant’s noise certification data adjustment and analysis methods.
(3) NPD Database: To facilitate data adjustments, many applicants convert measured noise certification test data into a Noise Power Distance (NPD) database as an equivalent procedure. The NPD database should be constructed utilizing data that are representative of reference conditions. For example, the selected NPD airspeed must be representative of the range of airspeeds for the configurations and gross weights which are anticipated to be used for reference conditions. Similar adjustments can be made to test conditions for distance, airplane altitude, engine thrust (power) and atmospheric sound attenuation to create a useful NPD database. Then, when the reference airspeed and other reference conditions are identified and approved, final noise adjustments can easily be made to the NPD database values to obtain reference EPNL values.
(4) Applicant’s Responsibility: The applicant is responsible for:
(i) Making data adjustments in accordance with a FAA approved noise demonstration compliance plan
(ii) Ensuring version control of all noise data analysis software and informing the cognizant FAA ACO of all revisions.
239. Section A36.9.1.1
Adjustments to the measured noise values must be made using one of the methods described in sections A36.9.3 and A36.9.4 for differences in the following:
(a) Attenuation of the noise along its path as affected by "inverse square" and atmospheric attenuation
(b) Duration of the noise as affected by the distance and the speed of the airplane relative to the measuring point
(c) Source noise emitted by the engine as affected by the differences between test and reference engine operating conditions
(d) Airplane/engine source noise as affected by differences between test and reference airspeeds. In addition to the effect on duration, the effects of airspeed on component noise sources must be accounted for as follows: for conventional airplane configurations, when differences between
test and reference airspeeds exceed 15 knots (28 km/h) true airspeed, test data and/or analysis approved by the FAA must be used to quantify the effects of the airspeed adjustment on resulting certification noise levels
a. Explanation
This section specifies the types of differences between test and reference conditions that require adjustment of measured noise values.
b. Supplemental Information
(1) Test Atmospheric Conditions: Noise certification tests are rarely, if ever, conducted during atmospheric conditions that comply with reference conditions (i.e., sea level ambient pressure, 77º Fahrenheit and 70 percent relative humidity).
(2) Sound Propagation: The sound that propagates along the path between an airplane and the noise measurement points is affected by the meteorological conditions of the air through which the sound passes.
Temperature and relative humidity affect the atmospheric sound attenuation of transmitted source noise. Noise test data are to be adjusted to reference conditions using an FAA-approved procedure.
(3) Homogeneous Path: Reference atmospheric conditions (see section B36.7(a)(5)) along the propagation path are considered homogeneous (uniform in nature and composition). The reference conditions are sea level (pressure altitude), 77° Fahrenheit temperature, a constant 70 percent relative humidity and zero wind.
(4) Required Adjustments of Noise Values: Measured noise values must be adjusted to the reference ambient temperature and relative humidity conditions specified in part 36 and to reference airplane position and engine performance conditions. Reference airplane position and engine performance conditions must be derived using FAA-approved airplane performance data at the reference ambient conditions. These adjustments account for the effects of atmospheric sound attenuation, sound propagation distance, airplane speed, and airplane source noise.
(5) Effect of Airspeed on Source Noise: Sections 2.3.4.7 and 2.3.4.9 of the appended ICAO TM provide guidance on the effect that airspeed has on airplane source noise. Applicants must obtain FAA approval for adjustment methods to account for the effect that airspeed has on source noise (e.g., when differences between test and reference airspeeds exceed 15 knots (28 km/h) true airspeed).
(6) Other Adjustments to Noise Values: Part 36 is intended to produce accurate, repeatable, and comparable values of EPNLr. Some adjustments inherent in the FAA approval of equivalent procedures may also apply to the overall adjustment of noise values from test to reference conditions. These adjustments may account for:
(i) Airplane flight path altitude;
(ii) Airplane flight path climb or descent angle;
(iii) Airplane airspeed;
(iv) Extrapolation of adjusted data for an NPD data base;
(v) Airplane configurations (or modifications) that differ from the reference airplane;
(vi) Background noise levels;
(vii) Engine thrust (power) setting;
(viii) Spectral irregularities;
(ix) Cutback thrust (power) application;
(x) Instrument and equipment calibrations;
(xi) Pseudo-tones.
(7) Manufacturer’s Data: Adjustment of noise values from test to reference conditions should be based on FAA-approved manufacturer's data. Manufacturer's data may include:
(i) Reference flight profiles during take-off with maximum gross weight;
(ii) Flyover, lateral, and approach engine thrust (power) or thrust settings at reference conditions;
(iii) Engine cutback thrust (power) requirements at reference flyover conditions;
(iv) Data defining negative runway gradients;
(v) Reference airspeeds during flyover, lateral, and approach tests at maximum gross weights.
c. Procedures
(1) Applicant’s Responsibility: The applicant is responsible for:
(i) Developing and obtaining FAA approval of noise data including those associated with equivalent procedures.
(ii) Obtaining FAA approval of noise data adjustment procedures and analysis methods.
(iii) Applying approved noise data adjustments used to determine values of EPNLr.
(iv) Documenting adjustment procedures not specifically identified in part 36.
240. Section A36.9.1.2
The “integrated” method of adjustment, described in section A36.9.4, must be used on takeoff or approach under the following conditions:
(a) When the amount of the adjustment (using the “simplified” method) is greater than 8 dB on flyover, or 4 dB on approach; or
(b) When the resulting final EPNL value on flyover or approach (using the simplified method) is within 1 dB of the limiting noise levels as prescribed in section B36.5 of this part.
a. Explanation
None.
b. Supplemental Information
(1) Simplified and Integrated Methods: When testing is conducted at other than reference conditions, noise data must be adjusted to reference conditions by one of the following methods:
(i) The simplified analysis procedure, whereby the PNLTM value of the lateral, flyover, and/or approach noise measurement is adjusted to reference conditions
(ii) The integrated analysis procedure whereby, each half-second spectrum of noise measurements throughout the 10 dB-down period is adjusted to reference conditions. The noise emission angle is kept constant in determining the reference emission time that corresponds to the test emission time. The use of the integrated procedure is not required by part 36 for adjustment of lateral noise data.
c. Procedures
(1) NPD Equivalent Procedures: Applicants that propose the use of NPD databases may be required to use the integrated method as part of an equivalent procedure approval process. Appropriate noise data adjustments from test to reference conditions must be incorporated into the integrated method prior to comparing values of EPNLr to the limits specified in Appendix B.
241. Section A36.9.2 Flight profiles
As described below, flight profiles for both test and reference conditions are defined by their geometry relative to the ground, together with the associated airplane speed relative to the ground, and the associated engine control parameter(s) used for determining the noise emission of the airplane.
242. Section A36.9.2.1 Takeoff Profile
Note: Figure A36-4 illustrates a typical takeoff profile.
(a) The airplane begins the takeoff roll at point A, lifts off at point B and begins its first climb at a constant angle at point C. Where thrust or power (as appropriate) cut-back is used, it is started at point D and completed at point E.
From here, the airplane begins a second climb at a constant angle up to point F, the end of the noise certification takeoff flight path.
(b) Position K1 is the takeoff noise measuring station and AK1 is the distance from start of roll to the flyover measuring point. Position K2 is the lateral noise measuring station, which is located on a line parallel to, and the specified distance from, the runway center line where the noise level during takeoff is greatest.
(c) The distance AF is the distance over which the airplane position is measured and synchronized with the noise measurements, as required by section A36.2.3.2 of this part.
a. Explanation
This section specifies the take-off flight test profiles for flyover noise measurements.
b. Supplemental Information
(1) Take-off Tests: The reference take-off configuration selected by the applicant must be within the approved airworthiness certification envelope. Special flight crew procedures or aircraft operating procedures are not permitted. The following figures explain the approved takeoff procedures used during noise certification flight testing for subsonic transport category large airplanes and jet airplanes.
(2) Take-offs with Thrust (Power) Reduction: Take-offs with thrust (power) reduction are permitted to be included in the reference flyover noise certification procedure (and in the reference lateral noise certification procedure for tests conducted before August 7, 2002) for subsonic transport category large airplanes and jet airplanes, regardless of category (see Figure A36.4). Maximum average engine take-off thrust (power) must be used from the start of take-off roll to the minimum altitude for initiation of thrust (power) reduction as specified in section B36.7(b); see the table below. Section B36.7(b)(2) limits the thrust (power) reduction to the greater of:
(a) that required to maintain one-engine-inoperative level flight, or (b) that required to maintain 4 percent climb gradient with all engines operating. After thrust (power) reduction a slight decrease in the climb gradient may occur due to the thrust lapse that results from increased altitude during the 10 dB-down period. Figure 2 illustrates an example of the effect of thrust (power) reduction on the PNLT time history.
Figure A36-4. Typical Takeoff profile
(3) Minimum Altitude for Thrust (Power) Reduction: During take-off, the engine thrust (power) may be reduced after the minimum altitude has been reached. The minimum thrust (power) reduction altitudes are given below:
Stage Number of Engines Bypass Ratio Minimum Thrust (Power) Reduction Altitude 1 a. More than 3 turbojet engines
b. 3 or fewer turbojet engines
c. Not powered by turbojet engines
a. All bypass ratios
b. All bypass ratios
c. Not applicable
a. 700 ft (214 m)
b. 1,000 ft (305 m)
c. 1,000 ft (305 m) 2 a. More than 3 turbojet engines
b. 3 or fewer turbojet engines
c. More than 3 turbojet engines
d. 3 turbojet engines
e. Fewer than 3 turbojet engines
f. Not powered by turbojet engines
a. Less than 2.0
b. Less than 2.0
c. 2.0 or more
d. 2.0 or more
e. 2.0 or more
f. Not applicable
a. 700 ft (214 m)
b. 1,000 ft (305 m)
c. 689 ft (210 m)
d. 853 ft (260 m)
e. 984 ft (300 m)
f. 1,000 ft (305 m) 3 a. More than 3 engines
b. 3 engines
c. Fewer than 3 engines
a. Minimum cutback altitude for Stage 3 airplanes is not dependent on bypass ratio.
b. Minimum cutback altitude for Stage 3 airplanes is not dependent on bypass ratio.
c. Minimum cutback altitude for Stage 3 airplanes is not dependent on bypass ratio.
a. 689 ft (210 m)
b. 853 ft (260 m)
c. 984 ft (300 m)
PNLT
POINT OF THRUST (POWER) REDUCTION TIME
B ALONE
A ALONE
FLYOVER NOISE TIME
HISTORY WITH THRUST (POWER) REDUCTION SPOOL DOWN
FULL POWER 10d 2(A) TAKE-OFF NOISE TIME HISTORY
POINT OF THRUST (POWER) REDUCTION
FULL TAKEOFF THRUST (POWER) FLIGHT PATH ENGINE SPOOL- DOWN PERIOD
REDUCED THRUST (POWER) FLIGHT PATH
NOISE MEASURING POINT 2(B) TAKE-OFF FLIGHT PATH OVER FLYOVER
MEASURING POINT WITH THRUST (POWER) REDUCTION
Figure 2: Noise Time History with Thrust (Power) Reduction
(4) Full Thrust (Power) Take-offs: Full Thrust (power) take-offs are also permitted as the reference flyover noise certification procedure and are required for the lateral noise certification procedure for tests conducted on or after August 7, 2002 (see sections B36.3 and B36.7(b)(3)). Maximum approved take-off Thrust (power) is to be used from the start of roll (Point A) at one end of a runway (Figure 3). Liftoff from the runway is at Point B, after which the landing gear is stowed, and flap positions adjusted. At Point C, the stabilized climb angle and airspeed are achieved while maintaining full take-off thrust (power). The airplane continues to climb until sufficiently past (Point F) to ensure that the 10 dB-down time noise value is measured at point K. Between Points C and F, the thrust (power), flight path and aircraft configurations are to be kept constant.
Altitude BR LOF Test Start
PT A PT B PT C PNLTM-10dB PNLTM PNLTM-10dB Test End PT F PT M PT K Flight Path Microphone PNLT Time 10dB-down Period PNLTM C F 10dB 21,325 ft (6,500 m)
Figure 3: Normal Full Thrust (Power) Take-Off
(5) Flight Path Intercept Take-off: The flight path intercept procedure is permitted as an equivalent procedure for take-off noise certification testing (Figure 4). Under this equivalent procedure, noise certification test runs may be conducted with the airplane “intercepting” the relevant portion of the noise certification flight test path. This procedure is used in lieu of performing an actual landing and takeoff for each test run. When using the flight path intercept procedure, the airplane test flight path, pitch, airspeed, thrust (power), etc., are to be identical to those that would have existed had the applicant conducted the take-off noise certification test from a static condition on the runway. See sections 2.1.1 and 3.1.1 of the appended ICAO TM.
Altitude 0 PT B PT C PNLTM-10dB PNLTM PNLTM-10dB Test End, Initiate Go Around PT F PT M PT K Flight Path Microphone 10dB down periodDistance Power Advance Stabilized Conditions PNLT PNLTM Time C F 10dB
(6) Flight Path: Figure 5 illustatrates the flight path tolerance for envelope within which the flight crew can fly between Points C and F. The FAA permits a + 20 percent tolerance in overhead test altitude and a + 10° lateral tolerance relative to the extended runway centerline. These tolerances permit the applicant to conduct testing during most wind conditions with minimal risk of re-testing being required due to off-target flight paths. In conjunction with the climb gradient and approach angle, these flight path deviation limitations define the takeoff “flight path” through which the aircraft is to fly during and throughout the noise measurements (during 10 dB-down period). During flyover and lateral noise measurements, the extended centerline is not visible and it may be more difficult to conduct flight within the approved flight path, especially during conditions of anomalous winds aloft. Several methods have been devised to assist, and provide direction to the flight crew in order to stay within the required flight path envelope. Indicators located in the airplane cockpit can provide flight path direction and indicate deviations from the extended runway centerline. Transmissions from the airplane position-indicating system (e.g., microwave position system, precision DMU, or DGPS) can also provide useful inputs.
Figure 4: Flight Path Intercept Take-Off
Allowable Vertical Deviation Flight Path Test End Microphone Location Test Begin PT C PT F
PT K PT M PNTLM-10dBPNTLM-10dB Extended Runway Centerline Overhead Test Target Altitude Allowable Lateral Deviation Overhead Test Window
Figure 5: Take-Off Flight Path
c. Procedures
(1) Target Test Conditions: Target test conditions are established for each noise measurement.
These target conditions specify the flight procedure, aerodynamic configuration to be selected, airplane weight, engine thrust (power), airspeed, and, at the closest point of approach to the noise measurement point, airplane altitude. Regarding choice of target airspeeds and variation in test weights, the possible combinations of these test elements may affect the airplane angle-of-attack or airplane altitude and therefore possibly the airplane noise generation or propagation geometry. See sections 2.1.2.1 and 3.1.2(a) of the appended ICAO TM for guidance on the choice of target airspeeds and variation in test weights.
(2) Simulated Thrust (Power) Reduction Evaluation: When thrust (power) reduction, as permitted under section B36.7(b), is used in conjunction with establishing the flyover noise certification level (or lateral noise certification level for tests conducted before August 7, 2002), a commonly used equivalent procedure allows for the noise certification level to be determined through the coupling of the NPD database with the approved engine spool-down data. As long as the minimum thrust (power) reduction height requirements specified in section B36.7(b)(1) are met, the thrust (power) reduction initiation height may be selected to ensure stabilized thrust (power) reduction conditions before the initial 10 dB-down point. As discussed in section 2.2.1.2 of the appended ICAO TM, this procedure can be implemented using constant stabilized thrust (power) throughout the 10 dB-down period. Flyover noise levels with thrust (power) reduction may also be established from the merging of PNLT versus time measurements obtained during constant thrust (power) operations. The 10 dB-down period may contain portions of both the full thrust (power) and reduced thrust (power) noise time histories. Section 2.2.1.1 of the appended ICAO TM describes the establishment of the flyover noise certification level when the 10-dB down PNLT noise time history contains portions of both full thrust (power) and reduced thrust (power). The FAA must
witness and approve all engine spool-down testing. See section B36.7(b)(1) for additional information regarding engine spool-down testing.
(3) Flight Test Procedures: Before the start of noise testing the FAA must approve flight path tolerances (see Supplemental Information Item 6 under this section). Between Points C and F (Figure 5), the engine thrust (power), airplane flight path and aerodynamic configuration must be kept constant during each approved certification flight test (except when take-offs with thrust (power) reduction are being demonstrated).
(4) Invalid Test Data: Noise measurements obtained when the airplane flies outside the approved flight path envelope between Points C and F (Figure 5) during a noise certification test are considered invalid, and the noise measurement is to be repeated.
243. Section A36.9.2.2 Approach Profile
Note: Figure A36-5 illustrates a typical approach profile.
(a) The airplane begins its noise certification approach flight path at point G and touches down on the runway at point J, at a distance OJ from the runway threshold.
(b) Position K3 is the approach noise measuring station and K3O is the distance from the approach noise measurement point to the runway threshold.
(c) The distance GI is the distance over which the airplane position is measured and synchronized with the noise measurements, as required by section A36.2.3.2 of this part. The airplane reference point for approach measurements is the instrument landing system (ILS) antenna. If no ILS antenna is installed an alternative reference point must be approved by the FAA.
Figure A36-5. Typical Approach profile