Chapter 6 GUIDELINES FOR HELICOPTERS NOT EXCEEDING 7000 LB EVALUATED UNDER APPENDIX J OF 14 CFR PART 36
CHAPTER TABLE OF CONTENTS
601 EXPLANATORY INFORMATION237
601(a) General237
601(b) Noise Measurements238
601(c) Noise Certification Test and Measurement Conditions238
601(c)(1) Atmospheric test conditions238
601(c)(2) Anomalous meteorological conditions239
601(c)(3) Winds speed239
601(c)(4) Anomalous wind conditions239
601(c)(5) Flight test conditions239
601(c)(6) Flight test conditions241
601(c)(7) Flight test conditions242
601(c)(8) Flight test conditions242
601(c)(9) Flight test conditions243
601(d) Noise Unit Definition243
601(d)(1) Units244
601(e) Measurement of Helicopter Noise Received on the Ground244
601(e)(1) Noise measurement system244
601(e)(2) Noise measurement procedures244
601(f) Adjustment of Test Results244
601(f)(1) Data adjustments244
601(g) Reporting of Data to the FAA244
601(g)(1) Data reporting245
602 EQUIVALENT PROCEDURES INFORMATION245
602(a) General245
602(b) Procedures for the Determination of Changes in Noise Levels245
602(b)(1) Modifications or upgrades involving aerodynamic drag changes245
602(b)(2) Testing of light helicopters outside Appendix J temperature and humidity limits246
601 EXPLANATORY INFORMATION [ETM 6.1] 601(a) General [ETM 6.1.1] Helicopters not exceeding 7000 lb (3175 kg) can be certificated under either Appendix H or J of the part 36. Helicopters exceeding 7000 lb (3175 kg) can be certificated only under Appendix H of part 36. Guidelines for helicopters certificated using Appendix H of part 36 are provided in Chapter 4 of this AC.
Unlike Appendix H of part 36 which requires takeoff, flyover and approach tests with noise measurements being made at three measuring points, compliance with Appendix J of part 36 is based on flyover tests only, with the noise data being obtained only at one microphone locat ed under the flight track. Flight pa th adjustments are simplified and the final results determined in terms of SEL instead of EPNL.
Also since the Appendix J procedure is based on flyover tests only, there are no trade-off provisions between flight conditions as allowed in Appendix H of part 36. However, if a helicopter not exceeding 3175 kg (7000 lb) fails to comply with the noise limit of Appendix J of part 36, certification of the helicopter under the Appendix H procedures is allowed.
601(b) Noise Measurements [ETM 6.1.2] See 301 of this AC for technical procedures generally app licable for noise certification tests of all aircraft types including those evaluated under the provisions of Appendix J of part 36. Procedures specific to Appendix J are presented in the following section.
601(c) Noise Certification Test and Measurement Conditions [ETM 6.1.3] 601(c)(1) Atmospheric test conditions AMC §J36.101(c)(2), (4), and (6) [ETM AMC No. 2 A4 2.2.2]
(1) Temperature/relative humidity test window Tests are permitted over the range of temperature and relative humidity specified in J36.101(c)(2) of part 36 and shown in Figure 49.
Reference condition (25°C; 70%) 10 dB/100 m Temperature (°C) Relative humidity (%) Test window Figure 49. Appendix J temperatu re/relative humidity test window
(2) Testing outside the temperature and relative humidity window If the limits of J36.101(c)(2) of part 36 cannot be met, but the tests can be conducted within the temperature/relative humidity limits specified in H36.101(c) of part 36, then the applicant may alternatively elect to use the equivalent procedure defined in 602(b)(2).
(3) Meteorological measurements Measurements of the meteorological conditions are required to be made using equipment approved by the FAA.
The temperature, relative humidity and wi nd measurements are required to be made in the vicinity of the noise measurement point at a height between 4 ft (1.2 m) and 33 ft (10 m). This allows the use of hand-held equipment but does not preclude the use of more complex measuring systems. Temperature and relative humidity may be measured by a hand-held psychrometer. This device measures the wet and dry bulb temperatures from which the relative humidity is obtained. Similarly wind measurements may be made using a hand-held device if its specifications comply with the provisions of J36.101(c) of part 36.
(4) Temperature and relative humidity Measurements of temperature and relative humidity should be made at intervals of not more than one hour to ensure that the test conditions remain in the required limits. It is advisable to make measurements for each flight in case it is required at a later time to verify the test conditions.
Section 6.2.2.2 provides an equivalent procedure to the specifications for temperature and humidity measurement given in J36.101(c)(2) of part 36.
601(c)(2) Anomalous meteorological conditions AMC §J36.101(c)(5) [ETM AMC No. 3 A4 2.2.2]
(1) OAT differential The presence of anomalous meteorological conditions can be reasonably determined by monitoring the outside air temperature (OAT) using the helicopter on-board temperature gauge. Anomalous conditions that could impact the measured levels can be expected to exist when the OAT at 492 ft (150 m) is higher than the temperature measured at a height between 4 ft (1.2 m) and 33 ft (10 m) above the ground by more than 2°C (3.6°F). This check should be made in level flight at a height of 492 ft (150 m) within 30 minutes of each noise measurement.
601(c)(3) Wind speed AMC §J36.101(c)(3) and (4) [ETM AMC No. 4 A4 2.2.2]
(1) Wind speed limitations Wind speed measurement points and limits are given in J36.101(c) of part 36. Wind speed measurement system specifications are given in J36.101(c)(3) of part 36. Measurements should be taken frequently and, if near the limit, at least prior to each flight to confirm that the requirements are met. Particular attention should be given to the crosswind component since this can often be the limiting factor during testing. If feasible, the reference flight path direction can be changed to reduce the impact of this requirement. These wind speeds should be recorded and included in the report of the noise certification program. Wind limits are intended to minimize the adverse effects of wind on helicopter noise generation and sound propagation.
601(c)(4) Anomalous wind conditions AMC §J36.101(c)(5) [ETM AMC No. 5 A4 2.2.2]
(1) Identification of anomalous winds Anomalous winds are difficult to quantify but, providing the helicopter can be easily flown within the flight path and airspeed limits defined in part 36, there is no excessive side slip or yawing of the helicopter and no indication of rough air, then the flights can be considered acceptable. In the case where wind effects are anticipated to be a likely problem, an agreement between the applicant and the FAA or designated observer should be reached prior to testing to determine the acceptability criteria. Normally such issues arise only with gusty wind conditions near the 10 kt (5.1 m/s) wind speed limit, high crosswind conditions or the presence of strong thermals.
601(c)(5) Flight test conditions GM §J36.105(b) [ETM GM A4 2.4]
(1) Flyover height A reference flyover height of 492 ± 50 ft (150 ± 15 m) above the ground at the noise measurement point is specified as indicated in Figure 50. The measured noise data must be adjusted for the effects of spherical spreading Page 239
Test Window between the helicopter test flight path and the reference flight path, and between reference airspeed and adjusted reference airspeed as specified in J36.205(b) of part 36.
(2) Flight path measurement The helicopter flight path is also required to be within ±10 of the vertical above the noise measurement point (see Figure 50). This requirement, along with the height test window, means that the helicopter has to fly through a height/off-track test window located directly above the noise measurement point. There is no requirement to determine the magnitude of the off-track distance, but it is necessary to show that the helicopter is within the required height and angular limits. The applicant may ther efore find merit in recording the off-track values for subsequent confirmation of compliance.
(3) Flight track: markings The helicopter has to fly on a straight path and be within ±10 of the vertical overhead the noise measurement point as shown in Figure 50. In order for this to be successfully accomplished the applicant should consider marking on the ground, in a manner that can be readily seen from the helicopter, the intended track and associated lateral limits. Brightly colored or day-glow markers or lights to mark the flight track are advisable. These markings will be very important in the case of a small helicopter where the on-board equipment may be the minimum required to comply with the airworthiness certification.
Reference height 150 m (492 ft) Flight track noise measurement station 15 m (50 ft) 10° 10° Test window Figure 50. Flight boundaries for flyover test condition
(4) Number of test runs A minimum of six flyovers, with an equal number of flights with headwind and tailwind components over the noise measurement point, are required. These test runs should be conducted in pairs, since the aim is to minimize the influence of wind speed and direction on the measur ed SEL. The tests in each pair should be conducted immediately one after another in order that the meteorol ogical conditions are as similar as possible for the two test runs. It should be possible to determine immediately after each test run if it meets the necessary requirements and thus relatively easy to establish when three pairs of valid test runs have been made. The applicant would also be advised to conduct one or two additional pairs of test runs to ensure that after all the test parameters have been examined a minimum of three valid pairs of test run results are available. If additional valid pair(s) of test runs are obtained these will be required to be included in the analysis to determine the arithmetic average SEL.
(5) Landing gear position If the helicopter has a retractable landing gear, the landing gear position for noise tests needs to be that used for Page 240
the cruise configuration.
601(c)(6) Flight test conditions AMC §J36.105(c), (d) and (e) [ETM AMC A4 2.4]
(1) Test period The test conditions have to be maintained, or held constant, over an adequate distance (time interval) to encompass the 10 dB-down period. The maximum A-weighted sound pressure level, dB(A) or L Amax, will normally occur when the helicopter is at, or just prior to, the position directly above the noise measuring point. Pre-test flights should be conducted to determine the 10 dB-down period and ensure this period is adequately captured by the noise measurement system. It is advisable that the helicopter flight test conditions are stabilized well in advance of the initial 10 dB-down point and maintained until well after the second 10 dB-down point to ensure a valid noise measurement is obtained.
(2) Maximum normal operating rotor speed In order that the noise levels are representative of normal operation, the rotor speed used must be the maximum normal value associated with flyover at the reference conditions. Also, since on most helicopters small changes in rotor speed occur during a stabilized flight, a ±1 percent rpm variation is allowed.
(3) Test weight Fuel, together with the weight of the pilot, flight observers and ballast, are normally used to set the weight of the test helicopter within the required test range of +5 percent/–10 percent of the maximum takeoff weight. Fuel burn
(i.e., decrease in fuel weight) should be documented to determine the actual test value. Care must be taken regarding the location of ballast to ensure it does not have any adverse impact on the applicable center of gravity limits.
Unlike in Appendix H, there is no requirement to conduct any test run above the maximum takeoff weight. Note that variation of the flyover noise levels within the a llowable weight limits is small and for this reason no adjustments for difference in test weights are required. The helicopter weight, or the quantity of fuel from which the weight of the helicopter can be calculated, should be recorded for each flight.
Note.— Conducting the tests as near as possible to the upper weight limit of +5 percent can also be useful in supporting noise certification of future increases in maximum gross weight of the helicopter by minimizing the likelihood of having to conduct new tests.
(4) Background noise Some initial pre-test flyovers should be performed to confirm helicopter noise levels exceed background noise by 15 dB(A) as specified in J36.109(f)(3) of part 36. The FAA has generally accepted that the requirement has been met if the maximum helicopter noise level exceeds backgr ound noise levels by 15 dB(A). If this requirement cannot be met when the flyover test is conducted at 492 ft (150 m), a lower flyover height approved by the FAA may be used. This normally will be required only in the case of lightweight/small helicopters or those that generate extremely low noise levels.
Variations in measured noise levels of up to ±1.5 dB(A) from flight to flight may typically occur. The applicant should therefore ensure that the difference between background noise and helicopter noise levels is adequate for the quietest flyover noise measurements anticipated. Such information may also be useful for adjusting the sensitivity of the noise measurement system. The level of background noise may be influenced by the location of the test site.
601(c)(7) Flight test conditions GM §J36.105(c)(1) [ETM GM A4 2.4.2]
(1) Adjusted reference airspeed The flyover adjusted reference airspeed (Var) is defined as the value at the test temperature that gives the same advancing main rotor blade tip Mach number (MAT) as associated with the reference temperature of 25C (77ºF). On most helicopters the controlling noise source is dependent on the advancing blade tip Mach number. The advancing blade tip Mach number is dependent on the temperature and thus the sound level varies with temperature. To avoid the need to make a source noise correction, as would be required for the flyover tests under Appendix H of part 36 unless an equivalent procedure is used, tests to meet the requirements of Appendix J of part 36 are required to be conducted at an adjusted reference airspeed, Var, which gives the same advancing blade tip Mach number at the time of the test as would occur if the test were conducted under the reference conditions. The speed of sound increases with absolute temperature so that tests conducted in temperatures below the reference value of 25C (77ºF) at the reference flyover height will result in a higher advancing blade tip Mach number, and a reduction in test airspeed will be required to obtain the reference advancing blade tip Mach number. Similarly when the air temperature at flyover reference height is higher than 25 C (77ºF), the flyover test speed must be increased. This requires knowledge of the OAT measured on-board the aircraft at the time of the test. The applicant should note that it is essential to test at the required airspeed value since there is no provision for the adjustment of data obtained at the wrong Mach number.
601(c)(8) Flight test conditions AMC §J36.105(c)(1) [ETM AMC No. 1 A4 2.4.2]
(1) Test speed for light helicopters For the purposes of compliance with Appendix J of part 36, the helicopter should be flown at the test speed (Var) that will produce the same advancing blade Mach number (M R) as the reference speed at reference conditions given in J36.3(a) and J36.3(c) of part 36.
The reference advancing blade Mach number (MR) is defined as the ratio of the arithmetic sum of the main rotor blade tip rotational speed (VTIP) and the helicopter true airspeed (VREF) divided by the speed of sound (c) at 25°C
(346.1 m/s) such that:
ܯோ ൌ ܸோாி ܸ ்ூܿ The test airspeed (Var) is calculated from:
ܸ ൌܿ ்ሺܸோாி ܸ ்ூሻെܸ ்ூܿ where cT is the speed of sound obtained from the on-board measurements of outside air temperature.
Since the ground speed obtained from the flyover test s will differ from that for reference conditions, an adjustment, ΔJ3, of the form ΔJ3 = 10 log (Var/VREF) will need to be applied. ΔJ3 is the increment in decibels that must be added to the measured SEL.
There are two additional requirements for light helicopter test speed. First, the airspeed during the 10 dB-down period should be close (i.e., within ±3 kt (±5.5 km/h)) to the adjusted reference speed (see J36.105(c)(1)(ii) of part 36).
The second speed requirement states that the level flyovers must be made in equal numbers with a headwind component and tailwind component (see J36.105(b) of part 36). For practical reasons, if the absolute wind speed component in the direction of flight, as measured at a height between 4 ft (1.2 m) and 33 ft (10 m) above ground (see J36.101(c)(4) of part 36), is less than 5 kt (2.6 m/s), then the effect of wind can be considered to be negligible.
In this case, the measured flyover may be used to satisfy a test run in either the headwind or tailwind direction if the flyovers are conducted in pairs. Each pair should consist of two flyovers performed one after the other in opposite directions over the reference flight track.
Any changes in rotor speed, which may occur with the flight airspeed, will also need to be taken into account in the above calculations to determine the adjusted reference airspeed. If this is likely to occur then this topic should be reviewed with the FAA to determine if any additiona l adjustments to the flight test speed are required.
Normally this is not a concern, since the rotor speed will be independent of flight speed.
The applicant should also no te that the calculated adju sted reference airspeed (Var) is the adjusted true airspeed (TAS). Additional information will be required to determine the IAS for use by the pilot. This will normally be based on calibration charts or adjustments for the airspeed measurement system showing the IAS/TAS relationship.
601(c)(9) Flight test conditions AMC §J36.105(c)(2) [ETM AMC No. 2 A4 2.4.2]
(1) Rotor speed The rotor speed can be varied on some helicopters, and on others variations in the rotor speed can occur with flight speed. In order that the noise levels are representative of normal operation, the rotor speed used must be the maximum normal value associated with flyover at the reference conditions.
Note.— It is not the intent to require noise measurements at any value but the maximum used during normal operations, and thus testing at the maximum tolerance rotor speed is not required.
On some helicopters two distinct rotor speed values are available. If both can be used for normal operations then the noise certification has to be conducted at the higher roto r speed. If the higher of the two speeds is restricted to special operations, or if the helicopter is configured so that it cannot be used at the reference conditions and/or during lower altitude flight, then, subject to approval by the FAA, testing at the lower rotor speed may be allowed.
On some helicopters it may be possible for the rotor speed to be changed by pilot action. In these cases noise certification will require the highest rotor speed specified in the RFM for the flyover flight condition at the maximum takeoff weight to be used.
For most turbine-engine-powered helicopters the rotor speed is automatically linked by the engine control/ governor system to the flight condition. If this results in a different rotor speed at the adjusted reference airspeed to that associated with the reference airspeed, then additional adjustments may be required to ensure the correct advancing blade tip Mach number is used for the tests. If this situation is likely to occur, FAA approval of the rotor speed and/or adjusted reference airspeed to be used should be obtained.
On some recently designed helicopters the use of lower ro tor speeds has been certifi cated for operations at low altitudes and/or in cruise flight. Since these lower rotor speeds are defined in the RFM and, if higher rotor speed values cannot be used at the reference conditions, exce pt possibly in an emergency, noise certification is conducted at the certificated lower rotor speed, subject to approval by the FAA. If, however, the helicopter simply incorporates a two-speed or multi-speed system, and either can be selected by the pilot, then the highest value is required to be used for noise certification.
601(d) Noise Unit Definition [ETM 6.1.4] Page 243
601(d)(1) Units GM §J36.109(b) [ETM GM 3.1] The noise levels are to be determined in terms of the SE L metric. The SEL is the time-integrated A-weighted sound level over the 10 dB-down period. This metric takes into account both the duration and the level of the sound.
601(e) Measurement of Helicopter Noise Received on the Ground [ETM 6.1.5] 601(e)(1) Noise measurement system AMC §J36.109(c) [ETM AMC A4 4.3]
(1) System and calibration requirements The noise measurement system and system calibration requirements are specified in J36.109(c) through J36.109(e) of part 36 for compliance with Appendix J of part 36. The noise demonstration compliance test plan must include a description of the system to be used for the noise measurement. The FAA must approve the measurement system and calibration procedures in order to ensure that accurate measurements and results are obtained.
601(e)(2) Noise measurement procedures AMC §J36.109(e) [ETM AMC A4 4.4]
(1) Sound level integration period The A-weighted sound pressure level must be integrated over the 10 dB-down period. When using an integrating sound level meter where the start and stop times are selected manually, the actual test integration period should be slightly longer than the true 10 dB-down period. This will not have any significant impact on the SEL value, providing the integration period is only a few seconds longer, since the noise levels will be more than 10 dB(A) below the maximum sound level value.
601(f) Adjustment of Test Results [ETM 6.1.6] 601(f)(1) Data adjustments AMC §J36.205(b) and (c) [ETM AMC A4 5.0]
(1) Height adjustment In order to account for the differences between the test heights (H) and reference height of 492 ft (150 m), which influence both the spherical spreading of the noise and the duration of the 10 dB-down period, the ∆1 adjustment is applied.
(2) Airspeed adjustment In order to account for the differences between the adjusted reference airspeed (Var) and reference airspeed (VREF), which influence the duration of the 10 dB-down period, the ∆2 adjustment is applied. Variations in the ground speed, and hence duration, as a result of wind at the test height also occur, but since test runs are to be made with equal numbers with headwinds and tailwinds (see J36.105(b) of part 36), this effectively cancels out this effect and no additional adjustments for duration are required.
601(g) Reporting of Data to the FAA [ETM 6.1.7] Page 244
602 601(g)(1) Data reporting AMC §J36.111 [ETM AMC A4 6.0]
(1) Reporting requirements Noise certification data reporting requirements are detailed in J36.111 of part 36. Compliance with stabilized test conditions, including test airspeed, average rotor speed and flyover height, should be reported for the 10 dB-down period. If an acoustic data recording is made, information about the recorder, including frequency bandwidth and sample rate and operating mode, should be recorded.
(2) Lateral position flight track data There is no requirement to determine the lateral off-track position directly above the noise measurement point, since it is necessary to show only that it is within the requirements defined in J36.105(b)(3) of part 36. Even so, an applicant may find merit in determining and reporting the lateral off-track distance, with the height information that is required, as a way to show compliance.
EQUIVALENT PROCEDURES INFORMATION [ETM 6.2] 602(a) General [ETM 6.2.1] The objective of a noise certification demonstration test is to acquire data for establishing an accurate and reliable definition of a helicopter’s noise characteristics. In add ition, part 36 establishes a range of test conditions and procedures for adjusting measured data to reference conditions.
602(b) Procedures for the Determina tion of Changes in Noise Levels [ETM 6.2.2] Changes in noise levels determined by comparison of flight test data for different helicopter model series have been used to establish certification noise leve ls of modified or newly derived versions by reference to the noise levels of the baseline or “flight datum” helicopter model. These noise changes are added to or subtracted from the noise levels obtained from individual flights of the “flight datum” helicopter model. The confidence intervals of the new data are statistically combined with the “flight datum” data to develop overall confidence intervals (see 305 of this AC).
602(b)(1) Modifications or upgrades involving aerodynamic drag changes [ETM 6.2.2.1] The use of drag devices, such as drag plates mounted beneath or on the sides of the “flight datum” helicopter, has proven to be effective in the noise certification of modi fications or upgrades involving aerodynamic drag changes.
External modifications of this type are made by manufacturers and aircraft “modifiers”. Considerable cost savings are realized by not having to perform noise testing of numerous individual modifications to the same model series. Based on these findings, it is considered acceptable to use the following as an equivalent procedure:
a) for helicopters to be certificated under Appendix J of part 36, a drag device is used that produces the aerodynamic drag calculated for the highest drag modification or combination of modifications;
b) with the drag-producing device installed, a flyover test is performed by using the appropriate noise certification reference and test procedures;
c) a relationship of noise level versus change in aerodynamic drag or airspeed is developed by using noise data, adjusted as specified in Appendix J of part 36, of the “flight datum” helicopter and of the “high drag” Page 245
configuration;
d) the actual airspeed of the modificati on to be certificated is determined from performance flight testing of the baseline helicopter with the modification installed; and
e) using the measured airspeed of the modification, certification noise levels are determined by interpolation of the relationship developed in item c).
602(b)(2) Testing of light helicopters outside Appendix J temperature and humidity limits [ETM 6.2.2.2] With the approval by the FAA, it may be possible to cond uct testing of light helicopters in compliance with the temperature and relative humidity test limits specified in H36.101(c) of part 36 (see Figure 51) instead of the limits specified in J36.101(c)(2) of part 36 . Temperature and relative humidity measurements must be made between 4 ft
(1.2 m) and 33 ft (10 m) above ground as specified in J36.101(c)(2) of part 36 and within 30 minutes of each noise measurement as required by H36.101(c)(7) of part 36. In such circumstances , it will be necessary to conduct a onethird octave band analysis of a noise recording of each flyover. The measured value of SEL must be adjusted from the test values of temperature and relative humidity to the re ference conditions defined in J36.3(a) of part 36. The adjustment procedure must be similar to that defined in H36.205(f)(2) of part 36 with the propagation distances QK and QrKr, respectively replaced by H, the height of the test helicopter when it passes ov er the noise measurement point, and the reference height, 492 ft (150 m).
Appendix 2 test window Reference condition (25°C; 70%) 12 dB/100 m Relative humidity (%) Temperature (°C) Figure 51. Optional Appendix H T emperature/relative humidity test window Page 246
Chapter 7 GUIDELINES FOR TILTROTOR AIRCRAFT EVALUATED UNDER APPENDIX K OF 14 CFR PART 36
CHAPTER TABLE OF CONTENTS
701 EXPLANATORY INFORMATION247
701(a) Background247
701(a)(1) Applicability247
701(b) General Information249
701(b)(1) Terms used in tiltrotor noise certification procedures249
701(c) Information on Specific Appendix K Texts249
701(c)(1) Definitions250
701(c)(2) Applicability250
701(c)(3) Noise evaluation measure250
701(c)(4) Reference noise measurement points250
701(c)(5) Maximum noise levels and trade-offs250
701(c)(6) Noise certification reference procedures251
701(c)(7) Test procedures252
701 EXPLANATORY INFORMATION [ETM 7.1] 701(a) Background [ETM 7.1.1] 701(a)(1) A pplicability GM §36.1(5) [ETM GM ATT F 1]
(1) Intent of Appendix K requirements The prescribed test requirements for the noise certification of tiltrotor aircraft is presented in Appendix K of part
36. It was adopted in the USA following the ICAO development effort by the CAEP5 Tiltrotor Task Group that specifically addressed the noise certification of the Bell/Agusta BA609, the first example of a civil tiltrotor aircraft, which was renamed the AW609 in 2011. Initially developed as ICAO guidelines, it is becoming the basis for noise certification of tiltrotor aircraft in the USA. The explanatory material in this chapter is intended to provide an insight as to how the guidelines were developed. Its use is anticipated for AW609 noise certification and will become an ICAO standard within the Annex after its initial application.
(2) Scope of Appendix K In anticipation of the AW609 becoming the first civil tiltrotor aircraft, these guidelines are being adopted in the USA as applicable standards. The ICAO Annex 16, Volume 1, Attachment F guidelines, published in 2002, provides a noise certificati on compliance testing methodology for tiltrotor aircraft that pr omotes uniform and consistent environmental protection for aviation.
When Working Group 1 (Noise) was originally task for developing tiltrotor guidelines, it was decided after careful deliberations, that the current Standards of Chapter 8 for helicopter in Annex 16 would offer a good basis for the guidelines since the helicopter configuration would characterize the worst case (noisiest) condition.
a) The noise from tiltrotors will be most prominent during departure and approach. In these situations tiltrotors will normally operate in or near the “helicopter mode”.
b) In the development of the guidelines, the noise of the Bell XV-15 tiltrotor aircraft (a prototype of the AW609) was observed. It was concluded that the char acter of the noise of this aircraft was much like that of a normal helicopter.
c) In horizontal flyover, the “helicopter mod e” will normally be the noisiest configuration.
d) The proposed guidelines are confined to tiltrotors th at can only take off vertically, excluding those with STOL characteristics. They will operate much like normal helicopters, with relatively steep takeoff and approach paths.
e) The level of available noise abatement technology for tiltrotors is considered to be the same as for helicopters.
f) Tiltrotor operations will often mi x with helicopter operations from the same heliport. Therefore there will be a desire to compare the noise from tiltrotors and helicopters.
(3) Transition phase noise-test point evaluation One item of strong interest is the tr ansition from one nacelle angle to the other, which may be associated with particular noise-generation mechanisms. For example, wh en one considers the tiltrotor transition from airplane mode to helicopter mode while decelerating, there is a ph ase in which the component of the speed vector that is perpendicular to the rotor changes from “top to bottom” to “bottom to top”. It would be conceivable that sometime during the transition phase, blade vorti ces would be ingested or another non-stationary effect would create additional noise.
A number of flyovers of the Bell XV-15 were listened to, one of which was especially set up to study the noise during transition. In this run, the tiltrotor (Bell XV-15) passed overhead at 492 ft (150 m) while transitioning from airplane to helicopter mode. No special phenomena were heard during this flight. In addition, during the other runs, in which there were demonstrations of hover, hover turns, sideward flight, takeoff, level flyovers at various speed/nacelle angle combinations, and approaches at 6° and 9°, no particularities were heard other than normal blade vortex interaction noise during both the 6° and 9° approaches. During the procedures to set the aircraft up for the various runs, several transitions were made from helicopter to airplane mode and back, which were listened to from different positions relative to the aircraft. No particular noise was heard.
Based on this experience and the arguments stated as follows, it has been decided not to attempt to define a special test point aimed at catching transition noise of tiltrotors.
The arguments for this are:
a) Experienced observers from industry claim they have never noticed any particular noise phenomena associated with the transitional phase. This was backed up by the specific observations of the Bell XV-15 referred to above.
b) The conversion rate is relatively slow, which means that during the whole conversion process, the flow field also changes very slowly.
c) If there were to be a transitional noise, it would probably be related to some form of blade-vortex interaction.
This phenomenon is covered under the approach procedure, and it might be hard to justify adding a measurement point to get some additional information.
d) Defining a reproducible and practicable procedure to catch the transition noise which nobody has ever noticed is virtually impossible.