Advisory Circular sdja: NOISE m m DtBti: 614187 AC No: 91-66 HELICOPTERS Initiatedby: AFS-840 cbrurgt:
c 1 l PUE?POSE. This advisory circular (AC) presents guidelines intended to assist pilots, operators, nranagerS,andotherinterestedpersonsintheestablis~t of effective noise reduction procedures when qerating helicapters. It is by no . mans totally coqrehensive. However, when the flight procedures andconcepts outlined herein are followed, significant noise abatemnt will be achieved, and public acceptance of helicopter 6perations should be enhanced in noise-sensitive areas 0 2 0 RELATEDREADINGMATERIAL.
a. AC 91036C, VFR Flight Near Noise-Sensitive Areas. -. L , -.
b. A(3 150/5020-l, Noise Control and Carpatibility Planning for Ai&ts.
c. AC 150/5020-2, Noise Assessmt Guidelines for New Heliports.
The above Advisory Circulars are free andavailable franthe U.S. Department of Transportation, Utilization and Storage Section, M-443.2, Washington, DC 20590 d Fly Neighborly Guide, published by the Helicopter Association International, 1919 Duke Street, Alexandria, VA 22314; (703) ,683~4646. This publication is free except for postage. ,
e. Federal Aviation Regulations (FAR) Part 150. (For sale- by the Swintendentof &cummts, U.S. Gwernmnt Printing Office, Washington, DC 20402.)
f, The FRA Office of Environmnt and Energy has issued technical reports containing helicopter noise measurement/flight test data. These data reports are caqiled for som specific types of helicopters. Tk Office of Environmnt and Energy also has published FM-FE-85-2, Aviation Noise Effects. (Contact National Technical Informtion Service (NTIS), Springfield, VA 22161, or FAA Office of Environmnt and Energy, Noise Abatemznt Division, 2XEE-100, 800 Independence Avenue, SW, Washington, DC 20591.)
3 0 BAcmmuND.
a. The Federal Aviation Administration (FAA) amtinually receives conplaints concerning low flying aircraft wer noise-sensitive areas. These canplaints have pranptea requests for regulatory action which muld prohibit low altitude flight wer certain identified noise-sensitive locations.
AC 91-66 6/4/87 .
b. FAR Part 36, as a-ted in 1969, does not address rules applicable to helicopters identified herein. However, since 1969theFAAhas consider& adopting existing noise standards for aircraftthatmuldaffectthe -ation of all helicopters. After careful evaluationof theeconanicinpact the considered pr~alswouldhaveon helicqters, theseproposalswerewithdrawn.
TkFAAhas continuedtoactivelystudytheissues surroundingnoiseabatemnt in connection with the certification and operation of lMicqters and to carpile technical data on noise abaNt in an efforttoreach anequitable solution to the acoustical iqact on the public.
4 0 OPEEUXTIONALooNsIIEEW!IoNs,
a. Operator/Pilot Responsibilities. One of thegreatestchallenges facing the heliwter industryisgainingpublic acceptanceof thehelicopter. The helicopter is considered anti-social by so11y3 segments of the public. In my cases, acceptanceof helicopteroperaticMsis thwartedbyanangry, soanetims uninfommd cmnunity who fear the perceived noise and safety problenrs inherent with helicopters. Publicreactionto a noiseenvir~tdepends uponmany cosrplex physical, econcmic, and psychological factors. Therefore, it is up to the operator to educate the public about the safety and usefulness of the helicopter and to equip the aircraft with sound-suppressing devices wkren they are available and needed. The pilot can make the public less hostile to the helicopter's day-to-day -ations by being aware of noise-sensitive routes and areas and by flying the helicopter in such a way that the sound of t& aircr&ft' causes the least possible annoyance to the public.
b. The Source of Noise. The acousticaliqactof aheliqter is a function of the size andthetype ofwlantusedas wellasthemvmtof the rotor blades through the atmosphere as they produce lift. Turbine-wed helicqters are generally quieter than pistan-wed helicqters with muffled engine exhausts, Turbine-poweredhelicoptersproducesounds often no louder than familiar surface transportation vehicles. The acoustical signature or sound of a helicopter is also due in part to the tiulation of sound by the relatively slakl-turning main rotor system.- This elation attracts attention rnmh as a flashing light attracts attention by being moL'e conspicuous than a steady one. The r&ulated sound is often referred to as blade slap. For a typical helicopter, blade slap occurs during partial power descents or when a blade intersects its crwnvortexsystemor that of another blade. when this happens, the blade experiences rapid changes in angle of attack. Either or both phenmnon generates sound. Blade slap is also generated during high speed level flight due to a shock forxmtion being created onthe advancing blade tip.
This r&e of flight should be avoided. This alone is an effective noise abatemnt qmating prom&r@ in lxpulated (noise-sensitive) areas. Figures 1 and 2 display typical noisy flight operational ara for light and &ium weight helicopters. Perceptible, mtinuous, and maximum slap areas should be avoided by the conscientious airman, One method would be to reduce airspeed 10 to 20 percentbelownormalcruising speeds.
\ 2 Par 3
FIGURE 1. MAIN RomR BL?lDE sL?sP ~~Y~-TypIcAL LIGErI! HEu@oE?l!ER 5001 t .
R/C FPM 0 1 I R/D 1
- 500- -1000 - m Slap Boundary 20 40 60 80 100 . 120 140 I I i I I 1 I C Airspeed, MPH - r -. -. b r* m Slap Boundary 5Oot m Continuous Slap t m Maximum Slap R/C FPM o I R/D Airspeed, Knots - Par 4 3
AC 91- 66 614187 ’
c. SoundMeasurement. Numrousm&hods areusedtodetennine the intensity of sound.
(1) Sound levels in the comnunity are often pressed using a unit called the A-weighted sound level. The term "weighted" refers to the filtering or Righting of sound to simulate the response on the hm ear. Since high frequency sound is mOre readily perceived than low frequency sound, this unit of nreasurmtfilters or weights the soundandconsiders how the hurrpln ear responds to canbinations of different pitch. The A-weighted sound level is referred to as dI3A or by the more recent term, AKm.
(2) Sound vure level EEL) is a masuremnt of the total amunt of the soundenergy (duration andmagnitude) from a single eventandexpressed in decibels MB). The sound exposure level is A-ighted and is used in calculating the helicopter noise contritxtion to the equivalent sound level (Leq) and the day-night sound level WL).
(3) The average sum of all noise events for a given period of timz is called equivalent sound level or Leq. The word equivalent is used so that a fluctuation sound level during a specified period of time can be carpared to a steady sound level for 'the same period of time. The purpose of Leq is to provide a single nmber value noise muremznt of comnunity noise exposure over a specific period of tirrre. a -. b r -. .,
(4) The day-niqht sound level UNL), is also expressed in dB. It is the sum of sound exposure for a given period but considers night penalties for increased annoyance factors of a -unity. It is similar toLeq, themain difference being a 10 dB correction factor wkrich is mlied to nighttim (10 p.m. to 7 a.m.) sound levels to account for increased annoyance during the night hours. The day-night sound level was introduced as a sirrple m&hod for predicting the effects on a population of the average long-term -sure to environmntal noise. It can be derived directly frcxn the A-ighted sound level.
(5) Fiqure 3 depicts masured noise levels expressed in dE3 as wed to some ground vehicles.
FIGURE 3. TRENDOF HELJCOPTERNOISELEVEXS INdB (These levels are notA-ighted, averaged, or otherwise corrected.)
loo - fi:i:, Diesel $$ Locomotive @ 100 Ft.
90- dB ‘/ Truck or 80 -9, Motorcycle @ 100 Ft.
Dist = 250 Ft.
Gross Weight, Lb.
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d. Meteorological conditions have a bearing on helicopter noise. A pilot may be able to adjust his flight schedule to take advantage of such conditions when noticeable. -Wind carries sound in the direction toward which it is blowing andmakes abackgroundnoiseof its own which tends tolowerthe surface .
annoyance factor of helicqter tiise. In same parts of the mrld, the wind directions are predictable onanear-dailybasis. When pilots areawareofthe surface wind direction and velocity, helicoptzs should be less objectionably . W flight operations are conducted downwind of populated or noise-sensitive aT=. The direct effect of humidity on somd propagation is of little inpartance, other: than the attenuation of high frequency sound. Humidity, in the form of visible moisture in the air (fog, drizzle, light snow), should be used 'by the pilot as an indicator that the wind gradient is small, resulting in increased sound prapagation. .Mnd, high-humidity conditions warrant the use of noise abatemnt procedures.
5 l OPERATlXWREQUIEl!LY
a. Recoqnize noise-sensitive areas and avoid them by flying as high as practical. Sound attenuates relative to the square of the distance to the keceiver. If the altitude is &ubled, the sound reaching the surface is a fraction of what it was. Increasing altitude is probably the most effective mans of noise abaterrrent: LOW flying creates the xzst problem even though the sound footprint is smaller. Avoiding these arms by a lateral offset.of 1,000 feet or more in conjunction with the higher altitude adjustxmt~will reduce or possibly eliminate noise cmnplaints. An increased number of exposures to noise sensitive areas will increase the Leq.
b. Noisiest a~roaches occur in the 550 to 65-knot airspeed range and at a 4000 to 6OO-foot per minute rate of descent. Pilot actions which can be taken to avoid most of this type of noise include:
(1) When practical, leave the last 1,000 feet of altitude at 10 to 15 knots higher airspeed than normal.
(2) Keep rotor torque as low as practicable, consistent with safety.
(3) Tune out min rotor blade-slap irrpulses with cyclic and collective by operating at airspeeds and rates of descent that keep the aircraft outside of areas that are identifiedas areas that produce blade slap.
(4) Use sound abatmt flight techniques when they have been identified as being quieter for conducting approaches.
c. Noisecreatedbythehelicapter~~atinsatahover or on the ground can bedirectedaway frcmthepopulace by turning thequietestquadrant in the direction of congestion. Noise directional angles can be determined by very sixply walking around the helicopter when it is operating and identifying thequietestquadrant.
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AC 91-66 614187 l
d. Iandinqs on grass or dirt areas are better than hard surfaces such as concrete. The soft path over which sound travels reduces sound levels.
e. Ambient noise is a catposite of sounds fran many sources (autanobiles, . , . trucks, buses, motorcycles, construction noise, aircraft, etc. 1 within an area and oontinually varies with ti.m as a result of different h&.s of activity.
This activity changes with the tim2 of day, day of the week, and the seasons.
An exarrple of an option available to a pilot muld be the use of a freeway route.
This would keep the aircraft noise over a high ambient noise area ~Which would cause less of a sound contrast, This wouldxesult, .in mst cases, in less annoyancetothe residents belw. A depiction of-relative ambient noise levels is shownon thechartbelow.
FIGMZ 4. AMBIENT NOISE (Not corrected or weighted) 1 I 1 10 20 Seconds Near Freeway Center
f. The noise footprint of a helicopter is described as a ground contour of equal sound levels and is relative to the ambient noise level that exists on the surface over which the aircraft is operated. Pilots should consider this factor 6 Par 5
6/4/87 . AC 91-66 when planning a flight over noise-sensitive areas. The ground noise pyr3asure footprint contour can bereducedconsiderab~yasdepictedin figure 5 by using the noise abatemnt approach procedure. When the footprint is smll and a waterway is an available route, it often will absorb the worst noise.
FIGURE 5. S-NOISE -ExPmuREFlooTpRzNT Noise-Abatement Approach 9 Most helicopter manufacturers today publish noise abatemznt procedures as a kppl-t to their flight manual for each specific helicoptkr. Such procedures normlly are categorized as General, Takeoff, Static, Enroute, and Approach and describe the manner in which pilots can operate their specific helicopter in the quietest way. Additionally, most mnufacturers have an acoustics department that studies the noise impact of each make and model.
Operators are encouraged to m&e mntact with these departmnts in order to keep their knowledge current on specific methods of noise abatemnt and to continue efforts to reduce public noise ccnrplaints.
h. Aircraft desicp is a factor under constant review as it pertains to mre efficient noise reduction, Manufacturers havemademjar advancements in aircraft and rotor system designs. Huweva, aircraft and rotor system cannot be modified to eliminate objectionable noise totally. Helicopter noise intensity is directly proportional to klicwter Right. While the nmibr of main rotor blades may change the frequency of noise, the perceived noise itself will rain basically the sam3.
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AC 91-66 614187 6 STATIC MOESo Static operational &es for the helicopter can often be adjusted to reduce noise. .
a. If the aircraft is equipped with a piston engine and exhaust pipes that directexhaustgasses in arearwar d direction, noise will be greater to the rear of tlxz helicopter. When qeratinginareaswheretherearelargegroups of people, the aircraft couldbeparkedwiththerearof tk helicqterpointed in a direction away frcxn them. Ifyouhaveotherwise determinedthatonequadrant is noisier than another, thequieter one shouldbepointed toward the crowd.
b If an airman intends to park a helicopter at a specific location for a long I;wiod of tinvt with the engine running (rotors turning), noise mid be reducedby reducing engineandrotor RPMfran normal cqeratingRPMtoareduced speed or to ground idle. The approved flight manual or qerat@s manual shauld be consulted to assure that there are no restrictions to prolonged reduced RPM operation. Transmission or engine oil pressure minimumvaluesmustbe observed.
7 EENEFITSO The benefits of quiet operations are unlimited to pilots and &se on the surface as well. Sam2 of them are:
a. The need for noise-related regulations is reduced.
b The krelicopter as abusiness tool~uldbenrore acceptable toa cm&ity where noise is x0st objectionable. -. b , _ rd
c. Point-to-point public transportation would be inproved.
d. Poiice, fire, public safety, and @ical services would be enhanced, and .
livesmuldbe saved.
e. Auto traffic may be re-routed over m3re desirable routes during peak rush hours because of radio station helicopter traffic reports.
f Television news crews array bring the newswxthy activities for public view Are rapidly.
9 Public acceptance of helicopters eventually wxld result in an increased n& of aircraft available to a -unity in the event of a natural disaster.
8 PLzwsANDPoLIcIEs, The FRA encourages ccnplia&e with the good neighbor policies established by professional heliqter associations concerning helicopter sound. Discussions of policy objectives are encouraged throughout the industry to continue efforts to ti helicopter sound less objectionable.
The FRA is continuing work to develop a helipoft noise Wel. More wmk is being done to quantify noise. Standard protocol is being developed for noise masurenrent along'nonnal helicqter approach paths. Additional noise abaterrrent operational procedures will result from these studies, which will be made public upon oonpletion.
&YS Acting D&or of Flight Standards 8 Par 6