Derisive clamor or the sound of freedom?
Aviatrix and storyteller Beryl Markham, in her book West With the Night, wrote, "We began every morning at that same hour, using what we were pleased to call the Nairobi Aerodrome, climbing away from it with a derisive clamour, while the burghers of the town twitched in their beds and dreamed perhaps of all unpleasant things that drone — of wings and stings and corridors in Bedlam."
In Markham's day, neither the government nor many citizens were concerned about airplane noise. Times have changed. Today, typing the topic "airplane noise" into any Internet search engine uncovers hundreds of sites that have been established to study, complain about, or attempt to abolish airplane noise.
Flying Friendly
When an airplane takes off or lands at a backwoods country airstrip, does it make any noise? Noise complaints are the number one issue that the AOPA Airport Support Network responds to on a daily basis. The biggest part of the problem is a result of urban growth and the fact that communities are allowing residential construction much closer to airports than in the past.
The AOPA Airport Support Network volunteers have a video called Flying Friendly, produced by AOPA in 1995. This video demonstrates that flying techniques have a tremendous impact on the noise generated by light aircraft. Those techniques include power and rpm settings for takeoff and landing, climb and descent profiles, and use of noise abatement procedures around airports. Contact your ASN volunteer and ask him or her to show this video at your next pilot meeting. — Mark Lowdermilk, ASN Manager
At AOPA Expo 2001, Andy Cebula, AOPA's senior vice president of government and technical affairs, cited airplane noise as one of the biggest threats to general aviation. What is noise? How much do our propeller-driven airplanes contribute to airport noise? Should the average lightplane owner be concerned about how much noise his airplane makes?
The first recorded aviation-related noise complaint occurred in 1928 when the proprietor of the Cackle Corner Poultry Farm in Garrettsville, Ohio, complained to the U.S. Postmaster General that the noise from low-flying airplanes was upsetting his chickens and that egg production was down. The postmaster forwarded the complaint to the local airline company — National Air Transport (NAT) — with the suggestion that it make a special effort to maintain altitude over the poultry farm.
In 2002 the noise-abatement summary from Torrance's Zamperini Field, a general aviation airport located near Los Angeles International Airport in the L.A. Basin, detailed an average of five complaints a week — for a yearly total of 264 noise complaints during 168,229 operations.
According to AOPA's Manager of State Affairs Keith Holt, three states — California, Florida, and New Jersey — generate the highest number of noise complaints. So what makes airplane noise?
Sound is defined as vibrations that travel through solids, liquids, or gases. Sound (audio) waves are a combination of condensation (compression or close packing of air molecules) and rarefication (expansion or spreading out of air molecules). The perception of sound and the measurement of sound involve two characteristics — intensity and frequency. Intensity is a measure of the strength of sound vibrations while frequency is the pitch or tone of the sound vibrations. Sound intensity is measured in terms of sound levels ranging from zero decibels (dB), the lowest level of human hearing, to 130 dB, the threshold of pain. The human ear can respond to frequencies ranging from a low of 20 Hertz (Hz, or cycles per second) to a high of 20,000 Hz. Humans are most sensitive to frequencies near the normal range of spoken conversations. Because of this, aircraft sound measurements are predicted and measured by using an A-weighted (dBA) scale that emphasizes frequencies near the normal range of speech communications, while deemphasizing high and low frequencies.
A common method of measuring airplane noise is to combine both noise intensity and duration in a sound exposure level (SEL), which is a measure of the total sound energy of a single event. Some airports measure noise by using a single-event noise exposure level, or SENEL.
Decibel equivalencies aren't easy to understand — the sound intensity between the pilot and copilot seats was measured at 90 dBA during a cross-country flight from California to Florida in a Lancair Columbia 300 (see " The Tortoise and the Hare, Round 2," April Pilot). The Lancair cabin's sound level is typical of small airplanes. At row 45 in a Boeing 757 the measured level in cruise flight was 79 dBA. The measured sound level at the front seat of a mid-1990s Toyota Camry traveling at 65 mph on a smooth road was 70 dBA — standing on the corner of a small town indicated 58 dBA until an average car passed at 35 mph — then the level rose to 68 dBA.
Because decibels are charted on a logarithmic scale, the amount of sound energy at 60 dB is roughly 10 times greater than it is at 40 dB; and 10 times greater at 80 dB than at 60 dB. The result of this is that there is approximately 100 times more sound energy at 80 dB than at 40 dB. A 10-dB change is perceived by the average person as a doubling, or halving, of a sound's loudness. Doubling the distance from a noise source results in a perceived change of approximately 6 dB.
The sound produced by propeller blade tips is often thought of as the greatest noise generator from propeller-driven general aviation airplanes, but this is only partly true. Propeller blade-tip speed is based on speed of rotation (rpm) and the length of the blade, with longer, faster-turning blades generating greater tip speeds and more noise.
Propellers are especially noisy during high-rpm takeoff operations. Anyone who has heard the ruckus that a Cessna 185 floatplane (engine takeoff rpm of 2,850) with a seaplane propeller generates as it accelerates for takeoff has experienced the window-rattling effects of near-supersonic propeller tip speeds.
A few airplane/engine/propeller combinations are exceedingly loud, but most airplanes are reasonably quiet, even during takeoff. A published study by the German government ( www.lba.de/englisch/technical/noisedata/noisedata.htm) shows that virtually every common GA airplane complies with the current FAA noise guidelines.
Until recently, the amount of noise created by a GA propeller was not a top priority for propeller design engineers. However, the German study shows that there are propellers available that can reduce propeller noise.
A second source of noise is the engine combustion sound that escapes past the exhaust valves and flows out through the exhaust system. Exhaust noise accounts for the majority of the sound heard on the ground when a GA airplane flies over at cruise power settings.
As with propellers, there has been little progress made in reducing exhaust noise. Many pilots believe that the large round or oblong metal cans under the cowling are mufflers, but this is not true. These cans don't muffle exhaust sound to any great degree; there are perforated metal tubes or funnellike shapes (often called baffles) welded inside most of the cans. The primary functions of these baffles are to provide the correct amount of back-pressure for proper gas flow through the cylinders, and to maximize the transfer of heat from the hot exhaust gases inside the can to ram air that is routed around the outside of the can and then into ducting to provide carburetor (or alternate air) and possibly cabin heat.
Engines that are equipped with turbochargers or turbonormalizers are markedly quieter than nonturbocharged engines because a great deal of the exhaust energy is extracted to drive the turbine wheel, leaving a low-energy exhaust stream to vent out the overboard pipe.
In Europe, where propeller-driven light-aircraft noise is much more tightly regulated than it is in the United States, there are at least three companies that have developed muffler systems to address the government mandates to reduce noise. In addition, the extractor-type exhaust system manufactured by Power Flow Systems has some muffling qualities, although the system is marketed primarily as a performance-enhancing modification.
Almost all light-airplane exhaust systems direct the exhaust noise downward — directly into the ears of those who are most likely to complain. Something as simple as an approved slide-on exhaust pipe tip that directs the exhaust noise sideways instead of downward would result in fewer noise complaints.
Noise complaints can be tricky. Zamperini Field has a noise-monitoring program that cost taxpayers $365,000 in 2002 — or about $1,364 per noise violation. Before the city locked up the records showing the identity of those who filed noise complaints, an AOPA Airport Support Network (ASN) volunteer investigated 18 months of records and found that 12 individuals made more than half the complaints. The resident who made the most complaints was an airport commissioner. Only 6 percent of the complaints could be correlated with an actual event recorded by the airport noise-monitoring equipment. It would be easy to dismiss this airport's noise study as an expensive program detailing the busy work of a few chronic complainers, but this attitude is as woolly-headed and ineffective as telling those who complain that the airport was there first. The airport may have been there first, but GA pilots are not being responsible when they act as if noise is not a critical issue.
If U.S. fliers don't voluntarily start paying attention to noise issues, the future is spelled out in black and white in the European Union rules on airplane noise. Starting this year in Germany, pilots may be required to include a noise report (which they will obtain at their expense from an official testing station) certifying the noise level of their airplane with the other required airplane documents.
Although Boeing has reported that the number of worldwide airplane noise reports has decreased by 95 percent since 1970, the 17,000 commercial airliners that ply the skies hauling passengers and cargo still generate the majority of airplane noise. Commercial airlines, following the prodding mandated by the implementation of FAR Part 161, have had to retire noisy airplanes or install noise-reduction modifications (hushkits). The manufacturers and the companies that build and operate airliners can point to this action as proof that they're responding to noise complaints. Since almost all GA airplanes comply with federal noise guidelines at the present time, there has been little demand for noise-reducing hardware such as mufflers and shorter-bladed propellers (which reduce blade-tip generated noise). European companies that have attempted to sell these products in the United States have had very limited success, yet this hardware does work.
The German study provides noise test data using the measurement criteria detailed in ICAO Chapter 10. In this study, measurements with the propeller/engine combinations listed in the airplane type certificate (a nonmodified airplane equipped as delivered from the manufacturer) can be compared with the measurements of the same airplanes when the original equipment has been replaced with propellers and/or silencer-type exhaust systems STCed to replace the original equipment. In some cases the results are striking; in others, not so dramatic.
A Cessna 182J was measured at 80.5 dBA in the original configuration. When the stock two-blade McCauley propeller was replaced with an MT composite three-blade propeller and a Gomolzig Quietflight exhaust silencer system was substituted for the factory system, the same tests showed a 7.6-dBA drop. Other airplanes didn't show such a dramatic decrease but each change did lessen measured noise levels.
Great, you say, the next time my airplane needs a new muffler I'll do my part by installing the Gomolzig system. Planning ahead helps since Gomolzig silencer systems aren't yet STCed for all makes and models, nor are the Quietflight systems as readily available as stock systems. In addition, they are more expensive. When Gomolzig, or another silencer system manufacturer, starts selling more systems, the price should eventually be competitive, but right now the market penetration is very small.
There are many tools GA pilots can use to lessen noise levels.
- Check with your airport administrator to find out if your airport has established noise-abatement procedures. Almost all airports have. Learn these procedures and abide by them. Give your flying friends a briefing on how the procedures work, and ask them to pass the word.
- AOPA's Airport Directory online lists noise-abatement procedures for many airports. Check to see if the listings are correct for your airport — if they're incorrect or not listed, e-mail those who update the listings ( [email protected]). During cross-country flight planning, check for published noise-abatement procedures at your en route and destination airports, and abide by the procedures. Respect the airport and the people who live nearby.
- Reduce propeller rpm as soon as it is safe and practical. For each 100-rpm decrease, there's a 3- to 4-dB decrease in noise.
- Climb out at best angle-of-climb (V X) airspeed. If this angle is too steep for your passenger's comfort, back off to the best rate-of-climb (V Y) airspeed. This accomplishes two things — it rapidly increases the distance between potential noise complainers and the airplane, and it lessens the amount of time the airplane is at those altitudes below which a safe turn back to the airport is not possible.
- If possible, fly tight patterns so you're close enough to fly low-power approaches, and avoid low, dragged-in approaches. This has two benefits — your approaches to landing will generate fewer noise complaints, and you'll gain proficiency that will come in handy if you are ever faced with a power-off landing situation.
- Flight instructors should help students understand the need to develop good-neighbor flying skills and monitor their adherence to procedures. Developing lifelong flying habits begins with the first lesson.
- Intersection departures increase the amount of noise that neighbors are exposed to — plan ahead and use the whole runway — it's safer and quieter.
- Airport administrators can take a lesson from the example set by the airport advisory board of Truckee-Tahoe Airport in California. This advisory committee has created and implemented safe and neighbor-friendly arrival and departure procedures, and made those procedures available on the airport Web site ( www.truckeetahoeairport.com). In addition, Truckee-Tahoe has an incentive plan that rewards pilots who sign up to comply with its noise-abatement plan.
It seems nonsensical that a local government would not question the sincerity of a resident who knowingly buys property near an airport, only to complain about the noise. But it is also nonsensical that a few pilots don't seem to care how their actions affect the people who live around airports and the other people who are flying or want to fly in the future.
You can do a lot to reduce an airplane's noise footprint — it only takes a few minutes to learn your airport's noise-abatement procedures, and it doesn't cost a dime.
E-mail the author at [email protected].
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