Interior Sound Levels in General Aviation Aircraft

Pilots are subjected to hazardous sound levels. A headset is adequate hearing protection for a projected eight-hour period, given the sound levels found in this study.

• By Ernesto Lamm, Nancy Lawrence
• Jul 12, 2010

General aviation aircraft are notoriously noisy inside the cabin, and many people are unaware of the potential threat of exposure to such high sound levels. OSHA requires that workers use some sort of personal protection equipment once sound levels exceed 85 decibels.

This study looked at two specific and common general aviation airplanes: the Cessna 172S model, which is a four-passenger, single-engine aircraft, and the Piper Seminole, which is a four-passenger, multi-engine aircraft with wing-mounted engines. Many people across the country utilize these two types of aircraft on a daily basis both for pleasure and to earn a living. The main difference between people who may use the aircraft for pleasure instead of business (such as a flight instructor) is the different duration of exposure to the ambient sounds. The flight instructor may conduct up to five training flights in one day, whereas the average student or recreational aviator will utilize the aircraft at a much lower rate, usually about two to three times per week or less.

Two of the biggest sources of noise are the sounds from the engine running while in flight and the sounds of the wind rushing around the aircraft as it moves through the air. Much as in a car, the aircraft engine produces sound as more power is demanded by the operator. Cars, however, are usually much quieter due to the exhaust equipment. Weight and cost restrictions do not allow for the average general aviation aircraft to obtain a sound level that is comparable to that of an automobile. In addition, just as when driving a vehicle down a freeway, the sound made by the surrounding air moving around the cabin increases as the aircraft speed increases. This wind noise is an issue, but, once again, weight, cost, and construction techniques limit the insulation manufacturers can install.

Equipment & Sampling
Two noise dosimeters were used for data collection. Each dosimeter was calibrated with a calibration unit at 114 dB before every flight.

In order to determine whether exposure to aircraft interior noise is a health hazard, sound samples were taken in two types of aircraft: a Cessna 172 and a Piper PA-44 Seminole. The aircraft used for the study are utilized and maintained by a flight training school.

Samples were taken in two different airframes so the study could include a comparison of how much a front-mounted engine differs, if at all, from a twin-engine design with the engines mounted away from the cabin. Eight samples per airframe were taken. During each flight, two sound-level meters were used--one to take samples from within an occupant's headset, the other to take samples from within the cabin to obtain readings as though a person were not wearing any hearing protection.

The cabin microphone was placed in the center of the cabin seating area approximately 6 inches behind the pilot's and co-pilot's ears. The microphone for the other dosimeter was placed within the headset to obtain a decibel reading with the occupant using hearing protection. The headset used for the study had the volume on the side of the microphone completely turned down to prevent interference with the readings from communications.

The samples per aircraft can further be broken down by aircraft power settings. Half of the samples in each aircraft were taken at relatively low power settings, while the other two were at relatively high power. For the purpose of this study, the high-power setting was when the aircraft was operating at 65 percent power or higher for more than half of the flight, and low power was when it was operating at less than 65 percent power for half of the flight or less.

The dosimeters were started and stopped at the same times for the cabin and headset readings for each flight. Because measurements were taken in the month of July in Arizona, some measurements were taken with cabin windows open. This improves cooling inside the cabin and incorporates the sound levels to which aircraft users are exposed during the summer months in order to remain cooler in the aircraft.

Data Analysis
Analysis of the data was conducted with a three factor analysis of variance (ANOVA) with two- and three-way interactions. The three factors are the type of aircraft (single engine versus multi-engine), power level (high versus low power), and location (headset versus cabin sound levels). The projected eight hour TWA used as the dependant variable in the ANOVA was automatically calculated by the dosimeters' internal software.

Results
Maximum sound level in dBA, along with a time-weighted average (TWA) and projected time-weighted average (PTWA), were measured during flights. Some of the maximum readings were obtained while completing the engine run-up procedures with the windows open. The average of all eight maximum readings was 101.3 dBA.

The analysis of variance yielded results that the only significant factor in reducing decibel levels was the use of a headset. As to why the aircraft types were found to be non-significant, it could be that the PA-44, which seems to have a quieter cabin when compared to the Cessna 172 by a person sitting in it on the ground, was probably in the same decibel range in flight as the Cessna 172 because of its ability to fly at higher speeds. This, in turn, might increase the aerodynamic sounds produced from the airframe interacting with the air in flight at higher speeds, much like a vehicle increasing its speed on a highway. Aircraft power settings, along with all of the interactions, both two- and three-way, were also found to be non-significant in sound level reductions.

The combined projected time-weighted averages for all flights with the cabin and headset measurements respectively showed for an eight-hour projected period, the use of a headset lowered the projected level by an average of 13 dBA. This was a significant drop that brings the values below OSHA levels.

Conclusion and Recommendations
The data collected and analyzed in this study reinforce the notion that pilots are exposed to sound levels higher than OSHA standards. A projected eight-hour exposure and eight-hour time weighted average were used in this study to simulate the effects of a worst-case day in the cockpit. This is because most general aviation pilots are not in the cockpit of an aircraft for eight hours a day and there is no set time for a given flight, so direct measurement of exposure is nearly impossible. Each pilot or flight instructor may complete zero to five flights per day, and each flight’s time can vary based on numerous variables.

The average for the projected time-weighted average for all of the aircraft cabin measurements came in at 86.26 dBA, which is less than what was found by Nygra (2006) in his study of Coast Guard helicopters. However, considerating the levels found in Nygra's study are for military grade, turbine-powered aircraft, the civilian piston engine propeller aircraft in this study featuring a lower dBA rating is consistent with what we expect.

The objective of this study was met by demonstrating that aircraft pilots are subjected to hazardous sound levels. Data collected also demonstrated that the use of a headset is adequate as hearing protection for a projected eight-hour period, given the sound levels found in the study.

Due to our limited ability to change the configuration of the aircraft, pilots should be encouraged to educate themselves through a hearing conservation program and utilize an aviation headset whenever inside an aircraft that has the engine running. Although it is not required by regulations to wear headsets, the regulatory agencies should increase the awareness programs that educate pilots on hearing conservation and the use of headsets.

This study used Quest Technologies' NoisePro Dosimeters and calibration unit and David Clark headset model H10-13.4

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