Direct Effects of Noise in the Airliner Cockpit

Cockpit noise levels directly affect the safety of the aircraft, crew, and passengers. When operating with high ambient noise levels, crew members have difficulty understanding each other and radio communications. In a pioneering study of noise in the airline cockpit, Dr. Richard Stone calculated that at a PSIL of 78, crew members three feet apart could understand each other only by shouting. Crew members one foot apart must shout at a PSIL of 88. In 1968, ALPA adopted a policy with two standards for maximum cockpit noise levels: (1) that PSILs not exceed 80dB in descent or 75 dB in any other regime of flight; and (2) that maximum cockpit noise levels not exceed 95 dB in any octave band below 600Hz or 85dB in any octave band above 600Hz. Little investigation of noise levels in air carrier aircraft has occurred since ALPA adopted this policy. The FAA, in Advisory Circular 20-133 maintains that "[t]oday's large, jet-powered, air-transport airplanes present few speech-interference problems for flight crews." This stands in stark contrast to Stone's findings which showed that the B-727 produces 82dB in a high speed descent. Even an airplane as quiet as the DC-9-30 exceeds ALPA guidelines in some frequencies.

Noise measurement in the cockpits of various aircraft using a hand held Radio Shack Digital-display Sound-level Meter produced interesting results. Boeing 737-300/400 aircraft registered 84-86 dBA in normal cruise flight at FL 270-290 (27000-29000 feet). The Fokker F-100 measured 94-96 dBA. These results represent an informal effort to collect some "ballpark" numbers. Pilots collecting the numbers used the meter right out of the box, without calibration. Tests occured during the course of a normal trip pairing; aircraft were not randomly selected. More formal tests performed by a major United States airline found 110 dBA during climb in the aft cabin of the DC9-30. It is interesting to compare these high noise levels with USAF auditory risk criteria. Applying USAF criteria to Flight Attendants on the DC-9 would limit their work day to less than 3 minutes. Pilots flying the Fokker F-100 would be limited to less than one hour of flight per day.

High ambient noise is only part of the noise problem that pilots may experience in the cockpit. Radio communications must be louder than the ambient noise in order to be heard. This is referred to as the signal-to-noise ratio (S/N). High ambient noise levels require higher radio volume. Even in a situation where the ambient noise level is acceptable, the pilot must carry the radio volume 10-15 dB higher to provide an acceptable S/N. Because intelligibility decreases at high volume settings, increasing radio volume does not necessarily help in understanding ATC communications. Crew members must speak at high volume to be heard by others in a noisy cockpit, potentially reducing intelligibility. High ambient noise levels may make it difficult to hear warning alerts in the cockpit. Crew member failure to hear a warning alert could result in the loss of an aircraft.