EUROCOPTER AND McDonnell Douglas Helicopter Systems (MDHS) have become embroiled in a row over, which of their products - the EC 135 and the MD900 Explorer - is quieter. Noise is one of the most competitive marketing issues among the new generation of helicopters.

Eurocopter says that noise-certification measurements conducted under French and German airworthiness authority rules show fly-over levels for the EC 135 of 81dBA for the Pratt & Whitney-powered version and 80.2dBA for the Turbomeca-powered version.

MDHS says that realistic comparisons will not be possible until the EC 135 undergoes US Federal Aviation Administration noise tests under identical evaluation conditions, which quote levels in environmentally perceived noise decibels (EPNdB). EPNdB figures are "weighted" to reflect the apparent levels of noise to the human ear, whereas dBA is a measurement of actual sound-wave pressure.

MDHS received completed FAA data a week before Heli-Expo '96 which show a level fly-over noise level of 83.1 EPNdB for the Explorer, 10.2 below the International Civil Aviation Organisation limit (ICAO). Fly-over noise is measured by the FAA at 500ft (150m) above ground level at 115kt (215km/h). Take-off noise, at maximum power, was 85.8 EPNdB, or 8.5 below ICAO limits. Approach and landing noise levels, at 68kt and 6° glide-slope, showed a level of 90.3 EPNdB - five below the limit.

MDHS says that the Explorer "...measures an average 8EPNdB quieter than FAA stage II and ICAO noise-emission requirements, which is the greatest average margin of compliance of any helicopter flying". Eurocopter claims that the EC 135 "...has the lowest noise emission of any helicopter in its class".

Source: Flight International