pressurize and ventilate the cabin. Bleed air was used to power a turbocompressor, which compressed the ram air to the proper pressure. That air was cooled by a Freon vapor compression-cycle air conditioner for temperature control before being distributed to the cabin. The arrangement was heavy, expensive, and inefficient because of the inefficiencies of the turbocompressor (E.Marzolf, retired, Douglas Aircraft Co., personal communication, April 29, 2001). Even more important was the high amount of maintenance that the systems required (R.Kinsel, retired, AlliedSignal, personal communication, May 7, 2001). For those reasons and because increasing the altitude of flights would have required an even larger turbocompressor and there was no discernable difference in quality between ram air and bleed air, it was decided in the late 1960s to use bleed air directly (E.Marzolf, op. cit.). New large passenger aircraft have since used bleed-air-based ECSs.

Essentially every large commercial passenger aircraft in use today is equipped with an alternative system, the APU. The APU provides a source of air that is independent of the propulsion engines. Although some APUs provide compressed air in much the same manner that the main propulsion engines provide bleed air, by extracting compressed air from the turbine-engine compressor, some APUs use a compressor, that is independent of the engine compressor, as described previously. Definitive data are not available, but the committee saw no evidence that contamination from the APU or brought into the APU is any less likely to affect cabin air adversely than is bleed air.

The committee did not investigate alternative ECSs in depth. To do so would take person-years of effort because reliable information is not readily available in the public domain, if at all. Although it is possible that alternative systems are being developed or could be developed, the committee saw no evidence that bleed-air-based systems cannot be designed, maintained, and operated to provide adequate clean air to the aircraft cabin. That statement does not imply that they always provide uncontaminated air to the cabin. As described in Chapter 3, there is evidence that air is sometimes contaminated; but the measurements that have been taken during routine operation show no contamination of concern arising from bleed-air systems (Nagda et. al. 2001). Data are available on only a few aircraft and conditions, but they are adequate to make the point that bleed air can be clean.

The committee does not want to discourage the pursuit of alternative systems, but it finds that the best method to ensure good cabin air is to have the Federal Aviation Administration (FAA) focus on ensuring that bleed-air-based systems are designed, maintained, and operated properly and that prob-