Cabin air contamination events – engineering aspects

Abstract

Cabin air ventilation in passenger aircraft is done with outside air. At cruise altitude, ambient pressure is below cabin pressure. Hence, the outside air needs to be compressed before it is delivered into the cabin. The most economic system principle simply uses the air that is compressed in the engine compressor anyway and taps some of it off as "bleed air". The engine shaft is supported by lubricated bearings. They are sealed against the air in the compressor usually with labyrinth seals. Unfortunately, jet engine seals leak oil by design in small quantities. The oil leaking into the compressor contains toxic additives. Deicing fluid, hydraulic fluid, and fuel can find their way into the cabin on various pathways. Fan air and bleed air ducts at the interface between engine and wing carry outside compressed air. The inside of the ducts shows differences. The brown stain in the bleed air duct appears to be engine oil residue. In comparison, the fan air duct is clean. This shows that oil leaves the compressor bearings. Ducting further downstream shows a black dry cover. The reason for the change in color seems to result from the different air temperatures: 400 °C at engine outlet and 200 °C further downstream behind the precooler. The water extractor is a part of the air conditioning pack. The inlet of the water extractor is covered with black oily residue, because the temperature is even lower at this point. The air conditioning air distribution ducts in the cabin are black inside from contaminated bleed air. Flow limiters have been found in ducts of the air conditioning system that are clogged from engine oil. Also, riser ducts feeding the cabin air outlets are black inside from engine oil residue. Cleaning on top of the overhead bins brings to light dirt that is clearly more than dust. The black residue known from the ducts settles also on the bin surface. In failure cases, Cabin Air Contamination Events (CACE) due to engine oil can fill the cabin densely with white smoke. Cabin ventilation can be described in a simplified way with a differential equation. Application of this equation shows that 10 minutes of oil contamination can be sufficient to fill a cabin with smoke. If contamination stops, most of the smoke is gone within the next 10 minutes and after further 10 minutes there is hardly any smoke left in the cabin. Fire is the biggest danger on bord. In case of smoke, when a fire can be ruled out and the smoke source cannot be isolated, diversion and descent to 10000 ft for direct ventilation with air from outside is required. The aircraft must be flown slower, but still range is reduced (fuel consumption is higher). This is taken care of in normal flight planning (to take care of a possible loss of cabin pressure). Oil concentration in the cabin is decreasing with engine size (if all other parameters are assumed to be equal). Pack burn is an attempt for decontamination of ducts and components. It releases oil fumes impressively to the outside, but cleaning is only partial. An investigation of layover times after a CACE shows that many aircraft are released back into service so quickly that proper maintenance is impossible. Some severe CACE have, however, left aircraft in maintenance for up to one month.