Ultrafine particles (UFP) appear to be a good indicator of engine oil contamination in bleed air of passenger jet aircraft. However, ultrafine particles can also result from environmental control system (ECS) transients, from exhaust ingestion, and in the cabin from ozone chemistry. CO2 appears to be a good indicator of exhaust ingestion and could help to distinguish exhaust ingestion UFP from engine oil UFP. Fine particle (FP) concentrations are increased by hydraulic fluid but establishing the correct bleed air baseline concentration is a challenge. The bleed air baseline concentration for FP is on the ground generally below ambient concentrations. Fine particles may be a useful indicator of oil contamination from low temperature bleed air (e.g. sourced from APU bleed air). High bleed air temperatures in the range of 315 °C cause only marginally elevated FP concentrations. Highly elevated UFP are not present in high temperature bleed air contaminated by oil but are generated when the contaminated bleed air is cooled. UFP are present throughout the bleed path when extracted sample flows are cooled to room temperature. Important chemistry occurs in the ozone converter but does not appear to impact UFP generation. Interesting for UFP measurements are corona discharge based sensors. Note: Temperature transients in the bleed air system generate elevated UFP concentrations from oil deposited previously on ECS heat exchangers and other ECS and bleed air system internal surfaces. These contamination events are not necessarily due to engine seal leakage. In conclusion: It appears that it should be feasible to detect engine oil contamination with a lower detection limit on the order of 1 ppm by mass of oil in inlet air with an appropriate combination of a UFP sensor together with some other sensors. This lower detection limit is more-or-less in the same range as the odor detection threshold. However, major progress towards implementation requires getting prototype sensor packages on airplanes to get experience using them and learning how they work in the aircraft operational environment. First steps in using sensors in airline service would not be particularly expensive.
