Cabin Smoke Events on ATR 72-212A Aircraft at Guernsey Airport (G-OATR G-ORAI)

Incident Overview and Aircraft Details In June and July 2020, two separate cabin smoke events occurred on ATR 72-212A aircraft, G-OATR and G-ORAI, during engine starts at Guernsey Airport. Both aircraft were operated for commercial passenger transport and were manufactured in 2019. The incidents resulted in no injuries or damage to the aircraft, though both crews and passengers were evacuated as a precaution. The aircraft were powered by Pratt & Whitney Canada PW127M turboprop engines, which utilize pressurized air from the engine compressor for cabin pressurization and other systems. ## Sequence of Events The first event involved G-OATR on 29 June 2020. After the engines were started and the aircraft was pushed back from the stand, the Senior Cabin Crew Member reported a faint haze or smoke in the cabin that increased in intensity. The flight crew shut down both engines, and the passengers rapidly disembarked through the rear cabin door. A PAN call was made to air traffic control, and the crew departed the aircraft to liaise with the airport fire and rescue service. The second event occurred on 22 July 2020 with G-ORAI. Following the first incident, the operator had reduced the maximum interval between flights from 14 to 7 days. However, G-ORAI, which had last flown five days prior, also experienced a slight haze or smoke in the cabin after engine start and pushback. The commander informed air traffic control of a rapid disembarkation, which took place after the engines were shut down and the propellers stopped rotating. ## Technical Investigation and Findings After each event, the engines were run at 80% power with the bleed air system and environmental control system (ECS) selected on, and smoke and haze were observed in the cabins. Examination of the P2.5/P3 air switching valves on all four engines revealed that they were either stiff to move or seized. Disassembly of the valves showed corrosion on the pistons and sleeves. The P2.5/P3 air switching valve is a spring-loaded component that allows air from a different compressor stage (P3 air) to pressurize the system during engine start when P2.5 air pressure is insufficient. Once the engine high-speed rotor speed reaches 40-45%, P2.5 air pressure should overcome the spring force and close the valve. If the valve remains open, high oil consumption and temperature can result, along with an oil smell in the cabin and smoke in the exhaust. The corroded components were replaced, and the valves operated normally during subsequent ground runs with no oil smells or haze in the cabin. ## Operator and Manufacturer Actions The operator determined that the smoke and haze in the cabin was due to the partial seizing of the P2.5/P3 air switching valves, which allowed oil vapor to enter the ECS. Corrosion was found in all the valves, which probably resulted from the reduced utilization of the aircraft due to the coronavirus pandemic and the aircraft being parked and operated from an airfield close to the sea where there was a relatively high level of airborne salinity. In response, the operator introduced a requirement for engine ground runs to be carried out on aircraft that have not flown for three days to ensure the correct operation of the bleed air system and ECS. The operator also introduced an inspection of the P2.5/P3 air switching valves every 42 days and replacement of the piston ring every 100 days. A 'Notice to Crew' was issued to require flight crews and maintenance personnel to start the engines with the bleed air system and ECS selected off. The engine manufacturer updated the Engine Maintenance Manual to include an inspection of the P2.5/P3 air switching valves following a period of storage, irrespective of duration and environment. The manufacturer also issued a requirement to replace the P2.5/P3 air switching valves during engine overhaul and initiated a redesign of the valve, which will consider the materials used in the valve assembly.