What happened
On 29 June 2020, an ATR 72-212 A, registration G-OATR, was preparing for a commercial flight from Guernsey Airport to Southampton. After starting both engines and being pushed back from the stand, the crew was notified by the cabin crew of a faint haze or smoke appearing in the cabin. As the intensity of the smoke increased, the commander shut down both engines. The crew coordinated a rapid disembarkation of the 50 passengers via the rear door and notified air traffic control.
A second, similar event occurred on 22 July 2020 involving another ATR 72-212 A, registration G-ORAI. Following the initial engine start and pushback, the cabin crew reported a slight haze or smoke. The commander subsequently shut down the engines and oversaw the rapid disembarkation of the 42 passengers on board.
The investigation
The investigation examined the Environmental Control System (ECS) and the engine air switching valves. Testing of the engines at 80% power confirmed that smoke and haze were present in the cabins during both types of events. Upon disassembly of the P2.5/P3 air switching valves on all four engines involved, investigators discovered corrosion on the pistons and sleeves. This corrosion caused the valves to become stiff or seized, preventing them from fully closing. When the valve remains open, contaminated air from the engine enters the cabin.
Findings
- The primary cause of the smoke events was the partial seizing of the P2.5/P3 air switching valves, which allowed oil vapour to enter the ECS.
- The corrosion on the valve components was likely caused by a combination of reduced aircraft utilisation due to the pandemic and high levels of airborne salinity at the coastal airfield.
- The operator had previously reduced the interval between flights to 14 days to maintain engine health, but this was insufficient to prevent the buildup of corrosion.