The Federal Aviation Administration issued an emergency Airworthiness Directive for U.S.-registered Boeing 737 Next Generation and Classic aircraft concerning engine-failure on jets recently in storage due to reduced airline schedules. The directive requires a further round of inspection for the affected aircraft, which reports indicate covers about 2,000 aircraft operated by U.S. carriers.
According to FAA, four cases of individual engines shutting down in flight have been attributed to "engine bleed-air fifth-stage check valves" becoming stuck open.
According to the agency, the engines may be affected by corrosion on those valves, which could lead to dual-engine failure.
The Boeing 737 NG and Classic are the 737 series that precede the current 737 MAX. Unlike the 737 MAX, the previous models were not grounded as a result of the two crashes in October 2018 and March 2019. However, many aircraft have been taken out of service due to reduced demand since the start of the COVID-19 pandemic.
Now that airline travel is resuming for many carriers, jets that have been idle are being returned to service.
FAA's AD applies to all Boeing 737-300, -400, -500, -600, -700, -700C, -800, and -900 models in storage on or after July 23, and any airplane that, as of that date has been operated for 10 or fewer flight cycles since its most recent period of storage.
According to FAA's summary, corrosion of the check valves internal parts during storage may cause the valve to stick in the open position. If the valve opens normally under takeoff power, it may become stuck in the open position during flight and fail to close when power is reduced, such as when the aircraft begins its descent. That may result in a stalling that may be unrecoverable, such that the crew may not be able to restart the engine.
Corrosion of these valves on both engines of an aircraft could result in a dual-engine power loss, without the ability to restart.The 737 NG has been the object of previous safety concerns: last December FAA fined Boeing over $3.9 million in a case involving defective "slat tracks", a component that attaches the jet's fuselage to the wing structure. Earlier this year, inspectors reported cracks in 737 NG jet's fuselage "skin", following an incident of a commercial flight experiencing loss of cabin pressure.