The FAA issued an emergency AD affecting more than 2,000 second- and third-generation Boeing 737s that have been parked since the COVID-19 pandemic took hold in March. The AD requires immediate inspection of the fifth stage bleed air check valve to check for corrosion. The corroded valves don’t close after the engine is throttled back from climb power. “If this valve opens normally at takeoff power, it may become stuck in the open position during flight and fail to close when power is reduced at top of descent, resulting in an unrecoverable compressor stall and the inability to restart the engine,” the FAA said in a news release. “Corrosion of these valves on both engines could result in a dual-engine power loss without the ability to restart.”
The agency said there have been four reported incidents and one of them may have occurred July 15 when an Alaska Airlines flight lost an engine and made an uneventful emergency landing in Austin. The airline told USA Today the 737 had an “engine shutdown issue” but didn’t elaborate on the cause. Most of the aircraft are still in storage so the inspections and potential replacement of the valves shouldn’t affect any operations. The 737 MAX is not included in the AD.
From a story published by FlightGlobal:
“CFM International, which makes the 737NG’s CFM56 powerplants, says the FAA’s order relates to a ‘Boeing external bleed air valve that interfaces with the engine. We are working closely with Boeing in their resolution of this issue.’ ”
https://www.flightglobal.com/airframers/737ng-engines-at-risk-of-corrosion-in-flight-shutdowns-following-storage-faa/139485.article
I can understand how an open bleed valve could cause the engine to flame out following a power reduction, and how it could prevent a restart. But I always understood compressor stalls were precipitated by a sudden decrease in gas density or velocity at the compressor inlet and not too dependent on downstream conditions.
What do I have wrong?
The 5th stage HPC (High Pressure Compressor) valve is a mechanical flapper type check valve which is normally open when engine is running. It will close when additional bleed air is required usually when throttles are retarded, 9th stage HPC valve opens to supplement 5th stage. The 9th stage air is higher pressure which will slam the 5th stage check valve closed so the 9th stage air is not allowed to enter the 5th stage, HPC airflow causing compressor stalls. The check valve also stops other bleed sources from entering the HPC i.e. opposite engine (if Cross Bleed valve is open) or the APU Bleed. There is a Dual Bleed caution light to indicate to the crew that engine bleed and APU bleed are turned on at the same time.
Thanks for the explanation.
It seems that a malfunctioning 5th stage check valve can allow high pressure (9th stage) air to flow forward into the compressor’s 5th stage under the right circumstances – a novel way to stall the compressor.