The Federal Aviation Administration (FAA) has issued an emergency airworthiness directive (AD) for around 2,000 US registered Boeing 737 NG and Classic aircraft.
According to the FAA, this AD was prompted after four recent reports of single-engine shutdowns; caused by engine bleed air 5th stage check valves stuck in the open position.
“Corrosion of the engine bleed air 5th stage check valve internal parts during airplane storage may cause the valve to stick in the open position.”
In the worst-case scenario, corrosion of these valves on both engines could result in a dual-engine failure; this would leave the aircraft without the ability to restart and consequentially could result in a forced off-airport landing.
This AD is valid for any 737 NG (600 to 900 series) and 737 Classic (737-300 to 737-500 series) that, for 7 or more consecutive days, has not been operated in flight or is considered to be in “storage.
In the USA, several airlines operate this type of aircraft; including United Airlines, Delta, Southwest Airlines and American Airlines. The B737 MAX remains grounded.
Everyone knows that planes in storage can have issues exactly like this. In my view this is the system working exactly as we hope it will. As soon as it became clear what the cause is, the FAA got the information out so that aircraft mechanics can check the 5th stage bleed valve for corrosion and then ensure that the 9th stage won’t open (and feed back through stage 5) for ferry trips to a maintenance facility. Planes that have not been stored don’t have any issue.
“Could result in an off airport landing” is a pleasant way of saying crash.