The Federal Aviation Administration on Friday issued an emergency Airworthiness Directive  (AD) for 2,000 Boeing 737s that have been parked.

The FAA issued the directive after inspectors found compromised air check valves when bringing the aircraft out of storage, agency spokesman Lynn Lunsford said. Corrosion on the “fifth stage bleed air check valve” could result in dual-engine failure, he said.

Emergency Airworthiness Directive (AD) 2020-16-51 is sent to owners and operators of The
Boeing Company Model 737-300, -400, -500, -600, -700, -700C, -800, -900, and -900ER series
airplanes.

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This emergency AD was prompted by four recent reports of single-engine shutdowns due to engine bleed air 5th stage check valves being stuck open. 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. 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. Corrosion of these valves on both engines could result in a dual-engine power loss without the ability to restart. This condition, if not addressed, could result in compressor stalls and dual-engine power loss without the ability to restart, which could result in a forced off-airport landing.

Inspections and Corrective Actions
(1) For any airplane that is in storage on or after the date of receipt of this AD, and any airplane that, as of the date of receipt of this AD, has been operated for 10 or fewer flight cycles since returning to service from the most recent period of storage: Before further flight, do the inspections
specified in paragraphs (h)(1)(i) and (ii) of this AD on the engine bleed air 5th stage check valve on each engine. If any engine bleed air 5
th stage check valve fails any inspection, replace that engine
bleed air 5th stage check valve before further flight. For each engine bleed air 5th stage check valve
that passes both inspections specified in paragraphs (h)(1)(i) and (ii) of this AD, do the actions 3 specified in paragraph (h)(2) of this AD on that engine bleed air 5th stage check valve before further
flight.

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(i) Rotate the flapper plates by hand at least 3 times. If the flapper plate moves smoothly,
without signs of binding or sticking, from the fully closed position to the stop tube using gravity force
alone, the engine bleed air 5th stage check valve has passed this inspection.
(ii) Measure the clearance between the flapper bushings at both locations on each engine
bleed air 5th stage check valve. If the clearance between the flapper bushings is a minimum of 0.004 inch (0.102 mm) at both locations, the engine bleed air 5th stage check valve at that location has passed this inspection.
(2) For each engine bleed air 5th stage check valve that passes the inspections specified in paragraphs (h)(1)(i) and (ii) of this AD, do the inspections specified in paragraphs (h)(2)(i) through
(iii) of this AD before further flight on the engine bleed air 5th stage check valve on each engine. If any engine bleed air 5
th stage check valve fails any of the inspections specified in paragraphs (h)(2)(i) through (iii) of this AD, replace that engine bleed air 5
th stage check valve before further flight.
(i) Do a general visual inspection of the flapper bushings for signs of cracks, fractures, and
missing bushing heads. If the flapper bushings do not show any signs of cracks, fractures, or missing
bushing heads, the engine bleed air 5th stage check valve has passed this inspection. Signs of corrosion are not a cause for replacing the engine bleed air 5th stage check valve if the engine bleed air 5th stage check valve did not fail any of the inspections specified in paragraph (h)(1) of this AD.
(ii) Using only hand pressure, try to rotate the flapper bushings in the flapper plates. If the bushings do not rotate in the flapper plate, the engine bleed air 5th stage check valve has passed this inspection.
(iii) Do a general visual inspection of the check valve for signs of the flappers rubbing against the valve body. If the flappers do not show any signs of rubbing against the valve body, the engine
bleed air 5th stage check valve has passed this inspection.
(i) Minimum Equipment List Relief for Certain Airplanes
For airplanes that have operated 10 or fewer flight cycles since the most recent period of storage prior to receipt of this AD, as an alternative to compliance with paragraph (h): If allowed by
the operator’s FAA-approved Minimum Equipment List, the airplane may be dispatched with one engine’s engine bleed air high stage valve locked closed. Thereafter, within 5 additional flight cycles,
inspect the engine bleed air 5th stage check valve on both engines as required by paragraph (h) of this
AD.

Source : Airworthiness Directive