Regulators Order Inspections After Downtime Is Linked To 737 Engine Incidents

WASHINGTON—Storage-related corrosion of Boeing-supplied valves on certain CFM56 engines has been linked to four engine shutdowns, prompting regulators to order inspections before the 1,140 aircraft are returned to service after extended downtime.

“Corrosion of the engine bleed-air fifth-stage check-valve internal parts during airplane storage may cause the valve to stick in the open position,” the FAA said in an emergency airworthiness directive issued late July 23. “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 directive requires any aircraft that has not “operated in flight” for at least seven consecutive days to be inspected. The check includes manually rotating valve flapper plates, checking bushings for cracks and ensuring certain parts are not rubbing together. Other regulators, including Australia’s Civil Aviation Safety Authority and EASA, immediately adopted the U.S. directive.

The FAA directive applies to Boeing 737-300s, -400s, -500s, -600s, -700s, -800s and -900s powered by CFM International CFM56-3s and CFM56-7s. Boeing provides the bleed-air system for the engines, which directs air from the engine compressor to support other aircraft systems. Boeing alerted operators of the issue on July 9.

Beyond involving aircraft that were out of service for an extended period of time, the FAA directive does not provide details on common links between the in-service incidents. The agency does not say when the incidents took place.

Aviation Week Intelligence Network Fleet Data shows that operators have 1,140 737s that meet the FAA definition for needing inspections because they have been on the ground for at least seven days. Operators in the U.S. have 25% of these aircraft. Another 1,005 are in long-term storage. 

The sudden removal of thousands of aircraft for extended periods during the COVID-19 pandemic-triggered downturn has created numerous challenges for operators and manufacturers. 

Typical storage guidance from airframe and engine OEMs assumes aircraft will be parked for months at a time and in conditions conducive to minimizing corrosion. But airlines have been forced to park aircraft at airfields all over the world—not just in dry, arid storage facilities—while waiting out the pandemic’s demand variations. 

In many cases, airlines realized guidance provided by suppliers did not cover the middle ground of parking aircraft for short periods. Manufacturers have been working to fill that gap and remind operators that environmental conditions—from humidity to failure to protect openings on aircraft from wildlife—can affect airworthiness.