FAA Alert Flags 737NG Thrust Reverser Issue

737 CFM56-7B engine on tarmac
Green-time lease requests for engines such as the CFM56 have been in high demand during the past 18 months.
Credit: Sean Broderick/AWST

WASHINGTON—A mandatory, repetitive test to validate that Boeing 737 thrust reverser sensors are working can cause the systems to malfunction, prompting the FAA to advise operators of some models to cycle the reversers after conducting the tests. 

In a special airworthiness bulletin issued Aug 3., the FAA recommends that reversers on 737 Next Generation models—all variants of the -600, -700, -800s, and -900—be deployed and retracted following each reverser upper locking actuator “integrity test.” The tests were mandated in 2019 to address confusion between procedures meant to set up a new locking actuator and modify an existing one.  

The procedure calls for the removal of material from part of a sensor to ensure it does not cause a flight deck caution light to mistakenly illuminate. But using the procedure on in-service aircraft can degrade the sensor, which could mean the light does not illuminate even if there is a malfunction. The 2019 directive removed the procedure from maintenance manuals and required functional tests for the 737NG fleet. Operators soon discovered that the integrity tests could cause the system to malfunction. 

“The FAA has received reports indicating that, in some cases, after maintenance personnel performed these tests, the thrust reverser failed to deploy on the first commanded deployment,” the agency explained in the bulletin. “Performing the integrity tests can introduce a pre-load in a component of the thrust reverser (locking pin of the synchronization lock), and if the residual value of the torque is high enough, the synchronization lock will fail to unlock resulting in failure of the thrust reverser to deploy when commanded,” the bulletin added. 

Deploying the reversers one time eliminates the risk, the agency said. 

“Including this additional step will result in release of this residual torque after this integrity test is performed, preventing it from potentially interfering with deployment of the thrust reverser the next time it is commanded to deploy,” the FAA said. 

The 2019 directive’s maintenance manual changes applied to the 737 MAX as well. But operators of the newest 737 variant were not required to conduct integrity tests because “those airplanes have not been subjected to enough wear to warrant the use by any operator of the sensor target trimming procedure that leads to the upper locking actuator lock indication failure that may exist on 737 NG airplanes,” the agency explained. 

Sean Broderick

Senior Air Transport & Safety Editor Sean Broderick covers aviation safety, MRO, and the airline business from Aviation Week Network's Washington, D.C. office.