Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
AIR-21-12
August 3, 2021
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin advises owners and
operators of The Boeing Company Model 737-600, -700, -700C, -800, -900,
-900ER series (737 NG) airplanes, that thrust reversers on which the
integrity test required by airworthiness directive (AD) 2019-18-03 is
done may not deploy due to being in a state with residual load, when
commanded the first time after the test is done.
At this time, the airworthiness concern is not an unsafe condition that
would warrant AD action under Title 14 of the Code of Federal
Regulations (14 CFR) part 39 (AD 2019-18-03 will not be superseded to
address this single issue).
Background
The FAA published Airworthiness Directive (AD) 2019-18-03, Amendment
39-19730 (84 FR 49005, September 18, 2019), which requires revising the
existing maintenance or inspection program to remove text that allows
the size of the thrust reverser upper locking actuator lock sensor
target to be changed, and, for certain airplanes, performing repetitive
integrity tests of the thrust reverser upper locking actuator. That AD
was issued to address the potential for an undetected unlocked
condition of the thrust reverser upper locking actuator locking
mechanism in flight, which could significantly increase the likelihood
of an in-flight deployment of the thrust reverser and consequent loss
of airplane control. After completion of this integrity test, it is
possible that the thrust reverser could be in a state with residual
load and that it may not deploy when commanded the first time after the
integrity test is done. This bulletin contains an additional,
recommended step for avoiding this state with residual load.
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 (e.g., on the first flight
after this test has been performed, when attempting to deploy the
thrust reversers, one or both thrust reversers might fail to deploy).
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. Since the very action of attempting to deploy
the thrust reverser results in releasing the wind-up torque, the second
time when one attempts to deploy the thrust reverser, it will deploy.
In other words, if the thrust reverser fails to deploy the first time
after performing this integrity test, the failure will only occur the
one time.
Recommendations
The FAA recommends that all owners and operators of the affected
airplanes deploy and retract the thrust reverser(s), as specified in
Boeing 737-600/700/800/900 Aircraft Maintenance Manual, Section
78-31-00, Task titled “Thrust Reverser System-Adjustment/Test.” We
recommend doing the actions after performing each integrity test
required by AD 2019-18-03, and prior to release of the aircraft from
maintenance for normal operation. 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.
For Further Information Contact
Rajendran Mohanraj, Aerospace Engineer, Propulsion Section, FAA,
Seattle ACO Branch, 2200 South 216th Street, Des Moines, WA 98198, at
phone and fax: (206) 231-3621; e-mail: rajendran.mohanraj@faa.gov.
For Related Service Information Contact
Boeing Commercial Airplanes, Attention: Contractual & Data Services
(C&DS), 2600 Westminster Blvd., MC 110-SK57, Seal Beach, CA
90740-5600; telephone 562-797-1717; https://www.myboeingfleet.com.