The US Federal Aviation Administration has finalized an airworthiness directive (AD) first proposed in October, calling on operators of 721 US-registered Airbus A320-family aircraft to perform a one-time inspection of the aircraft’s trimmable horizontal stabilizer actuator (THSA).

The AD resulted from multiple operators discovering and reporting incorrect THSA installation while complying with an Airbus service bulletin.

According to the FAA, the faulty installation “could lead to a degradation of the integrity of the THSA primary load path”, that could result in uncontrolled movement of the horizontal stabilizer “and loss of control of the aircraft.” 
The AD calls for operators within 600 flight hours or 750 flight cycles from 6 June to perform a one-time detailed visual inspection of the lower and the upper THSA attachments for correct installation and for the presence of metallic particles.

The FAA earlier this week proposed a somewhat similar AD for Boeing 737s following a Boeing design review and safety analysis of the horizontal trim units on all its aircraft. The review followed the January 2000 loss of an Alaska Airlines MD-83 and all 88 passengers and crew after the failure of the horizontal stabilizer trim assembly due to an improperly lubricated jackscrew.


Northwest Airlines, through comments submitted by the Air Transport Association, had requested that the FAA reconsider issuing the AD. “NWA agrees that an incorrectly installed THSA could be a safety concern, but asserts that accomplishing a one time inspection will not prevent improper THSA installations in the future, and does not understand what corrective action is being taken (or should be taken) to prevent similar installation problems in the future.”

Though it did not agree that the AD should be withdrawn, the FAA did state that “Airbus has informed us that the maintenance instructions have been revised and clarified to prevent confusion during any future installation of the THSA.”