The US Federal Aviation Administration has finalised an airworthiness directive for tailplane trim inspections after concerns about incorrect installations that could result in loss of control of aircraft.

Proposed in October last year, the AD resulted from multiple operators discovering and reporting incorrect trimmable horizontal stabiliser actuator (THSA) installation while complying with an Airbus service bulletin. The AD requires operators of 721 US-registered Airbus A320-family aircraft to perform a one-time inspection of the THSA.

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 stabiliser "and loss of control of the aircraft".

A320 curved winglet 01 W445
 © Airbus

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 recently proposed a 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 stabiliser trim assembly due to an improperly lubricated jackscrew.

Northwest Airlines, through the Air Transport Association, had requested that the FAA reconsider issuing the A320 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."

Although it does not agree the AD should be withdrawn, the FAA says: "Airbus has informed us that the maintenance instructions have been revised and clarified to prevent confusion during any future installation of the THSA."

Source: Flight International