DATE: March 20, 2003
AD #: 2003-06-51
Background: The FAA has received a report of severe vibration followed by a rapid nose down pitch change on a Learjet Model 45 airplane. Investigation revealed that the acme screw of the horizontal stabilizer actuator assembly was fractured. The actuator features a dual load path. The actuator assembly’s primary load path, the acme screw, failed. Loads should have been retained by the secondary internal retaining rod. However, the threaded nut on the secondary internal retaining rod had worked completely off, either latently before the fracture or from the effects of the fractured screw. The cause of such failure has not been determined.

There have been no previous reports of fractured acme screws of the horizontal stabilizer actuator assembly. However, there have been reports of cracks in the acme screw. As a result of these reports, the acme screw design was changed in February 2000 to reduce the probability of crack initiation. The fractured acme screw was an early design that did not have the design improvements incorporated to reduce the probability of crack initiation.

Structural failure of the horizontal stabilizer actuator assembly could result in possible loss of control of the airplane.

FMI: See the AD