File:AS261jackscrew 261.ogv

English: Longitudinal Trim System Description and Failure Sequence

This three-dimensional animation (with narrated audio) is divided into two segments. The first illustrates the nominal range of travel of the horizontal stabilizer from maximum airplane nose down to maximum airplane nose up position. The second depicts the reconstruction of the accident sequence.

This animation depicts nominal horizontal stabilizer motion at twice the actual primary motor or alternate trim motor rate. With this exception, elapsed times do not correlate to real time.

The nominal range of travel of the horizontal stabilizer trim system is shown from both exterior and cut away views. The motion depicted begins at the neutral position, proceeds to the maximum Airplane Nose Down (AND) position, continues to the maximum Airplane Nose Up (ANU) position, and finally returns to the neutral position. System components are identified in the cut away view.

The reconstruction of the accident sequence begins with the Alaska Airlines Flight 261 horizontal stabilizer takeoff setting of 7.0 degrees ANU in Puerto Vallarta, Mexico. Subsequent airplane nose down stabilizer trim motion from 7.0 degress ANU to 2.0 degrees ANU was due to the use of primary trim. Stabilizer motion from 2.0 degrees ANU to 0.4 degrees AND was commanded by the autopilot.

The horizontal stabilizer jammed at 0.4 degrees AND and remained jammed until the Acme nut threads failed. The lower mechanical stop subsequently contacted the Acme nut, restraining the horizontal stabilizer at 3.1 degrees AND. Fracture of the torque tube inside the Acme screw caused the the horizontal stabilizer to move to 3.6 degrees AND, where it contacted the fairing brackets. Shortly thereafter, fracture of the fairing brackets resulted in an unrecoverable loss of pitch control.