Saturday, May 18, 2019

737 Pitch Trim Incidents

Pitch trim is an abstract term to represent the ability to reduce column forces by moving the stabilizer and elevator. This post will dwell on stabilizer trim. 

All airplanes provide at least two means to trim the stabilizer, mostly using two seperate actuators motors. Except the Boeing 737, which relies on manual trim as the second actuator. 

The only stabilizer trim incident of interest that I can find (searching NTSB database) for 737 involved a jammed actuator and that the pilot was UNABLE to trim the stabilizer, due to clutch issues. 

Ethiopian ET302 encountered high opposing forces due to the mistrim; it has been assumed that the ET302 actuator was not jammed, and there has been no concern raised that the ET302 clutch would have oppressively opposed trimming manually, on top of the aero loads.

The 737 pitch trim system has been extremely reliable when looking at the US NASA accident database. I searched for accidents and incidents with stabilizer, trim, runaway and came up with numerous collisions on the ground (with A319, A321, 767, 777, A320, MD81, L1011, several 737 Vs 737, and even one with a Cessna 401). There were a couple of elevator malfunctions. A number of structural damage due to foreign objects or otherwise.  But only two that involved trim itself, and neither a runaway.

Alaska Flight 497

On October 10, 2001, at 1627 hours Pacific daylight time, Alaska Airlines flight 497, a Boeing 737-700, N615AS, experienced a jammed horizontal stabilizer trim actuator during the takeoff initial climb from the John Wayne Airport-Orange County, Santa Ana, California. The flight was destined for Seattle, Washington; however, the flight crew diverted to, and landed at, Los Angeles International Airport, Los Angeles, California, at 1658.
Stepping through the incident, the flight crew went to manual trim.
The crew disengaged the autopilot and found the electric trim control inoperative at both the pilot and co-pilot controls and the manual trim control was jammed and immovable. 
To emphasize the point, the flight crew observed that arriving maintenance personnel had no more success.
According to the pilot, maintenance personnel who met the airplane on arrival could not move the stabilizer control either and believed the actuator gearbox was jammed. 
The flight crew were insistent that the manual trim was not usable. That the simulator forces were much less than what they encountered.
In a subsequent company interview, on November 13, 2001, the pilot reiterated that the trim manual control had been jammed and immobile; neither he, the first officer (co-pilot) or the mechanics who met the flight, were able to force it to move. The pilot said he had trained for the jammed stabilizer emergency in the simulator; however, in the incident at hand he had applied "a lot more force" than was required in the simulator and was still unable to move the manual control.
The actuator failed, as was expected.
Post flight examination revealed the horizontal stabilizer trim actuator motor was seized. Further examination of the actuator motor revealed that the motor was mechanically seized. 
The actuator was replaced. Nothing else was replaced.
The actuator motor was replaced with a serviceable unit and the aircraft was ferried to Seattle for inspection. No other faults were found in the pitch trim system and the airplane was returned to service. 
The resolution was that the flight crew needed to use more force to break out the clutch.
According to a representative of Boeing Aircraft Company, with the motor seized, in order to manually trim the stabilizer, it would have been necessary for the flight crew to have exerted sufficient force on the trim wheel to cause the motor clutch to slip in addition to the force necessary to overcome normal system friction.


The 737 originally had two actuators: one for manual pitch trim and the other for autopilot pitch trim. Having two actuators was the original source of the two 737 cutout switches, elec main and autopilot. This 737-300 autopilot actuator failed when the circuit breaker "popped". The autopilot ran out of elevator authority. The pilot disconnected the autopilot and flew the airplane manually without further incident.

The source of the circuit breaker trip was never identified. There were some minor exceedances in the equipment, but nothing that would explain any overcurrent.

While the pilot complained of some difficulty controlling the airplane manually (electric pitch trim was operable and was not noted for any issue), there was no technical fault found to explain it. The investigation concluded that the captain was over-controlling.


737 stabilizer trim has been very reliable. There is no instance of runaway. The failure of the autopilot actuator caused no trim difficulty. Most troubling is that the jammed actuator left the crew unable to use manual trim, and that there was no follow-up action taken to explore that issue, it was simply signed off. The force necessary to slip the clutch past a jammed actuator has not been suspected for ET302.

Peter Lemme

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Peter Lemme has been a leader in avionics engineering for 38 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chaired the SAE-ITC AEEC Ku/Ka-band satcom subcommittee for more than ten years, developing ARINC 791 and 792 characteristics, and continues as a member. He contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.

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