Saturday, March 25, 2023

Severe Stabilizer Mistrim - Pitch Upset - Challenger 300 N300ER - One Fatality

A Bombardier Challenger 300, N300ER, encountered an upset when the flight crew disconnected the autopilot while suffering from a nose-up mistrim situation. The mistrim was the result from a failure of the autopilot stabilizer trim control. The upset resulted in a roller coaster exceeding +4g and -2g, eventually brought under control by the flight crew. Tragically, a passenger was fatally injured in the upset.

The flight crew made several mistakes unrelated to the upset. They also apparently disregarded the caution alerting them to the mistrim. The NTSB preliminary report provides an account of the accident. The final report will provide analysis and probable cause. This post goes through the preliminary report and offers some additional commentary and excerpts from an operation manual of the same type aircraft.

Airspeed Disagree
The flight crew are required to conduct a thorough preflight inspection of the airplane. A primary emphasis is to ensure flight instruments are not covered or blocked. The pitot tube cover is prominent and clearly marked "remove before flight". There is no excuse for the flight crew to have missed the pitot cover in their preflight inspection.

Rudder Limiter Fault

Rudder Limiter Fault is an advisory because there is no loss of function.  If there was loss of function, Rudder Limiter Fail (caution) would have been displayed.

The Rudder Travel Limiter provides a variable limit based on airspeed (higher airspeed, lower rudder travel limit; lower airspeed, higher rudder travel limit) to limit aero loads on the vertical tail structure.

There is no entry for the Rudder Travel Limiter in the Master Minimum Equipment List. The flight crew were correct in proceeding with the fault, as it had no flight deck effect. Generally, the flight crew should have not taken action to clear the rudder fault, leaving it to a maintenance action post-flight.

The flight crew apparently did not set the V-speeds prior to taking off, relying on memory for the rotation.

AP Stab Trim Fail
The flight crew encountered Autopilot Stab Trim Fail. The autopilot moves the stabilizer to keep the elevator in a neutral position. If the autopilot is unable to use stabilizer trim, the elevator will be commanded into a mistrim in order to maintain the commanded flight path. Mistrim refers to a steady-state condition where the elevator is not in a neutral position - it is offsetting the mistrim of the stabilizer.

AP Holding Nose Down
Autopilot Holding Nose Down is a mistrim alert, where the autopilot is holding nose down to offset a stabilizer trim position commanding airplane nose up.  This is a positive indication of mistrim. Disconnecting the autopilot will remove the autopilot nose down elevator command, resulting in the airplane pitching up. This is exactly what happened.

Mach Trim Fail
Mach Trim is required to offset "Mach Tuck". Mach Tuck is a result of the center of lift moving aftwards at high mach numbers, causing the airplane to pitch down. If not offset, the pitch down will cause the airplane to accelerate, worsening the nose down tendency which can be catastrophic. Mach trim is a stabilizer trim command (airplane nose up with increasing mach). It is likely that the fault that caused autopilot Stab Trim Fail also caused Mach Trim Fail.  The airplane flight path did not encounter high mach numbers and therefore this failure was of no consequence.

Autopilot Disconnect
The NTSB reports that the autopilot disconnected twice prior to the upset. In both cases, the autopilot disconnected because manual pitch trim was commanded. It is not normal to manually trim the stabilizer while the autopilot is in-command. The autopilot disconnect is expected whenever the stabilizer is trimmed manually. I cannot explain whether the pilot did this, why the pilot would do this, or whether some other failure was encountered. I assume the pilot took the action for some reason???

The NTSB report makes no mention of PRI STAB TRIM FAIL alert being activated. For some reason, the flight crew chose to follow this unannunciated failure checklist. The first action was to select Stabilizer Trim to OFF. This action would immediately disconnect the autopilot.

Pitch Upset
Autopilot disconnect with the stabilizer mistrimmed nose up would result in an immediate pitch up. It was entirely predictable based on the alerts displayed. The flight crew should have been prepared for the pitchup, also by reference to the flight control position indicator (which would show the elevator position nose down).

Instead, the flight crew reacted too late to the pitch up, and then by over-controlling pitch down, and then over-controlling pitch up.

Stall protection activated (stick pusher  - nose down column command) in the second pitchup excursion. 

The flight crew restored pitch trim to primary control and used manual inputs to trim the airplane.  I presume the rest of the flight was flown manually, without autopilot.

Severe mistrim is a dangerous situation. Autopilot disconnect with severe mistrim has resulted in loss of control and catastrophe. While the flight crew managed to regain control and recover the airplane safely, the loss of life was tragic. The flight crew should have expected an upset and prepared the cabin occupants before taking action. Furthermore, the pilots should have reviewed the appropriate checklists which would have prepared them for the upset. The NTSB final report will analyze these circumstances when determining probable cause.

Stay tuned!

Peter Lemme

peter @
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Peter Lemme has been a leader in avionics engineering for 42 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 also contributes to the Network Infrastructure and Interfaces (NIS) subcommittee.

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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