Bombardier Inc BD-100-1A10 N557XJ accident near San Francisco, California

Probable cause

An inflight upset resulting in serious injury to the cabin attendant following an autopilot disconnect for reasons that could not be determined based on the available information.

NTSB analysis

The airplane departed on a positioning flight with two pilots and one cabin attendant onboard. Information from the flight data recorder (FDR) indicated that the autopilot was engaged after takeoff as the airplane climbed through about 4,200 ft pressure altitude. About five seconds later, an AP STAB TRIM FAIL message appeared on the crew alerting system (CAS). The message was acknowledged by the flight crew 5 seconds later, as indicated by the deactivation of the master caution recorded in the FDR. Fourteen seconds after the appearance of the AP STAB TRIM FAIL message, an AP HOLDING NOSE DOWN message posted on the CAS, which the crew acknowledged about 6 seconds later.

According to the flight crew, the pilot-in-command (PIC) began to review the checklist in the quick reference handbook (QRH) and assumed control of the airplane from the second-in-command (SIC) after a positive exchange of controls. Both pilots reported that the autopilot disconnected shortly after they exchanged control, and the airplane immediately pitched up. The SIC reported that, after “several hard oscillations,” the PIC was able to recover the airplane into straight and level flight. The cabin attendant sustained serious injuries during the in-flight upset.

The PIC reported that he did not feel comfortable reengaging the autopilot and continued to manually fly the airplane. After receiving clearance to return to the departure airport, the flight crew landed the airplane without further incident and the cabin attendant received medical treatment.

The procedures in the QRH for both the AP STAB TRIM FAIL and AP HOLDING NOSE DN messages warned that an abrupt change in control force or an out-of-trim situation may be experienced when disconnecting the autopilot, and that the seatbelt sign should be selected on. The first step of the procedure then stated, “Flight controls… HOLD FIRMLY.” The extent to which the pilots read the relevant warnings or completed the checklist items associated with the posted CAS message(s) could not be determined, because the cockpit voice recorder (CVR) recording of the event was overwritten.

Although the flight crew indicated that the autopilot disconnected unexpectedly, there was no data to indicate that the autopilot disengaged due to a malfunction or failure, and postaccident testing did not reveal any mechanical anomalies with the autopilot system. It is likely that the autopilot disconnect was the result of an inadvertent control input by one of the pilots; however, based on the available information, the reason for the autopilot disconnect could not be determined.

At the time of the upset, the airplane was above 10,000 ft, the altitude at which company procedures permitted cabin attendants to remove their seatbelts and move about the cabin. Although the QRH checklist called for the seatbelt sign to be selected on, none of the crew members indicated that the cabin attendant was instructed to either stay in or return to her seat, an advisory that would have been prudent before an expected abrupt change in flight control forces.

The manufacturer identified several AP STAB TRIM FAIL occurrences from aggregate aircraft health management system (AHMS) information, which indicated that an AP STAB TRIM FAIL caution message could result from the engagement of the autopilot in an out-of-trim condition, which in turn, could trigger a flight guidance computer (FGC) monitor and result in the disabling of the autopilot pitch trim function. Whether this specific scenario occurred on the accident flight could not be determined based on the available recorded data.

Text from the NTSB. Open the NTSB docket for WPR22LA284 for the full record.