WPR22LA284

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.

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.

On July 27, 2022, about 1236 Pacific daylight time, a Bombardier Inc BD-100-1A10 (marketed as a Challenger 300), N557XJ, was involved in an accident near San Francisco, California. The cabin attendant was seriously injured; the pilot and co-pilot were not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 positioning flight. The pilot-in-command (PIC) reported that the airplane arrived at San Francisco International Airport (SFO) San Francisco, California, about 1130 from Shively Field Airport, Saratoga, Wyoming. Five passengers deplaned and a new cabin attendant joined the current crew. After a briefing and compliance with the required checklists for the airplane, the flight crew departed to the east to Page, Arizona, with the second-in-command (SIC) as the pilot flying. The PIC indicated that, as the airplane continued to climb through 20,000 ft msl, an amber CAS “AP STAB TRIM FAIL” message appeared. The PIC began to review a checklist to troubleshoot the amber message when a second amber CAS message, “AP HOLDING NOSE DOWN” appeared. The PIC assumed control of the airplane after a positive exchange of the flight controls. The PIC stated the autopilot then “disconnected on its own just as I was going to push the disconnect button.” The airplane immediately pitched up, and after “several hard oscillations,” according to the SIC, the PIC was able to recover the airplane into straight and level flight. During this time, the cabin attendant in the aft cabin crawled up to the cockpit and reported to the crew that she was seriously injured. The PIC reported that he did not feel comfortable reengaging the autopilot and continued to manually fly the airplane. After being cleared to return to SFO, the crew landed the airplane without further incident and the cabin attendant received medical treatment. No damage was sustained to the airplane or engines. The cabin attendant later reported that she received a preflight briefing indicating that she should be seated and belted until the airplane reached 10,000 ft msl. She stated that, about 15 minutes after takeoff, she unbelted and moved about the cabin to perform her duties; she was not seated or secured with a seatbelt at the time of the in-flight upset (company policy allowed cabin crew to maneuver around the cabin above 10,000 ft msl). The audition of the cockpit voice recorder (CVR) revealed that the event was not captured due to being overwritten on the continuous recording loop of the CVR. Although the FDR “Master Caution” parameter recorded the posting of multiple caution messages around the time of the event, the FDR did not record which caution messages were posted. The FDR did not contain any telemetry about the “AP STAB TRIM FAIL” or “AP HOLDING NOSE DOWN” annunciations. FDR Data  Figure 1. FDR data of flight track · At 1230:51, the airplane departed runway 28R at SFO. As the airplane climbed, the magnetic heading changed from 282° to 54° (right turn). · At 1232:19, the autopilot was engaged while the airplane was at 4,265 ft pressure altitude, 220 knots (kts) indicated airspeed (244 kts ground speed), and a magnetic heading of 54°; the left elevator was at 5° while the right elevator was at 7°. · At 1232:24, the master caution activated for 5 seconds. The autopilot remained engaged, and the master caution was active again at 1232:38 (for 6 seconds). · At 1236:39, both the Master Disconnect parameter and the Automatic Flight Control System (AFCS) Abnormal Disconnect parameter changed from “Not Disconnected" to “Disconnected” (abnormal disconnect is any autopilot disconnect caused by any method other than the AP button on the flight guidance panel [FGP]). At this time, vertical acceleration increased from 0.985g to 2.221g in less than a second. · At 1236:40, the autopilot state was changed from engaged to disengaged. Vertical acceleration (sampled 8 times per second) reached -0.426g during this second. · At 1236:41, vertical acceleration was 2.096g and reduced within the second to 0.176g. The autopilot remained off for the remainder of the flight. · At 1236:48, the pilot and copilot manual trim enabled was active seven times within the minute following the autopilot becoming disengaged. For each occurrence where the pilot manual trim enabled was active, a respective pitch trim command nose down flag was active. At each occurrence, the stabilizer trimmed nose down. · At 1238:21, the right primary flight display (PFD) flag showed it failed for 3 seconds. · At 1238:22, the master caution activated for 16 seconds. · At 1239:08, the right multifunction display (MFD) flag showed it failed for 14 seconds. · At 1242:50, the left MFD flag showed it had failed and remained in this state for the rest of the flight. At this time, the airplane began to return to SFO. The remainder of the flight was uneventful. · At 1302:12, the weight on wheels on the left and right main landing gear registered a change from AIR to GND. The airplane landed on runway 28R without incident and taxied normally off the runway. Quick Reference Handbook (QRH) The airplane’s manufacturer’s QRH contained procedures for responding to the presented CAS messages. The QRH available and used by the flight crew was last revised on June 14, 2022: AP STAB TRIM FAIL “Condition: Autopilot pitch trim has failed. Objective: Assume manual control. WARNING: An abrupt change in control force, or an out of trim situation may be experienced when disconnecting the autopilot. SMKG /BELTS should be selected on. CAUTION: Minimize changes to airspeed and configuration to minimize control forces and out of trim situation. (1) Flight controls ........................................................................................... HOLD FIRMLY (2) Autopilot ..............................................DISCONNECT using control wheel MSW switch (3) Retrim airplane if necessary. (4) Autopilot ...................................................................................................... DO NOT USE” AP HOLDING NOSE DOWN “Condition: Autopilot is holding control force in the direction indicated. Objective: Correct mistrim condition. WARNING: An abrupt change in control force, or an out of trim situation may be experienced when disconnecting the autopilot. SMKG / BELTS should be selected on. CAUTION: Minimize changes to airspeed and configuration to minimize control forces and out of trim situation. (1) Flight controls ........................................................................................ HOLD FIRMLY (2) Autopilot ..................................................................DISCONNECT using control wheel MSW switch (3) Retrim airplane if necessary. (4) Autopilot .....................................................................................................AS DESIRED” The flight attendant reported that the BELTS light was on during departure, but could not recall if the light extinguished at any point during the flight. In addition, the FDR data did not include any information about the BELTS light indication. According to the operator, company procedures allowed the cabin attendant to be moving about the cabin at the time of the accident. Manufacturer Reference Information of AP STAB TRIM FAIL and Autopilot According to the airplane manufacturer, The purpose of the AP STAB TRIM FAIL Caution message is to advise flight crew[s] that the autopilot is no longer able to command horizontal stabilizer trim. In isolation, this has no effect on autopilot engagement, i.e. the message itself does not cause the autopilot to disengage. Inability to command horizontal trim or an out-of-trim condition will not result in autopilot disengagement; in such cases, the autopilot continues to control pitch using only the elevator. The autopilot is de