Serious incident with a Cessna-172R southwest of Örebro Airport

Summary

On 11 December 2024, a serious incident occurred involving aircraft SE‑LGL, a Cessna 172R, in the airspace south of Örebro Airport. The aircraft had recently undergone maintenance during which three servos in the autopilot system were replaced and the software was updated. On the first flight following the maintenance, the pilot engaged the autopilot in altitude hold mode shortly after the climb to cruise altitude. Shortly thereafter, an amber mistrim alert was displayed on the primary flight instrument. When the autopilot was disen­gaged, a sudden and significant pitch‑up occurred, resulting in an upset condition. During this phase, the pilot believed that the autopilot was still engaged. The autopilot’s Electronic Stability and Protection (ESP) function was activated on several occasions and applied control inputs, which likely reinforced the pilot’s perception that the autopilot was still acting on the system.

Recorded data shows that the autopilot commanded nose‑down trim, but that the effect was the opposite, indicating that the elevator trim moved in the opposite direction to the auto­pilot’s commands. The incorrect trim setting resulted in high control forces, particularly at high airspeed and with high power setting. Eventually, the pilot reduced engine power, pulled the circuit breaker for the autopilot, regained control and was able to land normally without injury to persons or damage to the aircraft.

The investigation has not identified any mechanical defects that could explain the significant and sudden increase in pitch attitude. The most probable cause of the incident is that the configuration settings for the autopilot system’s elevator trim were incorrect.

The analysis shows that several organisational and system‑related factors contributed to the fact that the erroneous behaviour of the autopilot system was not detected before the flight.

Causes/Contributing Factors

The probable cause why the upset occurred was that the configuration settings for the auto­pilot system’s pitch trim were incorrect, causing the autopilot to command pitch trim in the opposite sense to that intended for the aircraft, i.e. nose-up instead of nose-down.

Several factors contributed to the configuration settings probably being incorrect:

• The Service Bulletin and the manuals to which it referred did not prescribe adequate functional checks following maintenance of the autopilot’s pitch trim function.
• The maintenance organisation did not take sufficient measures to ensure that, after maintenance, the configuration settings were in accordance with the prescribed settings.
• The maintenance organisation did not carry out any independent inspection, nor did it apply any other method to detect errors following the software updates and configuration settings implemented during the maintenance.

A contributing factor of the upset condition was that the pilot did not notice the autopilot system’s mistrim alert. This was because the alert provided by the autopilot system was not presented in a sufficiently clear manner to draw the pilot’s attention to the significant control forces present in the autopilot system.

Safety recommendations

SHK submits the following recommendations:

Garmin is recommended to

• examine the possibilities for presenting mistrim alert to the pilot in a clearer manner when a pitch trim servo is installed and trimming is automatic (see Section 2.5). (SHK 2026:17e R1)
• ensure that Service Bulletins relating to maintenance on autopilot systems are drafted in such a way that it is clearly specified which tasks are included in the maintenance, and that they ensure adequate checks are carried out following completion of the maintenance (see Section 2.6.1). (SHK 2026:17e R2)
• clarify the GFC 500 Maintenance Manual so that it includes an instruction describ­ing how the automatic electric pitch trim is to be checked after maintenance has been carried out on the trim system (see Section 2.6.4). (SHK 2026:17e R3)

The Combined Airworthiness Organisation is recommended to

• ensure that work orders for maintenance are clearly and correctly drafted, and that they ensure adequate checks are carried out following completion of the mainte­nance (see Section 2.6.2). (SHK 2026:17e R4)

The Maintenance Organisation is recommended to

• ensure, in connection with software maintenance on autopilot systems, that actions are documented in accordance with the instructions issued by the manufacturer of the autopilot system (see Section 2.6.3). (SHK 2026:17e R5)
• ensure that independent inspections are carried out, or that other methods of fault detection are applied, in connection with critical maintenance actions that include the installation and configuration of software (see Section 2.6.5).
  (SHK 2026:17e R6)