What happened
On July 26, 2017, a crew operating for the Polish Medical Air Rescue (LPR) was tasked with a medical transport mission to Czeszków. Approximately two minutes before landing at the patient's location, the crew observed a "WARNING AP Trim" message on the Crew Alerting Display (CAD).
Following the landing, the crew performed a subsequent flight from Czeszków to a hospital in Sieradz. During this leg, the autopilot functioned correctly. However, upon restarting the engine in Sieradz, the "WARNING AP Trim" message reappeared, accompanied by an acoustic gong in the crew's headsets. During this second instance, the crew found that the autopilot could no longer be engaged.
Adhering to standard operating procedures and checklist protocols, the crew decided to return to their base in Łódź. The return flight was conducted under Visual Flight Rules (VFR) without the use of the autopilot. The landing at the base was completed without further incident. Following the flight, the pilot attempted to test the autopilot system, but all autopilot control buttons remained non-functional. The issue was subsequently reported to the Technical Coordination Center.
The investigation
The investigation focused on the technical failure of the autopilot system. Following the incident, the aircraft was temporarily removed from service. Technical personnel from the Warsaw Babice maintenance station performed diagnostic checks on the autopilot system in accordance with the THALES ASMM EC-135 F911.
Maintenance crews inspected voltages at the CADA and CAEA APM sockets (H and L). The diagnostic process led to the decision to replace the APM 2000 autopilot module. The faulty unit (P/N 416-00297-007, S/N 168) was removed and replaced with a functional unit (S/N 1272) following the approved maintenance manual procedures. A successful test flight was conducted, and the aircraft was returned to full service on August 1, 2017.
Findings
- The primary cause of the incident was a malfunctioning electronic mode selection module within the APM 2000 autopilot block.