Introduction
On the morning of 25 June 2021, a Cessna 208B aircraft with registration PH-FST was parked inside a hangar at a maintenance organization. The aircraft was scheduled for maintenance tasks before planned flights that day.
Sequence of Events
When the aircraft was on the ground with the engine off, erratic and high Exhaust Gas Temperature (EGT) readings were reported. The maintenance organization suspected a faulty magnetic pick-up in the engine’s propeller governor. A mechanic, supervised by a licensed engineer, was tasked to replace the magnetic pick-up. After removing the old pick-up, the mechanic followed steps one through four of the maintenance manual to install the new one. Steps five through eight, a system voltage test to verify correct installation, were not performed because they were overlooked. After connecting the wiring and tightening the lock nut, the mechanic and the pilot conducted a static engine test run and performed the Single Red Line (SRL) check to confirm proper system and EGT readings. The system appeared to function correctly. The mechanic then left the aircraft, and the pilot taxied to the skydive organization's building to pick up parachutists. After boarding seventeen parachutists, the pilot taxied to runway 26 and commenced the takeoff roll. Shortly after takeoff, at approximately 400 feet above field level, the engine lost power without any warning. The pilot performed an emergency landing in a field. The aircraft sustained substantial damage to the fuselage, wings, landing gear, and propeller. One parachutist suffered minor injuries.
Maintenance and Installation Details
The maintenance procedure for replacing the magnetic pick-up consisted of eight steps. The mechanic performed the first four steps but did not notice steps five through eight, which included a voltage check to confirm correct placement. The design of the propeller governor’s toothed gear allowed the magnetic pick-up to be positioned between its teeth. The maintenance manual did not specifically mention or address this possibility. The final voltage check, if performed, would have identified an incorrect installation.
Findings and Probable Cause
The official probable cause is as follows: During the initial climb after takeoff, the aircraft suffered a complete loss of engine power due to uncommanded feathering of the propeller. This feathering resulted from damage within the propeller governor, most probably caused by incorrect installation of the magnetic pick-up prior to the flight. The incorrect installation led to contact damage between the pick-up and the rotating toothed gear. Debris from this damage restricted the free movement of the ball head assembly in the governor housing, allowing all oil to drain from the propeller dome. This caused the propeller to continuously move toward the feather position and eventually cease rotation. The increasing propeller pitch reduced engine power, which reinforced the condition. The distorted output signal from the damaged magnetic pick-up did not cause the engine to flame out. The design of the toothed gear in combination with the maintenance procedure allowed for the incorrect installation, and the final voltage check was not performed by the maintenance staff.