What happened
On July 20, 1999, during an instructional flight at Coulommiers aerodrome, the student pilot in command of a Cessna 177 RG, registration F-GETL, noticed abnormal stiffness in the aileron controls following a go-around maneuver. The instructor took control of the aircraft and decided to divert to Lognes aerodrome.
While the flight remained stable, the aircraft continued to exhibit difficult lateral control. As a precaution, the instructor avoided applying excessive force to the controls. The aircraft was successfully landed on the longer grass runway at Lognes. Notably, the heavy control feel persisted during taxiing but vanished immediately after the battery master switch was turned off at the parking stand.
The investigation
The investigation established that the autopilot system had been inadvertently engaged. The instructor confirmed this by observing that the autopilot switch was in the 'ON' position. The investigation found that the switch's proximity to the throttle lever made accidental engagement possible.
Furthermore, the investigation revealed that the autopilot system was known to be inoperative among the club's pilots, including the instructor. This information was shared only verbally and had not been documented via official club notices, flight logs, or markings on the aircraft itself. Despite the system being considered out of service, the servo-control mechanism had not been physically disconnected or rendered inoperative.