What happened
On 17 August 2018, a Cessna 525B CitationJet CJ3, registered N525L, departed Zurich Airport (LSZH) for Sion Airport (LSGS) as part of a maintenance test flight. The flight was intended to investigate unusual noises heard in the control columns related to the autopilot system. Shortly after takeoff, the aircraft's autopilot began disengaging repeatedly, triggering acoustic warnings in the cockpit.
As the flight progressed, the pilot struggled to maintain the assigned flight path and altitude. The aircraft deviated significantly from the Standard Instrument Departure (SID) route and failed to adhere to altitude clearances provided by air traffic control. During the descent into Zurich, the aircraft's rate of descent became so steep that the ground proximity warning system (GPWS) triggered a "Caution terrain!" alert. The pilot eventually landed the aircraft safely on runway 14 at 17:10 UTC.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined cockpit voice recorder data, air traffic control recordings, and radar tracks. Technical inspections of the aircraft's autopilot system and elevator cable tension were conducted by the maintenance company. The investigation also reviewed the pilot's communication with air traffic controllers, noting frequent use of non-standard phraseology and a lack of clear radio call signs.
Findings
- The investigation ruled out technical failures in the autopilot actuators or elevator cable tension, as all measurements were within permitted tolerances.
- The primary cause of the flight deviations was inadequate pilot situational awareness and a lack of focus on flight instruments due to preoccupation with the malfunctioning autopilot.
- The pilot's concentration on the autopilot led to significant lateral and vertical deviations from the authorized flight path.
- The pilot failed to follow standard aviation communication procedures, often providing incomplete responses to air traffic controllers.
- The high rate of descent during the final approach, which triggered the terrain warning, was a result of the pilot's inability to effectively monitor the aircraft's flight parameters while managing the autopilot issues.