What happened
On 2 December 2002, a Cessna 560XL (registration HB-VAA) operating a government flight from St. Gallen-Altenrhein to Berne experienced a severe loss of control during the climb over Lake Constance. Following takeoff, the pilot flying issued commands to retract the flaps. However, due to radio interference and an inoperative intercom, the co-pilot misheard the command and set the flaps to 7° instead of retracting them fully.
As the aircraft accelerated, the crew eventually noticed the incorrect flap setting during an after-takeoff check. When the pilot retracted the flaps at a speed of 254 KIAS—well above the maximum permissible speed for that setting—the aircraft's stabilizer underwent a rapid change in angle. This triggered a violent pitch-down moment, causing the aircraft to dive at a rate of 9,600 feet per minute and reach speeds of 304.7 KIAS. The crew managed to recover the aircraft using manual trim and performed a precautionary landing at St. Gallen-Altenrhein with no injuries to the 5 occupants.
The investigation
SUST examined the aircraft's flight data, cockpit voice recordings, and technical systems. The investigation established that the aircraft's mechanical systems, including the autopilot and flight controls, were functioning correctly prior to the event. The investigation also noted that the crew was communicating without headsets because the left-side intercom was inoperative, which increased the risk of misunderstanding. The technical review confirmed that the sudden pitch-down was a direct result of the stabilizer's reaction to the flap movement at high speed.
Findings
- The primary cause of the incident was the retraction of flaps at a speed exceeding the structural limits for the extended flap configuration.
- Communication between the crew was compromised by the lack of headsets and external radio interference.
- The pilot not flying failed to intervene or question the incorrect flap setting despite noticing the discrepancy.
- The aircraft's stabilizer movement, designed to compensate for pitch changes during flap movement, created an unmanageable nose-down force because the autopilot had already trimmed the aircraft for a different flight phase.
Safety action
Following the incident, the manufacturer, Cessna, issued an alert service bulletin advising all operators to ensure aircraft speed is reduced below the maximum flaps-extended speed (Vfe) before any flap retraction. Subsequently, a mandatory service bulletin was issued to implement a system that inhibits stabilizer angle changes and triggers a warning if the aircraft exceeds 215 KIAS during flap movement.