What happened
On January 2, 1996, a Cessna 560 Ultra, registered in Bermuda, was conducting a non-commercial business flight from Lugano to Augsburg. The aircraft, carrying two pilots and three passengers, was on an automated precision approach to runway 25 using the ILS. At the time, the area was experiencing icing conditions from the ground up to 5,000 feet.
As the aircraft descended to approximately 400 feet, the pilot transitioned from autopilot to manual control. While the crew believed icing was negligible, investigators later found approximately 2 mm of ice on the leading edges of the wings. At an altitude of about 50 meters, the aircraft suddenly rolled to the right and then to the left, causing the left wingtip to strike the ground approximately 130 meters before the runway threshold. The impact caused the landing gear to collapse, and the aircraft slid 230 meters along the safety strip between the runway and taxiway F, coming to a halt. The impact caused heavy damage to the aircraft, and one passenger sustained a minor back injury.
The investigation
The BFU examined the aircraft's systems, including the stall warning computer and the air data computer. Technical testing at the manufacturer's facilities in the United States revealed that while the stall warning computer was functioning according to its design specifications, it failed to account for the increased stall speed caused by wing icing. The investigation also reviewed meteorological data, which confirmed significant icing conditions were present during the approach.
Findings
- The primary cause of the accident was icing conditions during the approach to Augsburg, which led to ice accumulation on the wings.
- The aircraft exhibited undocumented flight characteristics, specifically a tendency to roll and drop unexpectedly when flying near stall speeds under icing conditions.
- A critical contributing factor was a design flaw in the stall warning system, which did not trigger a warning before the actual aerodynamic stall occurred because it did not account for the higher stall speed necessitated by the ice.
- The crew was unaware of these specific flight characteristics, as they had not been previously documented for this aircraft model.