What happened
On 9 May 2007, a Dassault Falcon 20-F5, registration N757CX, was performing a private flight from Gander, Canada, to London (Stansted) Airport. The flight crew, consisting of two commercial pilots, began noticing irregularities during the approach phase. While flying manually, the pilot observed that the lateral controls felt unusually stiff, specifically when attempting to roll the aircraft to the right.
As the aircraft descended towards Stansted, the situation deteriorated significantly. The pilot found that the roll control was increasingly restricted, eventually reaching a point where the control wheel became completely jammed. The crew was unable to move the wheel to achieve wings-level flight through manual input, forcing them to rely on the rudder to maintain a controlled descent. The pilots declared an emergency, informing Air Traffic Control that they had a jammed flight control. Despite the severe restriction, the crew successfully intercepted the localiser and landed the aircraft safely at Stanlend under windy conditions. No injuries were reported among the 2 crew members or 5 passengers on board.
The investigation
An investigation by the AAIB examined the aircraft approximately 36 hours after the landing. Upon removing floor panels in the forward fuselage, investigators discovered a large quantity of water accumulated in the belly of the aircraft. When the manual drain was accessed, a significant volume of water poured out.
Technical examination of the roll control circuit revealed that the aileron trim actuator, located low in the forward fuselage, had been affected. The investigation focused on how this water entered the aircraft and why it caused the control jam. While the aircraft had recently undergone a major maintenance check, the water was found to be relatively clear and likely originated from rainwater entering through a leaking door seal or an open door during rain on the ground.
Findings
- The primary cause of the control restriction was the freezing of water within the forward fuselage, which trapped the aileron trim actuator body.
- The accumulated water migrated to the area of the trim actuator during flight.
- As the aircraft flew through cold temperatures, the water froze, preventing the movement of the actuator and subsequently restricting the movement of the ailerons.
- The amount of water found was too large to have originated from the aircraft's icebox drain alone, suggesting a leak from an external source like a door seal.
- The aircraft had recently been repainted, and the lack of visible markings on the fuselage drains made them difficult to identify during inspection.