What happened
On 14 November 2023, a Fan Jet Falcon Series D, registration G-FRAK, was conducting aerial work over South-West England. While cruising at FL190, the crew noticed that the aircraft's pitch control had become restricted, preventing the pilots from applying sufficient nose-up input to maintain level flight during a turn. The crew stabilized the aircraft in straight and level flight and declared a MAYDAY, diverting to the nearest suitable airfield.
As the aircraft descended through 4,000 ft, the pilots felt a sudden "thud" as the control restriction cleared. The aircraft subsequently landed without further incident. There were no injuries to the two crew members on board.
The investigation
An investigation involving the operator and the aircraft manufacturer examined the aircraft's flight data and physical components. Investigators discovered evidence of trapped water within the control box compartment at frame 3 and the rear fuselage, specifically underneath the elevator control idler.
Testing revealed that during periods of heavy rain, water could enter the rear fuselage through the APU exhaust shroud. On this specific aircraft, the natural drainage path intended to move water to the lower fuselage was obstructed by a combination of manufacturing tolerances and the application of anti-corrosion fluid.
Data showed that while the aircraft was at FL190, the outside air temperature had dropped to minus 25°C. The investigation concluded that the trapped water had frozen into ice, which physically blocked the movement of the pitch control idler. The restriction vanished during the descent as the aircraft entered warmer air, causing the ice to melt.
Findings
- Rainwater entered the rear fuselage via the APU exhaust shroud during heavy rainfall.
- The accumulation of ice in the control box compartment caused the pitch control restriction.
- Drainage in the affected area was impeded by structural tolerances and anti-corrosion fluid.
- The ice melted as the aircraft descended into warmer temperatures, restoring full control functionality.