Water that pooled under the floor panels of a Falcon 20, froze and restricted the movement of the aileron trim actuator as the airplane was landing at London Stansted Airport is to blame for the airplane’s “frozen” controls, according to the UK Air Accidents Investigation Branch.
On May 9 last year, the Falcon 20F-5 departed Little Rock, Ark., for London Stansted Airport, with technical stops at Teterboro and Gander. While making a manual approach to Gander, the pilot flying the aircraft felt that the lateral controls were unusually stiff. He assumed that the ailerons had been mistrimmed. The airplane landed at Gander and was on the ground for 39 minutes.
The commander, who had more than 2,000 hours in the type, flew the third leg. The flight control check before the flight was normal. About two hours into the cruise, with the autopilot engaged, the crew noticed flickering of the aileron trim caption on the PFD and applied corrective trim. On the descent to Stansted the commander found that the lateral flight control problems had become worse. In a turn to the left, the autopilot was reluctant to level the wings. Later the airplane reached 45 degrees of bank and the commander disconnected the autopilot, intending to fly the airplane manually. However, the roll control was stiff; both pilots applied force to the control wheel but were unable to move it. The crew used rudder to level the wings.
The pilots limited bank angle to 10 degrees and advised ATC of the emergency. They were given vectors requiring only left turns until the airplane was able to establish on the ILS for Runway 23. The pilot, who was faced with a night landing in a strong gusting westerly wind, expressed concern about the lack of lateral control. However, he intercepted the ILS and maintained the course with rudder. The landing was successful, but more than 20 minutes after touchdown the control wheel was still jammed, and the crew’s attempts to move the ailerons from outside the airplane were unsuccessful.
About 36 hours after landing, the AAIB inspected the airplane. At that time the controls were free to move and normal in function. After checks of the control runs showed no anomaly they lifted the central floor panels to check the roll trim actuator. Investigators found a large amount of water in the belly of the forward fuselage. When an investigator dipped a hand into the water near one of the manual drains it opened and water poured out at a considerable rate for more than 10 minutes.
The source of the water was a matter of some debate. Based on the quantity of water in the fuselage (an estimated 20 liters), investigators concluded that it was most probably rainwater that entered through a leaking door seal or through a gap if the door had been left unlatched in heavy rain. The manufacturer believes that the source was leaks in the area of the icebox drain. Regardless, after departure from Gander on the last sector, the water would have frozen and trapped the aileron actuator in ice.
Once the water had drained, the area dried and water-soaked insulation been discarded, a test flight was carried out and there was no further malfunction. The aircraft later returned to the U.S. with no further difficulty. There have been no more reports of control restrictions.