Dassault has developed a modification for Falcon 20 business jets to increase aft-fuselage drainage after a flight-control icing incident involving a UK-registered aircraft.
The aircraft’s crew found that pitch control became restricted at 19,000ft and could not be moved nose-up sufficiently to maintain level flight during turns, according to the UK Air Accidents Investigation Branch.
It says the Falcon was flying over southwest England at the time of the event, on 14 November last year, without elaborating – the aircraft has previously been used for military exercises such as target towing.
After the pilots rolled the jet wings-level, they believed the restriction had increased, and opted to divert after declaring an emergency.
Investigators have determined that, during heavy rain, water could enter the fuselage via the auxiliary power unit exhaust shroud.
“This allowed the rainwater to collect in the control box beneath the elevator control idler in the rear fuselage,” the inquiry states, adding that the intended drainage path was constricted by reduced gaps between components and application of anti-corrosion fluid.
The aircraft had spent 40min at 19,000ft, sufficient time for the water to freeze and limit the movement of the elevator control.
As the jet descended during the approach, the warmer air enabled the ice to thaw. The inquiry says the control restriction “cleared with a thud” and the aircraft landed without further incident. Neither of the two crew members, the only occupants, was injured.
The aircraft (G-FRAK) had been parked outside overnight for 23 of the 41 days prior to the incident – and 11 of the 12 immediately before – during a period of exceptional storm rainfall.
Its operator has fitted a gutter rail above the APU exhaust to direct rainwater away, while Dassault has developed a service bulletin, due to be published this month, to increase drainage in the box beneath the pitch-control idler.
