Stuck rudders on three different Cessna Citation 560XL business jets in December have prompted action by the US National Transportation Safety Board.
In a 15 March recommendation letter to the Federal Aviation Administration, the board says it has "serious concerns" over the problem based on its ongoing investigations of the three incidents, two of which involved aircraft belonging to fractional provider NetJets. No-one has been injured nor have any aircraft been damaged by the problem to date.
The 560XL line includes the Cessna Excel, XLS and XLS+ models.
Post-incident investigations have revealed that water entering the aircraft's tail cone "stinger" through torque tube openings may be accumulating, despite drain holes, and freezing around rudder cables, jamming the rudder control system.
In the first incident, the pilots of a NetJets 560XL on approach to Toledo Express airport in Ohio experienced jammed rudder pedals and were unable to align the aircraft's nose with the runway centreline after turning off the yaw damper at 600ft (180m) above the ground. The pilot landed, maintaining directional control on the runway with differential braking.
"The risk associated with the accumulation of ice can lead to an accident because the pilot may not be able to prevent the airplane from exiting the runway at high speed," says the NTSB. "Further, in the event of an engine failure, the use of the rudder to maintain directional control is crucial at any time."
Cessna in 2005 had issued a service letter recommending operators drill a 5mm (0.201in) hole in the bottom of the stinger to prevent water accumulation. Due to aerodynamic pressures however, the holes were actually drawing water into the cone rather than letting it drain, says the NTSB. All three of the incident aircraft had the drilled holes, although one was smaller than the recommended diameter.
A new service letter issued by Cessna in February calls for a 19mm hole to be drilled in the bulkhead of the aft fuselage frame instead of the smaller hole farther aft in the stinger, allowing water to drain away from the control cables. The NTSB is asking the FAA to make modification mandatory through an airworthiness bulletin, a move Cessna supports.