History of the flight
The aircraft was en route from Gamston Airport to Leeds East Airport on 12 May 2018 when the pilot noticed a vibration through the rudder pedals. This was shortly followed by an uncommanded yaw and a violent jolt through the pedals. Several seconds later, a loud bang was heard and the aircraft pitched nose down. The pilot regained control and observed that the rudder appeared displaced to the right and detached from its upper hinge. He informed Leeds East ATC and landed without further incident on Runway 24, accepting a slight crosswind.
Engineering investigation
Inspection revealed that the rudder upper hinge pin and bearings were missing. The lower hinge lug had detached from the leading edge of the rudder, leaving the rudder held only by its control cables. Distortion was noted on the inboard edge frames of both elevators, but the elevators functioned with some restriction and increased stick loading.
Discussion
The damage to the elevators indicated that the rudder had oscillated in the slipstream, requiring the pilot to physically restrain the rudder pedals. Without the upper hinge support, the oscillation overloaded the lower hinge lug, causing it to part from the rudder. The rudder then remained attached only by its control cables, providing ineffective yaw control. There was a risk that the elevator could have been jammed by the loose rudder, leading to loss of pitch control.
According to the illustrated parts catalogue, a washer should be fitted directly under the head of the pivot pin and a second washer between the split pin and hinge. During the investigation, it was found that no upper washer was present on the upper hinge assembly. The pin head diameter is slightly less than the outer diameter of the bushes; without the upper washer, the pin and bushes could work loose. It is not known why the bushes worked loose, but their loss would have been prevented by the specified upper washer.
Safety action
The Civil Aviation Authority was informed and is considering appropriate safety action. The Light Aircraft Association published an article in its magazine advising Cub owners on the assembly and integrity of rudder hinge pins.