The pilot of the Beechcraft King Air reported that just prior to
touchdown, the elevator control forces changed abruptly, causing an
uncommanded pitch down. The nosewheel struck the surface of the
airstrip and the aircraft bounced twice. During the subsequent
landing roll, the nose landing gear collapsed and the propellers
struck the ground.
An investigation by the operator found that an elevator servo in
the autopilot system was malfunctioning. The unit would not
disengage when the autopilot was turned off. The mode in which the
servo operated on the test bench was consistent with the control
forces experienced by the pilot during the landing. Because of the
aircraft's proximity to the ground, the pilot had been unable to
counter the unexpected forces in time to prevent the nosewheel from
striking the ground prematurely.
A manufacturer's product improvement service bulletin, issued
soon after the aircraft was manufactured, had not been implemented
on this servo unit. The modification was to improve the mechanical
integrity of the engage clutch assembly and eliminate the
possibility of the clutch not disengaging due to mechanical
interference between various parts of the clutch assembly and the
servo casting.
The operator forwarded its investigation report to BASI and the
Civil Aviation Safety Authority and recommended that the Service
Bulletin be upgraded to ensure that all servos of the type still in
service were modified.
The Civil Aviation Safety Authority informed BASI that it
believes that the occurrence was an isolated incident involving
this model servo unit and accordingly, airworthiness directive
action is not considered to be warranted at this time.