What happened
On June 12, 2005, a Pilatus PC 6 B2-H2 Turbo-Porter, registered F-BKQU, was taxiing toward the runway at Royan Aerodrome for a skydiving mission. The aircraft was carrying a pilot and nine skydivers. During the taxi, a skydiving instructor seated near the door noticed abnormal vibrations coming from the right-hand trailing edge flap and alerted the pilot.
In response to this observation, the pilot decided to cancel the flight to inspect the flight controls. While taxiing toward the parking area, the control linkage for the right flap became disconnected from the flap itself. The flap, which remained attached only by its rotation axis, moved into a vertical position. There were no injuries or fatalities during the incident.
The investigation
The investigation focused on the mechanical failure of the flap assembly. Investigators recovered the assembly screw and spacers from the taxiway, but the assembly nut and the securing cotter pin could not be located. The recovered screw featured a hole designed for a cotter pin, yet no evidence of previous cotter pin usage was found on the screw's bore.
Technicians noted that the aircraft had been returned to service the same day after replacing the missing nut with a Nylstop nut. The investigation examined the maintenance history and the manufacturer's requirements, noting that two different types of nuts—split nuts and Nylstop nuts—had been used interchangeably for installing flaps and linkages in the past.
Findings
- The primary cause of the separation was a non-compliant maintenance operation.
- The manufacturer's maintenance manual and illustrated parts catalog specifically require the use of split nuts secured with a cotter pin for the installation of flaps and control linkages.
- The investigation established that the assembly hardware was not installed according to the required manufacturer specifications.