What happened
On 8 May 2005, a modified Rans S6-ESD XL, registration G-MZNV, was conducting a private flight departing from Brimpton Airfield. Approximately ten minutes into the flight, the pilot heard a loud bang, after which the right cockpit door detached from the airframe. Following the loss of the door, the pilot reduced the airspeed to 60 mph. As the aircraft's handling characteristics remained normal, the pilot notified Brimpton Airfield and performed a successful return to the airfield. There were no injuries to the pilot or the passenger, though the aircraft sustained minor damage to the underside of the right wing and the leading edge of the tailplane due to the door striking the airframe.
The investigation
Investigators examined the door attachment mechanism and found that while the forward hinge remained attached to the tubular airframe, the rear hinge had broken off. A lock nut from the forward attachment bolt was discovered on the cockpit floor, though the bolt itself, the door, and the rear hinge were not recovered. The investigation established that the door likely moved upwards and outwards due to air pressure once the forward bolt became unsecured, eventually pivoting on the rear hinge until the hinge failed.
Further inspection of the aircraft revealed that the owner had previously installed additional washers between the hinge and the door assembly to address excessive sideways movement. Additionally, a check of the left door's attachment bolts revealed that the bolt ends were nearly flush with the lock nuts. A subsequent random inspection of ten similar aircraft by the Popular Flying Association (PFA) found that on nine of them, the bolts protruded through the lock nuts by only about 6 mm, which was significantly less than the acceptable standard of 1.5 threads.
Findings
- The primary cause of the incident was the failure of the lock nut on the forward door attachment bolt to remain secure.
- The loss of the nut allowed the attachment bolt to migrate out of the assembly, leading to the door unlatching and eventually detaching from the rear hinge.
- The use of additional washers to compensate for manufacturing tolerances likely reduced the amount of thread protruding through the lock nuts.
- The condition of the bolts on the left door suggested that the right door's forward bolt may not have had sufficient thread protrusion to ensure positive locking.