What happened
On July 23, 2011, a Pilatus Porter PC6-B2H2, registration F-GOAG, was performing a skydiving operation at the Saumur Saint Florent airfield. The aircraft, operated by a regional skydiving center, was conducting its 13th jump of the day. After dropping six skydivers at an altitude of 1,200 meters, the pilot initiated a descent with reduced power and the elevator trim in a normal nose-up position.
During this descent, the pilot noticed abnormal elevator behavior, noting that the control column required significant physical effort to move. The pilot reported that heavy manual force was necessary to flare the aircraft for landing. Upon returning to the parking area, an inspection revealed that the right elevator had partially detached from the horizontal stabilizer.
The investigation
Investigators examined the components of the elevator assembly, which is connected to the horizontal stabilizer at two points: an inner end via a plate and two screws, and a point two-thirds along its length via a spherical bearing and a single screw. The investigation established that the right elevator had separated from the stabilizer at the outer connection point. This separation subsequently caused a secondary failure at the inner connection, resulting in a broken plate and sudden deformation.
Laboratory analysis of the outer connection revealed that the securing screw had been lost due to progressive loosening. The investigation found that the nylon insert of the "Nylstop" nut was worn, failing to provide the necessary self-locking function. Furthermore, the anchor lug for the safety wire was found to be free to rotate on the structure. Vibrations caused the screw to rotate and loosen, which placed the safety wire under bending stress, eventually leading to its fatigue failure and subsequent expulsion of the screw.
Findings
- The primary cause of the incident was the loss of the securing screw connecting the elevator to the horizontal stabilizer.
- The loss of the screw resulted from its progressive loosening and the subsequent failure of the safety wire.
- Contributing factors included the failure of the Nylstop nut to maintain its locking capability due to wear, and the ability of the safety wire anchor lug to rotate around the rivet axis.