What happened
On 11 July 1999, a PIK 20E motor glider, registration G-SOAR, was conducting a private cross-country soaring flight. After a period of flight via Stokenchurch, the pilot decided to head toward Chalgrove Airfield to start the engine. While established on a long baseleg for Runway 36 at approximately 1,000 feet, the pilot successfully engaged the engine. Shortly after changing course toward Bideford, a loud bang occurred, followed by an immediate engine overspeed.
The pilot quickly shut down the engine using the ignition master switch and discovered that the propeller was missing. Despite the loss of the propeller, the pilot successfully retracted the engine pylon and performed a forced landing approximately one mile west of Chalgrove Airfield. There were no injuries to the pilot during the incident.
The investigation
Investigators examined the aircraft and found that the propeller, hub, and drive belt had separated entirely from the engine. The loss of these components caused the propeller disc to pitch downward, leading to the rotating blades striking the fiberglass fuselage behind the cockpit. These strikes caused longitudinal splitting and partial penetration of the fuselage structure. The investigation noted that the blades passed very close to the cockpit canopy, presenting a significant risk of serious injury to the pilot.
Technical examination of the propeller stub shaft revealed that the retention nut had loosened and separated from the shaft. This allowed the entire hub assembly, including the bearings and the propeller, to slide forward. While the shaft showed some old bruising, the more recent damage was consistent with the assembly migrating forward during the separation sequence. The investigation could not definitively determine why the nut had loosened, though it was noted that the nut's design (a left-hand thread) should have caused it to tighten under normal propeller rotation.
Findings
- The complete separation of the propeller hub assembly from the stub shaft was the primary cause of the incident.
- The loss of the assembly caused the propeller disc to tilt, resulting in structural damage to the fuselage via propeller strikes.
- Although the nut was designed to tighten during rotation, it had backed off the thread, allowing the assembly to migrate forward.
- Previous history of similar issues on PIK motor gliders had been addressed by airworthiness directives involving the inspection of the retention nut and lock plate.
- The log book for G-SOAR showed that the lock plate had been replaced during a recent inspection only 15 flying hours prior to the accident.