What happened
On 16 October 2016, a pilot operating a Phoebus A1 glider, registration ZS-PYC, departed Worcester aerodrome for a cross-country flight. After an aero-tow launch to 3000 feet AMSL, the pilot released into strong thermals, climbing to 6000 feet AMSL while navigating toward Audenberg Mountain and eventually the N1 crossing.
While flying near Keeromberg Mountain under a cloud base, the aircraft encountered severe turbulence. The pilot reported intense pitch oscillations that made maintaining a stable attitude difficult. During a maneuver to turn south and exit the cloud cover, the aircraft experienced even more violent movements, with the nose pitching up and down by at least 45 degrees. Following a loud cracking sound, the right wing separated from the fuselage. The aircraft entered a vertical dive, prompting the pilot to jettison the canopy, release safety straps, and egress the aircraft. The pilot deployed a parachute and landed safely at the base of the mountain, sustaining minor injuries.
The investigation
An examination of the wreckage by a specialist glider repairer focused on the structural integrity of the glass fibre and balsa sandwich construction. Investigators found that the wing tips had sheared off at nearly identical points on both sides, though the port wing failed at the root. The investigation ruled out issues such as improper tail plane connection, control linkage failure, or incorrect weight and balance.
Physical evidence, including scrapes on the pilot's shins from hitting the canopy, indicated the aircraft was subjected to extreme G-forces. The investigation examined whether the pilot had exceeded the maximum permissible airspeed (VNE) or if the oscillations were a result of pilot-induced oscillation (PIO) during recovery efforts.
Findings
- The primary cause of the structural failure was excessive control inputs by the pilot during the attempt to recover from severe pitch oscillations.
- These rapid and large movements of the tail plane likely caused the aircraft to reach its structural limit loads, leading to wing flutter and subsequent failure.
- The aircraft was experiencing severe turbulence and intense vertical oscillations prior to the wing separation.