Pilatus B4 disintegrates during inverted-flight attempt — VNE exceeded, no bail-out

On 16 October 1984 the pilot of a Pilatus B4-PC 11AF was fatally injured at 400-500 m AGL near Müllheim, Switzerland, when the aircraft disintegrated. After aerotow release the pilot half-rolled to inverted flight; the nose dropped 20° below horizon. Instead of rolling back upright, the pilot dived out without airbrakes. VNE was significantly exceeded; both wings failed. The pilot carried a rescue parachute but made no attempt to leave the cockpit; fatal. Cause: overspeed; contributing — insufficient preparation, recovery decision, no airbrakes deployed.

1. Aerotow to aerobatics: The pilot aerotowed from Amlikon to about 1100 m above ground for aerobatic training, intending to practise inverted flight north of the airfield.
2. Unstable inverted entry: After release, the pilot flew briefly straight ahead, then rolled the glider inverted via half-roll. The nose dropped 20° below horizon immediately and continued to steepen; the inverted attitude was not held.
3. Aerobatic figure beyond training: Per BAZL guidelines, higher aerobatic figures (incl. inverted flight) should be practised with an instructor on a powered aircraft for ~1 hour before transferring to a glider — this had not happened. Per Cumulus Amlikon's club rules, higher figures additionally require approval from the flight-instructor conference — this approval was not sought. Inverted flight was not part of the pilot's aerobatic-extension course (which covered loop, reversement, spin, steep turns).
4. Lower harness belt unbuckled: SUST possibly-contributing factor. The lower (groin) strap of the 5-point harness was not connected at the buckle; SUST suggests inattention before takeoff. This may have allowed the pilot's seating position to shift during inversion, and/or allowed the lead-shot ballast cushion (placed between seat pan and seat cushion) to slide and impair the controls. SUST does not draw a firm conclusion.
5. Recovery by dive instead of roll: Faced with an unstable inverted attitude with the nose already dropping, the pilot chose to recover by diving out (toward upright via a steep descent) rather than completing the half-roll back to upright. SUST lists this as a contributing factor: "Fehlentscheid beim Übergang in die normale Fluglage".
6. Airbrakes not deployed: During the resulting steep descent toward the ground, the pilot did not extend the airbrakes (which on the B4 are cleared up to VNE). SUST contributing factor.
7. VNE exceeded — wing breakup: VNE (240 km/h) was significantly exceeded. Torsional overload at high speed tore the leading-edge skin between ribs 3-6 and sheared the rivets. The left wing was torn upward and broke off; the right wing was torn off, taking the main spar fitting with it; the fuselage broke at the main bulkhead. The aircraft disintegrated into four main parts at about 400-500 m AGL.
8. No bail-out attempt — fatal: The cockpit forward section, with the pilot still strapped in, fell to the ground. The pilot carried a rescue parachute but made no attempt to leave the cockpit. The pilot was fatally injured; the glider was destroyed. SUST does not speculate on why the bail-out was not attempted.