History of Flight
On 11 December 2002 at 1525 UTC, a Grob G115E Tutor, registration G-BYUG, operated by the Royal Air Force University of Glasgow Air Squadron, was conducting a training flight. Both occupants were wearing parachutes and helmets as standard practice. The flight intended to train on the effect of flying controls. The aircraft departed Glasgow and climbed to 2,000 feet without issues. A request for further climb was granted. As the throttle was opened fully, the instructor noticed a slight vibration and engine noise, but engine instruments showed no abnormalities. At approximately 2,800 feet, the vibration intensified, making instruments difficult to read. Seconds later, a loud bang emanated from the engine. The oil filler access panel opened, emitting smoke, and a burning smell was present. The propeller stopped turning.
The instructor transmitted a mayday call to Glasgow approach, which initially caused confusion as to whether it was a practice emergency. After clarification, the instructor chose a forced landing rather than abandoning the aircraft, as the aircraft was near a built-up area and Loch Lomond. During the glide descent, a field was selected at 1,500 feet, but the rate of descent was higher than expected. The instructor then selected a smaller field oriented into wind. The wind was estimated at 090° at 20 knots. After turning final, flaps were deployed. The aircraft touched down about one-third into the field, braked, skidded, struck a ridge, passed through a hedge, hit fence posts, crossed a farm track, and stopped in a barbed wire fence.
Both pilots exited the aircraft and were met by rescue vehicles about 15 minutes later. They suffered minor back injuries from the landing.
Engine Description
The Lycoming AEIO-360-B1F is a four-cylinder, horizontally opposed, air-cooled engine. On the Grob 115E, it drives a wooden variable-pitch three-bladed Hoffman propeller. The engine includes an oil system for inverted flight lubrication.
Engine Examination
Initial inspection showed the No 4 connecting rod and piston pin had broken free and punched a hole through the crankcase. The engine was removed and strip-examined at a specialist overhaul organisation under AAIB supervision. The No 4 connecting rod had detached at its big end; the piston pin remained attached to the little end but was pulled from the piston. The connecting rod was wrapped around the crankshaft cheek between the No 3 and 4 crankpins, jammed between camshaft and crankshaft. The piston pin had rotated about 90° from its normal position and punched a large hole in the crankcase. The No 4 piston showed cracking and fractures near the piston pin location holes.
Metallurgical examination of fragments revealed the prime failure was at the big end of the No 4 connecting rod. Either loss of a retaining nut from one of the two big-end bolts, or failure of the bolt itself, initiated the failure. The nut was not recovered; bolt fragments did not permit determination of their pre-failure condition. The other big-end bolt had failed by high-stress bending fatigue, and its retaining nut was engaged by only one thread. After the first bolt failure, the big-end cap detached, trapping the connecting rod against the rotating crankshaft. The piston pin was pulled upward, breaking away from the piston. All piston fractures were due to overload, not pre-existing.
The rest of the engine showed no pre-existing defects. No evidence of oil loss was found; heat damage was limited to the No 4 crankpin area. The inverted oil system was satisfactory. The engine had 892.4 hours since new, with no work on the No 4 connecting rod. Time between overhaul is 1,600 hours.
This failure is similar to one reported in AAIB Bulletin 11/98 involving a Lycoming O-540-E45C engine on a Trislander, where a No 6 connecting rod big-end retaining nut had unscrewed, though the reason for detachment was not determined.