Power Loss During Touch-and-Go Leads to Forced Landing at Oxford Airport (G-PFTE)

History of the Flight

The aircraft, a Tecnam P2008-JC (registration G-PFTE), was being flown by a student with an instructor on a training flight practicing circuits and touch-and-go techniques at Oxford Airport. The activity had been in progress for approximately 1 hour without incident. As the student was climbing from a touch-and-go, the instructor became aware of a reduction in power. The aircraft was at about 300 ft above ground level and started to sink. The instructor took control and verified full power was set and the choke was off. He noted the carburettor heat was also off. At this point the engine was at 2,000 rpm and was audibly at a lower pitch than normal but appeared to be running smoothly. The instructor took an immediate decision to carry out a forced landing and configured the aircraft accordingly. He landed in a field about 10° left of the runway centre line. There were no injuries and the aircraft was undamaged.

Cause

Examination of the aircraft found the left carburettor throttle butterfly valve spring was missing. This led to an imbalance between the two carburettors and resulted in the power loss. The cable operated throttle system relies on the butterfly valve spring tension to open the throttle as power demands are made when the hand control is pushed forwards in the cockpit. To close the throttle, the hand control is pulled rearwards against spring pressure. Without the spring there can be a tendency for the throttle cable to ‘buckle’ rather than move the butterfly valve lever, which is what happened in this case. An engine ground run was carried out with the spring missing, which replicated the power loss symptoms experienced during the incident. The pilot suspects the spring broke and detached during the flight.