What happened
On 27 March 2012, a BN2A MK.III-2 Trislander, registration G-BDTO, was performing a scheduled passenger flight from Alderney to Southampton. While cruising at 5,000 feet, the pilot experienced a loud bang and intense vibrations, which were identified as a failure of the number 2 tail-mounted engine. Following the event, the pilot noticed decreasing oil pressure and observed that the engine cowling had opened on the left side.
During the subsequent emergency procedures, the pilot attempted to feather the propeller of the inoperative engine but was initially unable to move the control lever through the feather gate. This left the propeller in a windmilling state, causing the aircraft to descend at approximately 200 feet per minute. Eventually, the propeller blades moved to the feather position, allowing the pilot to control the descent and perform a successful visual landing back at Alderney Airport. There were no fatalities and no injuries among the 7 passengers and 1 crew member on board.
The investigation
An inspection of the aircraft revealed significant damage, including a missing section of the engine cowling and heavy oil staining. Upon removing the engine, investigators found that the number 2 cylinder had detached from the crankcase because the mounting studs had failed. The failure caused the connecting rod to flail, damaging the surrounding crankcase and dislodging the starter motor.
Metallurgical analysis of the failed studs showed that the failure was initiated by corrosion pitting in the thread root of a cylinder mounting stud. This pitting led to the development of fatigue cracks under the cyclic loads of engine operation. The investigation also noted that the propeller failed to feather initially because the guide tube for the Teleflex control cable had been distorted by the released starter motor.
Findings
- The primary cause of the engine failure was the failure of a cylinder mounting stud due to fatigue cracking initiated by corrosion pitting.
- The loss of the cylinder mounting studs was exacerbated by the absence of a protective cadmium coating on the fine threads of the stud.
- The aircraft's inability to maintain altitude was due to the propeller remaining in a windmilling state, which increased drag.
- The initial failure of the propeller to feather was caused by physical damage to the control cable guide tube caused by the engine component failure.