What happened
On 16 July 2006, a Europa aircraft, registration G-BWCV, was cruising at 3,500 ft near the Severn Estuary when smoke and a burning smell entered the cockpit. The engine stopped abruptly, though the propeller continued to rotate due to a free-wheel mechanism. The pilot, drawing on gliding experience, attempted a forced landing in a small field near Portbury, North Somerset. During the approach, the pilot had to navigate around trees and power lines while managing a high workload, including the loss of electric trim due to the master switch being turned off.
In an attempt to decelerate more quickly, the pilot retracted the single main landing gear. However, this action caused the propeller to break and the flaps to retract, which actually reduced the aircraft's drag and increased the rate of descent. The aircraft subsequently struck a dense hedge, causing extensive damage to the composite fuselage structure fore and aft of the cockpit. Both the pilot and the passenger sustained minor injuries.
The investigation
An investigation by the AAIB examined the engine installation and the wreckage. Evidence of black rubber deposits, slippage, and scorching was found on the drive belts near the alternator. While the belts themselves did not break, they had slipped significantly.
Upon inspection, the alternator's main bearing was found to have seized. A metallurgical analysis determined that the failure was caused by a lack of grease within the sealed unit, which led to the failure of the bearing cage under overload. The engine had been refurbished approximately ten years prior, and the history of the specific alternator component could not be verified.
Flight testing was also conducted to quantify the descent characteristics of a similar aircraft. The tests confirmed that a free-wheeling propeller can result in a significantly higher rate of descent compared to a propeller at idle.
Findings
- The engine stopped because the alternator main bearing seized.
- The seizure was caused by a lack of lubrication (grease) within the bearing.
- The seizure caused the drive belts to slip, producing smoke that entered the cockpit.
- The free-wheeling propeller mechanism significantly increased the aircraft's rate of descent during the engine failure.
- The pilot's decision to retract the landing gear inadvertently reduced drag, further increasing the descent rate.
Safety action
- The Popular Flying Association (PFA) was recommended to review the use of dual belts on NSI EA-81 engines to mitigate the consequences of an alternator seizure.
- A recommendation was made to advise owners of aircraft with free-wheeling propellers about the risk of a high rate of descent following engine failure.