What happened
On 6 December 1998, a private flight involving two microlight aircraft departed from East Fortune, Scotland. During the flight, the pilot of G-BUVX, a Streak Shadow, lost visual contact with his companion. After climbing to 2,500 feet to search for the other aircraft, the pilot descended toward a farm strip at Sheriffhall.
As the pilot approached the airfield, he reduced the throttle to idle for a standard circuit approach. Upon attempting to increase power to fly parallel to the runway, the engine began running roughly before failing completely. Because the aircraft utilized an electrical pitch trim system powered by the alternator, the loss of engine power also resulted in a loss of electrical power, leaving the pilot unable to re-trim the aircraft.
While attempting to glide through the base and final legs, the pilot realized the aircraft lacked the glide performance to reach the runway threshold. After maneuvering to avoid a line of trees and clearing a stone wall by approximately 15 feet, the aircraft touched down in a recently ploughed and sown field roughly 200 metres short of the landing strip. The impact was severe, resulting in no injuries to the pilot, but causing the nose and left main landing gears to break, the nosecone to shatter, and the tail boom to buckle.
The investigation
The investigation examined the engine's behavior during the descent and the pilot's decision-making process. The investigation noted that the weather conditions—characterized by freezing temperatures and high humidity—were highly conducive to icing. The pilot's engine had been running at idle for a prolonged period during the descent, which is a known risk factor for engine issues in two-stroke powerplants.
Findings
- The primary cause of the engine failure was likely carburettor icing, a phenomenon the pilot acknowledged despite having a manufacturer-fitted warming kit.
- The pilot's decision to descend to 500 feet significantly reduced the available altitude for an emergency glide.
- The pilot attempted to execute a standard glide approach as if the engine were still operational, which underestimated the actual glide performance of the aircraft.
- Running the engine at idle for an extended period likely contributed to the failure and increased the risk of spark-plug fouling.