What happened
On 26 May 2010, a Cessna 177A G-BTSZ was conducting a local area flight from Cranfield with one pilot and two passengers. Approximately 15 minutes after takeoff, the engine began to misfire. The pilot attempted to use carburettor heat, but the issue persisted. Upon checking the fuel gauges, the pilot discovered that both tanks were empty.
After establishing a glide, the pilot notified air traffic control of the intention to perform a forced landing at a nearby disused airfield in Bedfordshire. During the approach to the ploughed ground north of Tempsford Airfield, the nose landing gear collapsed. The three occupants sustained minor injuries and were able to exit the aircraft without assistance. The aircraft sustained damage to the propeller and the landing gear.
The investigation
The investigation focused on the cause of the engine power loss and the subsequent fuel depletion. While the pilot had visually estimated sufficient fuel was on board, the investigation confirmed the tanks were empty due to an in-flight leak originating from the engine fuel strainer drain.
Testing of the fuel filter drain mechanism revealed that the valve could stick in a partially open position, creating a leak rate of between 18 and 24 gallons per hour. The mechanism was found to be contaminated with a large deposit of waxy material, a common byproduct of water-contaminated fuel.
Findings
- The engine power loss was caused by the accumulation of water-contaminated fuel residue within the engine fuel strainer drain mechanism.
- This residue prevented the drain valve from closing fully after pre-flight checks, leading to an unobserved fuel leak during flight.
- The pilot's fuel estimation was based on a visual assessment of fuel depth through the filler necks, which lacked tabs to assist in accurate measurement.
- The aircraft was equipped with older-style fuel filler caps that are more susceptible to water ingress than newer designs.