What happened
On August 26, 2018, a Cessna 172H, registration LN-RAL, was performing a private flight from Arendal Airport Gullknapp to Voss Bømoen. After an initial leg and a two-hour stop, the aircraft departed Bømoen for the return trip to Gullknapp. During the flight, the pilot noticed that the fuel levels were dropping toward empty much faster than anticipated.
As the aircraft approached Gullknapp at approximately 1,100 feet, the engine began to sputter. Despite attempts to restore power by adjusting the mixture and checking the primer, the engine failed. Realizing there was insufficient altitude to reach the airport, the pilot executed an emergency landing in a field roughly 1.5 NM northwest of the airfield. The landing was successful, and there were no injuries to the pilot or the passenger, and no damage was sustained by the aircraft.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the aircraft following the incident. Upon inspection after landing, the pilot discovered that the fuel cap on the left wing was loose and resting on top of the wing. Further examination of the fuel filler flange revealed that a metal tab had become bent outward. This deformation allowed the notches in the fuel cap to slip past the securing tab, effectively unsealing the tank.
Findings
- The investigation determined that the fuel cap became unseated during the flight, leading to a continuous leak.
- The primary cause was a mechanical degradation of the filler flange; over years of use, the metal tab had been pushed out of position.
- It is believed the cap may have been inadvertently loosened during a fuel check at the previous airport.
- The pilot continued the descent toward the destination despite noticing the abnormal fuel consumption, rather than maintaining altitude to preserve glide distance.
Safety action
- The investigation highlighted the importance of monitoring wear and tear on aging components, as the aircraft was nearly 50 years old.
- Maintenance providers and pilots must ensure that structural fatigue or deformation in fuel systems is identified and corrected to maintain airworthiness.