What happened
On 24 September 2011, an AirLony Skylane UL was conducting a training flight near Zottegem, Belgium. While cruising at 2000 feet, the engine began to run roughly. The crew, consisting of an instructor and a student, attempted to rectify the issue by switching from the left fuel tank to the right. This action caused the engine to stop completely.
Following the engine failure, the instructor took control of the aircraft and identified a suitable field for an emergency landing. The aircraft approached the field at 100 km/ h with flaps deployed. Upon touchdown, the nose wheel sank into the soft, recently ploughed soil, causing the aircraft to tip over. There were no injuries to the two occupants, but the aircraft sustained enough damage to be written off.
The investigation
The investigation focused on why the engine failed despite the crew believing they had sufficient fuel. Before departure, the pilots had checked the fuel levels via a translucent indicator, noting a fuel height of approximately "two fingers." Based on this visual, they estimated they had roughly 10 liters remaining in each tank, which they believed provided sufficient endurance for the flight.
Technical inspections of the aircraft revealed that the engine stoppage was the result of fuel starvation and a subsequent loss of fuel priming in the feed line. Investigators found that the visual fuel indicators were not graduated, leading the crew to rely on a proportional estimation that proved inaccurate.
Findings
The primary cause of the accident was fuel starvation resulting from the unreliability of the fuel quantity indicators. Because of the wing's dihedral, the specific wing profile, and the placement of the fuel line port, the fuel level visible through the indicator was not linearly proportional to the actual volume of fuel remaining. Consequently, the actual fuel quantity was insufficient for the intended flight duration.