What happened
On July 7, 2023, a Piper PA-25-235, registration OY-CYY, was conducting a series of glider towing operations near Arnborg (EKAB) under visual flight rules. During the third series of flights for the day, the aircraft was climbing to approximately 1,500 feet when the engine stopped without warning.
Following the engine failure, the pilot maintained a glide speed of roughly 80-85 mph. After the glider pilot disconnected from the towline, the pilot of OY-C11 attempted to return to the airfield. While initially attempting to reach EKAB, the pilot observed that the aircraft's sink rate was increasing, making the airfield unreachable. The pilot then identified a nearby field for an emergency landing. However, the aircraft could not clear a tall hedgerow separating the field from a nearby motorway. The aircraft struck the wire and tree hedge at an altitude of approximately 1-2 meters, resulting in the total destruction of the aircraft. The pilot sustained minor injuries.
The investigation
The investigation by AIB Denmark focused on the aircraft's technical state, the pilot's decision-making, and the fuel management practices used during the towing operations. Investigators examined the aircraft's fuel system and found that the fuel tank was empty at the time of the crash. The investigation also reviewed the pilot's methods for estimating remaining fuel, noting that the aircraft's fuel gauge was unreliable and often indicated a higher level than what was actually present.
Furthermore, the investigation looked into the operational practices of the local glider towing group. It was noted that there were no formalized procedures for calculating fuel consumption, and the pilots relied on various unofficial indicators, such as the number of completed tow cycles or tachometer time, to decide when to refuel.
Findings
- The primary cause of the accident was that the method used to calculate remaining fuel led to the aircraft running out of fuel.
- The pilot relied on a "rule of thumb" estimating 6 liters of fuel per tow, but the actual average consumption was approximately 6.68 liters per tow, an 11% increase.
- The aircraft's fuel gauge was unreliable, providing inaccurate readings of the remaining fuel volume.
- There was a lack of consensus and formalized procedures within the towing group regarding fuel monitoring, with pilots using inconsistent metrics like number of tows or engine time.
- The pilot's decision to attempt a return to the airfield rather than landing in a nearby field may have been influenced by the risk of the aircraft flipping in a grain field, though the increasing sink rate eventually necessitated the emergency landing in the hedgerow.