What happened
On July 30, 2004, a Cessna 185 amphiibious aircraft, registration PH-DUK, was conducting a series of instructional flights at Lelystad Airport. The flight involved an instructor and a student pilot. Following two completed 40-minute training sessions, a third flight was initiated. During the return leg of this third flight, while flying at 1,000 feet near the Knardijk area, the engine RPM began to fluctuate severely. Despite the crew attempting to troubleshoot the issue and activating the emergency fuel pump, the engine failed. The instructor took control of the aircraft and performed an emergency landing in a freshly plowed field. There were no injuries and the aircraft sustained no damage.
The investigation
The investigation focused on the fuel state and the recent modifications to the aircraft. The Cessna 185 had recently undergone an engine upgrade, replacing a 260 hp engine with a more powerful 300 hp unit. Investigators examined the fuel levels recorded during the day's operations. Prior to the first flight, the fuel level was measured at approximately 170 liters. After the second flight, the level was approximately 95 to 100 liters. The crew proceeded with the third flight without refueling.
Post-incident inspections revealed that only 8 liters of fuel remained in the tanks, which was considered unusable due to the location of the fuel pickups. The investigation scrutinized the instructor's fuel consumption calculations, which had estimated a consumption rate of 60 liters per hour, whereas subsequent findings indicated actual consumption rates were significantly higher, ranging between 70 and 80 liters per hour during various flight phases.
Findings
- The primary cause of the engine failure was fuel exhaustion.
- The recent installation of a more powerful 300 hp engine likely increased the aircraft's fuel consumption rate beyond the instructor's previous estimates.
- The instructor's reliance on an optimistic consumption calculation (60 liters/hour) failed to account for the higher actual burn rate (up to 80 liters/hour) during climb and instructional maneuvers.
- Inaccurate fuel gauge readings necessitated manual dipstick measurements, which were subject to estimation errors.
Safety action
- Pilots are reminded to avoid over-reliance on fuel gauges, which are often inaccurate, especially at low fuel levels.
- Fuel management calculations should always account for the possibility of higher consumption rates due to engine condition or richer mixture settings.
- Pilots must account for unusable fuel when calculating reserves and should always adopt the most pessimistic estimate when discrepancies in fuel measurements arise.