What happened
On the evening of April 6, 2003, a Cessna 172-H, registration EC-HIC, departed from Logroño-Agoncillo for a planned 30-minute pleasure flight. The aircraft was carrying a pilot and two passengers. Approximately 15 minutes into the flight, while cruising at 5,000 feet near Nalda, the engine experienced a momentary surge. The pilot, attributing the event to a wind gust, descended to 3,000 feet.
At 3,000 feet, the engine began to sputter on two separate occasions. The pilot initially applied carburetor heat, which temporarily resolved the issue, but the engine failed to respond to the same measure during the second occurrence. Suspecting fuel starvation, the pilot switched the fuel selector from 'BOTH' to 'LEFT', which provided a brief period of two minutes of engine operation before the engine stopped completely.
Facing an imminent engine failure, the pilot attempted to reach an ultralight airfield at Prado Salobre but determined it was unreachable. An alternative cereal field was selected for an emergency landing. During the approach, the aircraft passed beneath electrical cables and brushed the tops of trees. The aircraft touched down on its main gear, but a maneuver to avoid a fence caused the nose gear and part of the right wheel to enter a ditch, resulting in light contact with a fence post.
All three occupants (the pilot and two passengers) exited the aircraft safely without injury. The aircraft sustained minor damage, including damage to two metal fence posts.
The investigation
The investigation focused on the engine's intermittent performance and the fuel system's integrity. Post-accident inspections of the Cessna 172-H included checks of the control linkages, cylinder compression, and spark plugs, all of which were found to be within normal parameters. The fuel filter was drained and showed no signs of water or debris.
Investigators examined the fuel delivery system, specifically the line between the fuel selector valve and the fuel strainer. While the line initially appeared to restrict flow during testing, subsequent inspection revealed no obstructions or dirt. A significant discrepancy was noted regarding fuel levels: while calculations suggested the aircraft should have had between 30 and 40 liters of fuel remaining, only 18 liters were recovered from the tanks during disassembly.
Findings
- The engine's initial sputtering was likely caused by carburetor icing during the descent, as the application of carburetor heat initially restored power.
- The subsequent engine failure was most likely caused by insufficient fuel levels, as the remaining fuel was very close to the aircraft's unconsumable volume (11.5 liters).
- Inaccurate fuel quantity estimation was a contributing factor, as the pilot relied on visual checks and manual refueling via canisters, which are prone to error.
- The fuel gauges may have provided imprecise readings, leading the pilot to believe more fuel was available than was actually present.