What happened
On November 27, 2011, a Cessna 172S, registration HB-CQW, conducted a private VFR training flight originating from Hausen am Albis. The flight plan involved maneuvers at 11,000 ft QNH. During the descent phase of the return leg, the pilot noticed a discrepancy between the expected and displayed fuel levels. Despite this observation, the flight continued toward the destination.
As the aircraft approached the arrival sector of Hausen am Albis at approximately 2,900 ft, the engine began to run unevenly. The pilot attempted various troubleshooting steps, including adjusting the mixture and checking fuel selectors, but the engine eventually lost all power. The pilot performed an emergency landing in a field at the eastern edge of Kappel am AlSB at 14:43. The aircraft sustained no damage, and the pilot was uninjured.
The investigation
SUST investigators examined the aircraft's fuel system and the pilot's pre-flight procedures. The investigation revealed that the actual fuel quantity at takeoff was significantly lower than the pilot's manual measurement suggested. Technical inspections of the Cessna 172S found that the fuel level sensors had been installed in a reversed orientation during a previous maintenance event in 2008. This error caused the sensors to provide inaccurate readings when fuel levels were low. Furthermore, the investigation found that the pilot had performed a flight over the incident site a few hours after the landing, after refuelling the aircraft and without authorization from the authorities, which hindered the initial technical assessment.
Findings
- The primary cause of the incident was an engine failure resulting from fuel exhaustion.
- The fuel level sensors were installed incorrectly (mirrored), which prevented the cockpit gauges from accurately reflecting low fuel levels.
- The pilot failed to correctly determine the actual fuel quantity during the pre-flight inspection.
- The pilot continued the flight despite recognizing a discrepancy between the expected and displayed fuel amounts.
- A "LOW FUEL" warning on the aircraft's warning display went unnoticed, likely because the pilot's seating position obscured the display and the pilot was experiencing high stress levels.
- The pilot's situational awareness was impaired, leading to a failure to identify nearby alternative landing fields.