What happened
On August 1, 1997, a Robinson R2ly B helicopter, registration EI-TKI, was conducting a private flight from Dublin Airport to a home base in Hazelwood, Co. Sligo. The flight, which included the pilot and his young daughter, involved several route changes and diversions through Athlone and Longford.
As the aircraft approached the landing site near Lough Gill, the pilot transitioned into a hover over a field to perform final checks. During this hover, the engine lost power and the low rotor speed alarm activated. The pilot managed to maneuver the aircraft sideways to clear a fence, but the aircraft struck the ground heavily. The engine had already stopped upon impact. While the two persons on board escaped without injury, the impact caused significant distortion to the aircraft's undercarriage cross tubes.
The investigation
The AAIU investigation established that both fuel tanks were empty at the time of the incident, with the carburetor bowl nearly dry. While the low fuel warning light was found to function correctly during post-accident testing, the main fuel gauge had been providing inaccurate readings. Specifically, the gauge was over-reading by approximately 1/8 of a tank.
Technical examination revealed that a slightly loose nut on a connector at the rear of the fuel gauge cluster caused the discrepancy. When this connection was disconnected, the gauge indicated a full tank even when no fuel was present. Furthermore, the investigation found that the aircraft's fuel logs did not account for fuel used during recent ground runs following an annual inspection. The investigation also noted that the R22 fuel tanks are deep and dark, making visual verification of fuel levels through the filler opening extremely difficult, especially in bright sunlight.
Findings
- The primary cause of the engine stoppage was in-flight fuel exhaustion.
- The pilot was misled by a fuel gauge that over-read due to a loose electrical connection.
- The pilot did not follow the flight manual's safety instruction to land for refueling before the main tank gauge reached 1/4 full.
- Pre-flight visual inspections were insufficient to detect the discrepancy between the gauge and the actual fuel levels due to the difficulty of seeing into the dark tanks.
- The fuel indicating system on this aircraft type is not fail-safe, as increased resistance in the circuit causes the gauge to show a higher fuel quantity than actually exists.
Safety action
- The manufacturer should review the fuel indicating system to ensure that increased circuit resistance or an open circuit results in a decrease in the indicated fuel level.
- R22 operators and pilots should consider using a calibrated dipstick to verify fuel levels, as visual checks through the filler opening are unreliable.
- The manufacturer should consider providing a dedicated fuel contents dipstick for the R22 or adopting the method used by UK operators.