What happened
On a recent evening in Manitoba, a pilot was conducting agricultural spraying operations in a Piper PA-25-150. After refueling and reloading with chemical at the Rivers airport, the pilot flew approximately 10 nautical miles southwest to a wheat field. While performing the second-to-last pass of the field, the engine suddenly lost all power.
At an altitude of roughly 300 feet and an airspeed of 90 knots, the pilot attempted to reach a nearby gravel roadway by turning the aircraft. During this maneuver, the pilot applied carburettor heat and attempted an engine restart, but the engine failed to respond. As the pilot continued the turn, the airspeed dropped to the stalling speed, causing the aircraft to descend uncontrollably into a farm field. The impact caused the aircraft to cartwheel and slide, resulting in a fuel-fed fire. The pilot sustained serious burns during the escape from the wreckage, and the aircraft was destroyed.
The investigation
Investigators examined the engine, the fuel supply, and the aircraft's control systems. The fuel used was confirmed to be the correct aviation-grade 100LL, and samples from the aircraft's gascolator showed no water contamination. A teardown of the engine revealed no mechanical failures, no issues with compression, and no problems with the magnetos or fuel delivery.
Analysis of the weather conditions indicated that the temperature and dewpoint at the time of the accident were highly conducive to carburettor icing at any power setting. Furthermore, investigators found that the carburettor heat control cable had failed. The cable had been weakened by fatigue cracking and repetitive wear caused by vibrational contact with the outer shielding. This wear likely went undetected during the aircraft's annual inspection, which had occurred 60 flight-hours prior.
Findings
- The engine power loss was likely caused by carburettor icing.
- The pilot's attempt to reach a roadway caused the airspeed to decay until an aerodynamic stall occurred.
- Due to the low altitude at the time of power loss, there was insufficient time for the application of carburettor heat to clear the ice.
- The carburettor heat control cable was compromised by fretting wear and fatigue.
- The aircraft was not equipped with a fuel tank bladder, a modification recommended by the manufacturer to mitigate impact damage.