What happened
On August 12, 2009, an Air Tractor 802F, registration VH-LIH, was returning to Covilhã after participating in a wildfire suppression mission near Atalaia do Campo. During the return flight, approximately 18km from the aerodrome, the aircraft experienced a sudden and significant reduction in engine power. The pilot attempted to restore power by adjusting the throttle and activating the ignition system, but the engine remained stagnant at approximately 62% Ng.
Seeking an emergency landing site, the pilot identified an agricultural field free of vegetation. During the approach, the pilot had to maneuver to avoid medium-voltage power lines. To increase gliding capability, the pilot set the propeller to the feather position. Although the aircraft successfully touched down on the field, the pilot was unable to stop the aircraft before the end of the terrain. The aircraft crossed a small stream and collided violently with a grove of medium-sized pine trees.
Following the impact, a fire broke out in the engine area. The two occupants, the pilot and a communications operator, were able to evacuate the aircraft uninjured. A nearby helicopter was able to discharge water, quickly extinguishing the fire and preventing further damage to the aircraft or the surrounding forest.
The investigation
The GPIAAF examined the aircraft's engine, fuel system, and the wreckage. Investigators analyzed the Fuel Control Unit (FCU) and the compressor outlet pressure (P3) line. The investigation also reviewed the aircraft's maintenance history and the pilot's flight maneuvers during the emergency.
Findings
- The primary cause of the power loss was a fracture in the P3 pressure line connecting the compressor outlet to the FCU. This fracture prevented the FCU from receiving accurate compressor outlet pressure signals, causing the unit to deliver fuel amounts appropriate for a much lower engine regimen, regardless of throttle position.
- A microscopic analysis of the fracture surfaces revealed hammering marks, indicating a pre-existing fissure was present before the event.
- The investigation found that the left wing fuel tank appeared to be empty, which could have contributed to insufficient fuel pressure in the FCU feed line.
- The pilot's focus on attempting to recover engine power reduced the time available for precise track and speed calculations.
- The presence of power lines necessitated avoidance maneuvers that contributed to an unstabilized approach.
- The use of a small flap setting, while increasing gliding capability, required a higher approach speed, which contributed to the aircraft overshooting the intended landing area.
- The investigation noted non-compliance with a service bulletin (SB Nr 14269) which recommended replacing the specific pressure line part during maintenance when access was available.