What happened
On July 16, 2012, an Air Tractor 802, registration EC-HMD, was performing aerial work near Ibiza, Spain. The aircraft had departed from Ibiza Airport to assist with firefighting efforts on the island of Mallorca. While climbing at an altitude between 800 and 90 and 900 feet, the pilot noticed a significant loss of engine power. Although the engine did not stop completely, the power reduction was severe enough to prevent continued flight.
The pilot immediately jettisoned the aircraft's water payload and attempted the in-flight engine restart procedure. Despite activating the fuel boost pumps and verifying sufficient compressor RPM, the pilot could not regain necessary power. The pilot subsequently executed an emergency landing in a clearing within a forested area. During the landing, the aircraft struck several trees, causing significant damage to the wings, propeller, and the front of the fuselage. The pilot escaped the wreckage without injury.
The investigation
The CIAIAC investigation focused on the engine's Fuel Control Unit (FCU) and the maintenance history of the aircraft. Investigators examined the engine's maintenance records, which showed that the airframe and engine had undergone a 100-hour inspection recently. However, the investigation revealed that the implementation of a manufacturer's Service Bulletin (SB 14389) to replace the FCU bellows had been delayed. The operator had opted to implement the modification gradually, and the specific engine on EC-HMD was scheduled for the upgrade only after the firefighting season ended.
Laboratory analysis of the removed FCU bellows by the manufacturer confirmed that corrosion on the outer surface of the bellows had progressed to a pre-existing internal defect, causing a leak. This leak resulted in a loss of pressure signal, leading the engine to revert to minimum fuel flow power. Furthermore, the investigation noted that the aircraft was not equipped with a Manual Override (MOR) device, an optional system that allows the pilot to manually control fuel flow during an FCU failure.
Findings
- The primary cause of the power loss was the corrupting of the P3 pressure signal due to a leak in the FCU bellows caused by corrosion.
- The corrosive environment of the Ibiza/Ses Salines area, characterized by high humidity and salt content, likely accelerated the degradation of the bellows.
- The operator's decision to delay the Service Bulletin implementation beyond the manufacturer's recommended timeline contributed to the occurrence.
- The lack of a Manual Override (MOR) system prevented the pilot from manually regulating fuel flow to maintain sufficient power for continued flight.
- The pilot's emergency procedures, including feathering the propeller and extending flaps, were consistent with the aircraft's flight manual for power-off landings.