What happened
On September 11, 1989, an SA 330J Puma helicopter, registration HB-XUB, was performing external load operations near Zermatt, Switzerland. The mission involved transporting mast components for a chairlift to the Blauherd area. After several successful rotations, the pilot attempted to lift a heavy component weighing approximately 2,400 kg using a 14-meter sling.
As the aircraft attempted to climb, the pilot was unable to lift the load significantly above the ground, reaching only about 6 meters. While hovering, the pilot heard a distinct squealing sound from the engines, followed by a flame erupting from the exhaust of the left engine. The pilot immediately reduced pitch and released the external load to prevent ground personnel from being struck.
In an attempt to reach a nearby landing site while avoiding the mast structures, the pilot maneuvered to the left. However, approximately nine seconds after the initial fire, the right engine also caught fire. Lacking sufficient power, the helicopter could not reach the intended landing area and instead descended into a rocky terrain. The aircraft struck the boulders, causing the tail boom to buckle and the helicopter to tip onto its side. The pilot escaped the heavy damage to the aircraft without injury.
The investigation
The investigation focused on the rapid succession of engine fires and the mechanical state of the fuel control units (FCU). Investigators examined the maintenance history, noting that the aircraft had recently undergone inspections in the United States. The inquiry also reviewed the pilot's training records and the operational procedures for external load transport used by the operator.
Findings
- The primary cause of the accident was the simultaneous burning of both turbines at a low altitude.
- Both engines had suffered from repeated thermal overstressing, likely due to previous operations exceeding permitted power limits.
- Significant misadjustments in the fuel control units (FCU) allowed for excessive fuel flow, and the seals/security wires for these units were missing.
- A defect in the right engine's FCU prevented proper fuel regulation during single-engine operations.
- The aircraft was operated with a single pilot during an external load mission that required a two-person crew.
- There were deficiencies in pilot transition training, specifically regarding simulated emergency power limit testing.
- The cockpit ergonomics made it difficult for the pilot to monitor critical performance instruments while looking downward to monitor the external load.