Background
The aircraft involved was an MD 500N helicopter, registered as HB-ZBI. It was owned by Swissrotor S.à.r.l. and operated by Helit Hélicoptères. The helicopter was built in 1992 and powered by a single Allison 250-C20R/2 turbine engine.
Flight History
On the day of the accident, the helicopter departed La Chaux-de-Fonds for Sion with a student pilot, an instructor, and two passengers. During the flight near the Sanetsch Pass, the engine chip light illuminated. Upon arrival at Sion, the instructor inspected the magnetic plugs and found metal particles. After consulting with the maintenance company, the instructor removed the particles and decided to return to La Chaux-de-Fonds without conducting a ground test.
Accident Sequence
At 1500 local time, the helicopter departed from the reporting point Whisky at Sion. Shortly after overflying power lines located 60–70 m above ground, two dull detonations were heard. Immediately, the engine-out signal sounded. The forward speed was about 40 knots. The instructor initiated an autorotation, avoiding buildings, and attempted an emergency landing in an orchard. On impact, the right skid broke and the helicopter rolled onto its right side. The tail boom detached from the fuselage.
Injuries and Damage
The instructor and student pilot sustained slight injuries. The two passengers, seated on the rear bench, suffered serious injuries; one passenger was airlifted to the cantonal hospital of Sion. The helicopter was seriously damaged.
Engine Examination
The engine was examined after the accident. No external damage was visible, but it could not be run on a test stand due to turbine blades rubbing against the casing. Metal particles were found on the chip detectors and in the oil filter. The compressor stators and all compressor blades showed considerable abrasion. The number 1 bearing was severely damaged; all rollers were present but three fell out during disassembly. The fuel control unit tested normal. The bleed valve was beyond tolerances but had no direct influence. A metallurgical examination found that the number 1 bearing material met specifications. The primary cause of the bearing damage was considered to be separator and roller wear, leading to local heat generation and secondary damage to stators and turbine wheel.
Probable Cause
The loss of engine power was primarily caused by the severe deformation of the Number 1 bearing.