What happened
On October 27, 1999, an MD 500N helicopter, registration HB-ZBI, was performing a flight from La Chaux-de-Fonds to Sion. The aircraft was carrying a full load, including a flight instructor, a student pilot, and two passengers. During the initial leg of the flight near the Sanetsch Pass, the engine chip light illuminated. Following this, the instructor landed in Sion, where an inspection of the turbine magnetic plugs revealed metallic debris. After consulting with the maintenance provider, the instructor decided to proceed with the return flight to La Chaux-de-Fonds without performing a required ground run.
Shortly after departing Sion at 15:00 local time, while flying near high-voltage power lines at an altitude of approximately 60–70 meters, the crew heard two loud bangs. The engine-out signal immediately followed. At a forward speed of roughly 4/40 kt, the instructor initiated an autorotation. The pilot managed to avoid nearby buildings and performed an emergency landing in an orchard. Upon impact, the right skid broke, causing the helicopter to overturn on its right side. The impact resulted in two serious injuries to the passengers, while the instructor and student pilot sustained minor injuries.
The investigation
The investigation focused on the mechanical state of the Allison 250-C20R/2 engine and the maintenance procedures followed prior to the accident. Investigators discovered significant amounts of metallic particles in both the forward chip detector and the oil filter. Detailed examination of the engine revealed that the turbine blades were rubbing against the housing and that the compressor stators and blades showed considerable abrasion. Furthermore, the first turbine wheel had suffered melting on more than 50% of its blades due to extreme heat.
Technical analysis determined that the primary cause of the power loss was the severe deformation of the number 1 bearing. This failure led to intense localized heat, which subsequently caused the secondary damage to the turbine and stator components. While the metallurgical properties of the bearing components met specifications, the origin of the initial bearing failure could not be determined.
Findings
- The engine failure occurred during a critical phase of flight at low altitude and low airspeed.
- The failure of the number 1 bearing was the primary driver of the engine power loss.
- The crew did not perform the 30-minute ground run required by the manufacturer's maintenance manual following the illumination of the chip light during the first leg of the flight.
- The manufacturer had previously reported similar failures in the separator roller pockets of this engine type, leading to a subsequent redesign of the bearing.