What happened
On January 22, 1998, at approximately 15:00 local time, a Lama SA3/315B helicopter, registration HB-XMR, was involved in an accident in Lausanne, Switzerland. The aircraft was positioned on Rue du Vallon for a scheduled external load operation involving the transport of an electronic cabinet.
As the pilot engaged the rotor clutch, a loud noise was heard, followed by the helicopter abruptly rotating approximately 80 degrees on its skids. Debris was projected into the surrounding area. The impact of the failure caused severe damage to the cabin. One blade from the main rotor assembly was thrown approximately 100 meters from the wreckage, while the remaining two blades stayed attached to the rotor head. The pilot sustained minor injuries to the head and hands, while the flight assistant remained unharmed.
The investigation
Investigators examined the wreckage and the history of the specific rotor blade involved (S/N 11439). The blade had been in service since 1978 and had accumulated 4,947 flight hours across various aircraft. Laboratory analysis of the broken components was conducted by the Federal Laboratory for Materials Science and Research (EMPA).
Technical examinations focused on the structural integrity of the blade's spar and the attachment points. The investigation reviewed the maintenance history, noting that the blade had previously undergone repairs and was subject to various installation cycles on different Alouette III and Lama helicopters. Metallurgical analysis was used to determine the crack initiation point and the progression of the failure.
Findings
Technical analysis established that the failure was caused by a fatigue fracture originating at the first bolt hole of the rotor blade spar. The investigation identified that the fracture was initiated by corrosive agents, specifically chlorides, which had penetrated the area.
It was determined that during the blade's first two years of service, it operated without a proper sealant at the spar and fitting interface. This allowed corrosive substances to enter the assembly. Although the blade was later repaired and sealed, these trapped chlorides caused pitting and intergranular corrosion at the radius of the bolt hole. These corrosion pits acted as stress concentrators, leading to the development of fatigue cracks.
Data indicated that by the time the helicopter was on the ground with the engine stopped, the crack had already consumed 62% of the spar's cross-section. During the final engine start, the increase in centrifugal force caused the crack to grow an additional 13%, reaching a critical threshold that prevented the spar from sustaining flight loads, resulting in the immediate separation of the blade.
Safety action
Following the accident, the manufacturer issued service bulletins (Telex Services 01-64 and 01-29 for Alouette III; 01-30 for Lama) instructing operators to perform dye penetrant inspections on the susceptible areas of the rotor blades after cleaning. Subsequent instructions (01-65 and 01-30) recommended visual inspections using a magnifying glass every 25 flight hours, with dye penetrant testing required if any doubt arises.