Aircraft and Flight Details
The accident involved a Lama SA315B helicopter, registration HB-XMR, privately owned and operated. The pilot, a Swiss national born in 1960, held a professional helicopter pilot license with 7,102 total flight hours, including 3,100 hours on type. The flight was a commercial external load transport operation under VFR. The accident occurred on 22 January 1998 at approximately 1500 local time (UTC+1) in Lausanne, at an altitude of about 600 m above sea level. The helicopter was on the ground with the rotor engaging at the time of the accident.
Sequence of Events
On the day of the accident, the helicopter departed from Epagny at about 0945 to Villeneuve, accompanied by a flight assistant, for a material transport from a rooftop to the ground, which proceeded without issue. Around 1300, the helicopter moved to Ecublens for an antenna relocation operation, also normal. At about 1430, the helicopter landed at Rue du Vallon in upper Lausanne for a planned transport of an electronic cabinet. The pilot shut down the turbine while waiting for clearance from personnel in Pully. Shortly before 1500, the start signal was given, and the pilot started the turbine. During rotor engagement, the pilot heard a dull sound, then the helicopter abruptly rotated about 80° to the right on its skids as numerous parts were thrown around. After the helicopter stopped, the pilot realized the cabin was destroyed and the rotor and transmission assembly had separated from the helicopter. The pilot sustained minor injuries to his head and hands and was rescued by his flight assistant, who had observed the accident from the ground and taken cover behind concrete blocks. The main rotor, with two blades still attached, was found near the destroyed cabin, while the third blade had been thrown about 100 m away.
Examination of the Blade
The failed main rotor blade, serial number 11439, was manufactured in 1978 and had accumulated 4,947 flight hours across several helicopters and operators. Its service history included repairs and transfers between different aircraft. Following the accident, blade fragments and the two other blades (serial numbers 5390 and 5465) were sent to the Federal Laboratory for Materials Testing and Research in Dübendorf for fracture analysis. Two reports were issued on 17 July 1998 and 9 June 1999.
Examination revealed that the attachment bolts in the blade root had lost preload over time, though interference fit between steel fitting and spar remained. Visual inspection showed deposits suggestive of corrosion at the lower part of the fracture. Metallurgical investigation found that the fatigue fracture originated from two different points in the first attachment bore and covered over 70% of the cross-section. The fracture initiation site was in the contact-free zone of the attachment bolt, at the bore's transition radius. Metallography revealed corrosion pits throughout the lower bore edge, from which intergranular cracks and fatigue cracks emanated. Analysis of corrosion products showed decreasing chlorine content from the outer surface inward, indicating chlorine strongly promoted the corrosive attack. The strength of the spar material met factory certificate values, with a possible slight increase due to aging.
Fracture mechanics analysis showed that the critical stress intensity factor K_IC of about 43 MPa√m would be reached at nominal rotor speed due to centrifugal force alone once the crack reached about 62% of the cross-section. At that point, the critical cross-section could no longer transmit additional flight loads. During the time the helicopter was still on the ground, the crack grew from about 62% to 75% of the net cross-section, corresponding to visible crack growth of about 15 mm on the left side and about 3 mm on the right side of the fracture surface. Therefore, before the final landing, the visible crack length on the left side was at most about 48 mm and on the right side at most about 32 mm. At the fracture origin, a region of intergranular corrosion about 0.15 mm deep containing chlorine was found, but further crack growth likely occurred without chlorine influence.
Cause
The accident was caused by the fracture of a main rotor blade due to a fatigue crack in the area of the first bore, initiated by corrosive substances.