What happened
On May 27, 1978, a joint emergency training exercise was conducted in Bern, Switzerland, involving the Bern Professional Fire Brigade, the municipal sanitary police, and the Swiss Air-Rescue (SRFW). The drill simulated an evacuation from a 20-story building using a vertical rescue net suspended from a Bölkow 105 CBS helicopter, registration HB-XGM.
During the first three evacuation flights, the crew successfully transported personnel from the rooftop to a landing zone. However, during the fourth flight, which carried five individuals including a flight assistant, the polyester sling attached to the helicopter's landing gear failed. As the helicopter began its descent toward the landing area, the sling tore, causing the rescue net to fall approximately 10 meters onto a grassy area. The impact resulted in three fatalities and two serious injuries among the occupants of the net.
The investigation
Investigators examined the condition of the aircraft and the specialized rescue equipment. The Bölkow 105 CBS showed no mechanical defects, and its weight and balance were within limits. The investigation focused heavily on the 8-meter-long polyester sling used to suspend the net.
Laboratory testing of the broken sling fragments revealed that the material had suffered significant abrasion. While the original breaking strength of the polyester was measured at over 5,800 kg, the strength of the section containing the abrasion was reduced to approximately 3,840 kg. Subsequent flight tests confirmed that when the sling is wrapped around the landing gear's skid tubes, the strap can shift downward due to rotor wash and aerodynamic forces, causing it to rub against the sharp inner edge of the landing gear's sink protection plate.
Findings
- The primary cause of the accident was the inappropriate attachment method of the load-bearing sling to the helicopter.
- The polyester sling tore because it rubbed against a sharp metal edge on the landing gear's sink protection.
- The design of the attachment allowed the strap to slide into a position where contact with the metal edge was inevitable during flight.
- At the time of the accident, there were no official airworthiness requirements or certification standards for this specific type of person-rescue net equipment.
- The crew and developers of the net were unaware that the attachment method could lead to such severe abrasive damage.