What happened
On July 9, 1963, a Bell 204B helicopter, registration HB-XBN, was performing external load transport operations near Zweilütschinen, Bern. During two separate flight segments—the first at approximately 09:35 and the second at approximately 10:50—two heavy loads consisting of prefabricated chalet parts detached from the aircraft's cargo hook. The detached components fell into the forested mountain slopes.
The investigation
The investigation focused on the mechanical failure of the attachment mechanism. Investigators examined the cargo hook assembly to determine why the loads became uncoupled from the helicopter during flight. The inquiry established that the release lever of the load attachment device was susceptible to accidental activation.
Findings
Technical analysis revealed that the release lever was far too sensitive to movement. The primary cause was a defective tension spring within the release mechanism. Specifically, the counter-spring for the release lever exerted only about 200 g of pressure, whereas a standard functional component should have provided approximately 1200 g of resistance. Because this spring was too weak, even minor jolts or sudden stresses during flight were sufficient to trigger the lever and release the cargo.