What happened
On 6 February 2002, a Eurocopter AS 350 B3, registered ZS-RWB, was conducting a commercial mission for Spectrum Air. The flight originated from Lanseria Airport, carrying a specialized piece of equipment known as a "HEM bird" attached to the aircraft's cargo hook. The mission involved flying a specific test line west of Krugersdorp under favorable weather conditions, characterized by clear skies and a 10-knot northern wind.
After departing Lanseria, the pilot performed various test functions at an altitude of 1,000 feet above ground level. During the transit, the onboard camera was used to monitor the stability of the load, which appeared secure throughout the initial phase of the flight. As the helicopter maneuvered to intercept a survey line at 250 feet and 40 knots, the load was observed to have shifted slightly during a turn, though this was initially deemed acceptable. However, while flying straight and level approximately 10 seconds later, the pilot experienced a sudden release of the load from beneath the aircraft.
The investigation
Following the incident, the aircraft returned to Lanseria Airport. A post-flight inspection of the helicopter and the cargo hook assembly was conducted. The physical examination of the equipment showed no obvious signs of mechanical failure, suggesting the release may have been an accidental or inadvertent triggering of the mechanism. The aircraft was subsequently moved to a specialized facility for a more detailed technical inspection of the cargo hook assembly.
Findings
Investigators identified several potential factors that could have led to the inadvertent release of the cargo hook:
- Improper installation of the cargo sling assembly, which may have caused an overload condition. If the load was positioned against the keeper rather than the saddle, the internal linkage could have been subjected to excessive forces.
- Incorrect adjustment or installation of the manual or mechanical cargo sling release cable, potentially creating unintended tension on the release arm.
- Accidental activation of the release mechanism by the pilot, either through the electrical release button on the cyclic control stick or the manual release lever on the collective pitch lever.