Accident Description
The public use helicopter was engaged in lifting a platform measuring 20 feet by 14 feet by 6 inches thick, weighing approximately 1250 pounds, on a 100-foot long line. The airline transport pilot had successfully lifted and transported three similar platforms with the accident helicopter just before the accident flight. However, the pilot had difficulty controlling the load on two flights and landed to check the helicopter for damage after the third flight.
On the accident flight, the pilot reported a severe jolt to the airframe while the helicopter was approximately 200 feet above ground level (AGL), and the load was approximately 100 feet AGL. This was followed by an immediate loss of engine power. Ground witnesses observed the platform swinging from right to left, fore and aft, and rising vertically several times. The platform rose one final time and "slammed down" on the long line, breaking the platform in half. Witnesses then saw white vapor coming from the helicopter. The load was jettisoned, and the helicopter descended very rapidly, impacting on a frozen lake.
Investigation Findings
Postaccident examination of the engines disclosed no evidence of any preimpact mechanical anomalies. An inspection of the external cargo hook and its support structure in the belly of the helicopter revealed a sheared bolt at the top of the hook support assembly, which allowed the assembly to move and change shape. Witness marks indicated that the hook assembly overrode its container ring and struck the fuel crossover lines, throttle control bell cranks for both engines, and the throttle position push/pull tubes. Both fuel crossover lines were pulled from their respective fuel tanks, and the throttle position bell cranks were found in the engine idle cutoff position, with the crossover lines crushed against them.