Mcdonnell Douglas Helicopter 369E N311VT accident near Waimea, Hawaii

Probable cause

Impact of the lift cable after it exited the helicopter during liftoff with the main rotor (MR) blades, which resulted in the separation of an MR blade tip and the vertical vibration of the helicopter.

NTSB analysis

The commercial pilot reported that, after completing an external cargo lift operation, he landed at a remote location to jettison the lift cable and to board the two ground workers for a ferry flight back to their home base. Shortly after liftoff, the pilot felt a moderate aberration in the cyclic flight control, followed by a significant vertical vibration. The pilot subsequently observed that the main rotor (MR) blade track had a substantial blade spread. Subsequently, the pilot conducted a forced emergency landing to a nearby suitable area.

A postaccident examination of the helicopter revealed that one MR blade was missing about 9 inches of its blade tip, consistent with impact with an object of substantial mass, possibly a cable; the damage was not consistent with separation of the blade end due to a preexisting condition. Two other MR blades exhibited scuff marks and scratches along their leading edges with areas that had defined parallel scratches, consistent with cable impact.

The pilot reported that he released the lift cable before the flight, and it was not recovered. The pilot added that, after he released the cable, the ground crew placed the coiled cable in the rear compartment that had no doors. Examination of the lift cable release mechanism revealed no damage to the component, and additional testing revealed no anomalies that would have precluded normal operation of the lift cable mechanism.

There is no evidence that the lift cable remained attached to the hook during the accident flight; therefore, it was likely in the passenger compartment at liftoff as reported by the pilot. Therefore, based on the damage to the MR blades, it is likely that the cable exited the helicopter during liftoff and subsequently impacted the MR, which resulted in the separation of an MR blade tip and the vertical vibration of the helicopter.

Text from the NTSB. Open the NTSB docket for WPR17LA005 for the full record.