K 1200 N314 accident near Pine Grove, Oregon

Probable cause

An in-flight breakup resulting from contact of the left rotor system with the right rotor system after an in-flight failure of a servo flap from a left rotor blade.

NTSB analysis

The pilot had been conducting fire suppression activities for the United States Forest Service using a water bucket suspended at the end of a 140-ft longline. The pilot informed the controller that he would need to return to the base for fuel after another bucket of water, which was the last communication between the controller and the pilot. The helicopter was subsequently located in a river at the dip site. No mayday call was received.

Onboard global position system (GPS) data revealed that the helicopter had arrived at the dip site and was hovering at an altitude of 138 ft above the water, which would place the helicopter about 40 ft above the treetops. The water bucket would have been near or in the water. No evidence showed any signs of rotor system contact with the trees. However, evidence did show that the helicopter rolled to the left as it descended and the 140-ft longline wrapped around the fuselage during the descent. The helicopter then struck the river in an inverted position and continued to roll to the left until it came to rest on its right side. The lack of damage to the longline demonstrated that both rotor systems and pylons had separated from the fuselage while the helicopter was in flight. The left rotor blades were found 560 ft away from the main wreckage.

Postaccident examination revealed that the damage observed on the right rotor blades, right hub, and right pylon occurred after they were struck by the left rotor blades while the helicopter was above the dip site. The damage observed on the left rotor system occurred when it hit the right rotor system while in flight. This intermeshing contact resulted in the in-flight separation and breakup of the left rotor blades.

The failure of the left white blade servo flap started as a fracture of the lower skin at the spar-to-afterbody transition and peel separation of the upper skin before transitioning to fracture of both upper and lower skins at the transition. The transition between separation modes, onset of reverse bending damage at the outboard closeout, and the extent of damage to the left white blade servo flap compared to the servo flaps from the remaining three blades indicates that the left white blade servo flap was cracked at its inboard end prior to the collision between the left and right rotor systems. The inboard end crack grew progressively and compromised the structural integrity of the servo flap leading to the failure and eventual separation of the afterbody.

Failure of the left white blade servo flap resulted in a loss of control of the left white blade. However, it could not be determined why the left white blade servo flap failure ultimately resulted in a collision between the left and right rotor systems in this accident. A past event involving a servo flap separation demonstrated that the loss of a servo flap does not always result in catastrophic consequences. It is likely that flight control inputs, including the pilot’s responses to an abnormal vibration in the rotor system, were a factor to the catastrophic outcome of the servo flap failure in this accident. The lack of flight data precluded analysis of the control inputs leading up to the collision between the left and right rotor systems.

Text from the NTSB. Open the NTSB docket for WPR20LA283 for the full record.