History of Flight
On August 24, 2020, about 1747 Pacific daylight time, a Kaman K-1200 helicopter, registration N314, was substantially damaged in an accident near Pine Grove, Oregon. The pilot was fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 133 external load flight under an exclusive-use contract with the United States Forest Service (USFS). It had been conducting water bucket/fire suppression activities for the White River fire at Mt. Hood National Forest.
The air attack controller, in radio contact with the pilot, reported that the pilot indicated he needed to return to the heliport to refuel after “one more bucket.” No further transmissions were received from the pilot. About 15 minutes later, the air attack controller contacted dispatch and learned the helicopter had not returned. The controller flew to the dip site and observed the helicopter lying on its right side in the river. No “mayday” call was received.
Onboard GPS data showed that about 1745, the helicopter was 0.75 miles southwest of the accident dip site. It returned to the dip site and began slowing and descending. The last recorded data point was about 138 ft above ground level at 0 knots, about 15 ft from the main wreckage location.
Aircraft Information
The Kaman K-1200 helicopter featured two counterrotating, side-by-side, intermeshing rotors with two blades per rotor (total four blades). The rotors were out of phase by 90° and tilted outward. The left rotor turned counterclockwise, the right clockwise when viewed from above. Each matched set of blades consisted of an “A” blade (white tip) and a “B” blade (red tip). The helicopter was not equipped with, nor required to have, a flight data recorder.
The rotor system used servo-flaps to control blade pitch changes. The Kaman K-1200 Maintenance and Servicing Instructions describe the servo-flap as mounted on each blade near the 3/4 radius, controlled by rods that transfer cockpit inputs through azimuth assemblies. The servo-flap controls pitch and acts as an aerodynamic stabilizer.
The helicopter was maintained under the manufacturer’s approved airworthiness inspection program. Recurring inspections for rotor blade servo flaps included preflight and 100-hour/annual or zone one progressive inspections. The last zone one inspection occurred on February 25, 2020.
Wreckage and Impact Information
The dip site was lined on three sides by trees about 100 ft tall. No evidence of in-flight impact with trees was found. The entire airframe was present with no evidence of in-flight breakup. The nose was crushed aft and inward, consistent with contacting the river in an inverted attitude. The tailboom showed deformation under the horizontal stabilizer, causing downward bending of the aft portion. The right horizontal stabilizer fractured and separated but was found nearby. The 140-ft long line remained attached to the cargo hook and wrapped around the fuselage, consistent with the fuselage rolling left around the line. The water bucket was found about 40 ft upstream, with minor damage.
The engine and transmission remained attached. Both left and right transmission pylons separated from the main transmission center housing. Engine examination revealed no mechanical malfunctions that would have precluded normal operation.
Rotor blade damage included root end fractures on left main rotors and contact marks on left and right rotors. All blade dampers separated; one was recovered. Left and right blade sets showed damage consistent with leading and lagging beyond stops.
Flight control continuity was established for cyclic, collective, and pedal controls. Control tubes and bellcranks exhibited impact fractures but no disconnection.
Left Rotor System: The white blade fractured into three sections, with the largest found about 560 ft from the wreckage. The servo-flap afterbody completely separated from its spar and was found 192 ft away. The red blade fractured in two locations, with most found about 500 ft away and the tip 570 ft away. The underside had a dark triangular rub mark and impact marks consistent with scuffing.
Right Rotor System: The white blade remained attached but fractured in four locations. The red blade fractured in one location, with most of it found 81 ft away. The right red servo-flap had cracks at the outboard end along bond lines.
Servo Flap Examination
All four servo flaps were examined at the NTSB Materials Laboratory. The left white servo-flap afterbody separated generally at the spar-to-afterbody transition, with features consistent with alternating stresses. The separation started at the lower skin inboard end and progressed outboard. The inboard closeout bonding tab was missing; much of the bond area appeared unbonded. Crack propagation traced to an origin at the lower inboard region of the bonding perimeter.
The outboard closeout was cracked along upper and lower bond lines, consistent with the onset of high-amplitude reverse bending loads. The other three servo flaps exhibited cracking along the bond line between the closeout and aluminum fitting, with varying extent. Sectioning of the right red servo flap showed outer and inner skin plies fractured beneath a surface crack.
Additional Information
In April 2009, the same helicopter (then registered N361KA) experienced an in-flight separation of a left white blade servo-flap. The pilot landed safely with no additional damage.
On June 16, 2010, a K-1200 helicopter in Donnelley, Idaho, experienced a servo-flap afterbody separation. The NTSB determined the probable cause was collision of two main rotor blades for undetermined reasons, resulting in loss of control.
A manufacturer demonstration on a whirl stand, with an exemplar blade set missing the inboard closeout from a red blade servo flap, showed no signs of flutter, out-of-track conditions, or load exceedances at various rotor speeds up to 270 rpm and torque up to 150%.