Accident Details
On December 28, 2012, at approximately 1015 Pacific standard time, a Bell Helicopter Model 407 (registration N407KS) experienced a hard landing during ground handling operations at Renton Municipal Airport (RNT), Renton, Washington. The helicopter was operated by the King County Sheriff's Department as a public aircraft. The commercial pilot was not injured. The helicopter sustained substantial damage to the tail rotor system, fuselage, and landing gear. Visual meteorological conditions prevailed, and no flight plan had been filed.
Sequence of Events
The helicopter was positioned on a raised movable platform approximately 18 inches off the ground. The back wheels of the dolly were chocked to prevent movement. As the pilot prepared for departure and increased throttle, the helicopter and platform began to rotate. The pilot attempted to stop the rotation by retarding the throttle but was unsuccessful. The helicopter shifted to the right, causing the right landing skid to come off the platform. The rotation ceased, and the pilot shut down all systems. The tail rotor system, fuselage, and landing gear sustained damage.
A witness reported observing the helicopter become airborne about 1 foot, shift forward, and then come down hard on the front of the platform, causing the platform to rotate. The helicopter then spun, tipped rearward, and the tail rotor struck the ground.
Aircraft and Equipment
The platform was locally fabricated by the King County Sheriff's Department. It comprised a platform with 8 sets of casters (16 wheels total). These carts are used to move aircraft in and out of the hangar. With the exception of the 4 rear fixed casters, the remaining 12 casters swivel. The fixed casters (one wheel in each corner) were chocked before aircraft operation.
Other operators at Renton Municipal Airport use similar landing platforms equipped with six casters. These operators chock all four corner casters to prevent platform movement.
Post-Accident Actions
Following the accident and subsequent investigation, the operator reported that chocking only the two rear wheels did not provide adequate stability for the platform. The operator has since modified their landing platforms by installing scissor jacks on all four corners in addition to chocking the wheels.