Thursday, August 3, 2017

CarbonCub CCK-1865, N625EX: Accident occurred August 12, 2016 at Todds Strip Airport (5AK5), Wasilla, Alaska

The National Transportation Safety Board did not travel to the scene of this accident.

Additional Participating Entity:
Federal Aviation Administration / Flight Standards District Office; Anchorage, Alaska

Aviation Accident Final Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

Aviation Accident Data Summary - National Transportation Safety Board: https://app.ntsb.gov/pdf

Investigation Docket - National Transportation Safety Board: https://dms.ntsb.gov/pubdms

http://registry.faa.gov/N625EX

NTSB Identification: ANC16LA059
14 CFR Part 91: General Aviation
Accident occurred Friday, August 12, 2016 in Wasilla, AK
Probable Cause Approval Date: 07/26/2017
Aircraft: WIGHT CARBON CUB CCK-1865, registration: N625EX
Injuries: 2 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The airline transport pilot was conducting a personal cross-country flight in the experimental amateur-built, tailwheel-equipped airplane. The pilot reported that, while landing on a turf and gravel airstrip, the airplane veered sharply left as the tailwheel touched down. Despite applying full right rudder and brake controls, the pilot was unable to correct the airplane’s track, and the right wing and horizontal stabilizer impacted the ground. The pilot stated that, after the airplane stopped, he was unable to straighten the tailwheel with the rudder control inputs because the tailwheel was bent. 

Postaccident metallurgical examination of the tailwheel suspension system revealed fractures within the welds of the assembly and a fatigue fracture at the bolt hole for mounting the tailwheel. The crack fracture surfaces exhibited orange and dark red corrosion, suggesting that the cracks were preexisting, and the fracture surfaces were consistent with overstress. The assembly also exhibited general twisting deformation, primarily as the result of deformation in the pivot bracket.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The failure of the tailwheel suspension assembly during landing due to preexisting fatigue fractures in the suspension system, which resulted in a loss of directional control.

On August 12, 2016, about 1800 Alaska daylight time, a tundra tire-equipped, amateur-built, experimental, Wight (Cub Crafters) Carbon Cub CCK-1865 airplane, N625EX, sustained substantial damage while landing on runway 7 at Todds Strip Airport, a private airstrip situated near Wasilla, Alaska. The airline transport pilot and one passenger onboard were uninjured. The airplane was registered to, and operated by, the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight that was not operating on a flight plan. Visual meteorological conditions prevailed at the time of the accident for the flight that originated from Summit Airport, Summit, Alaska, about 1630, and was destined to Todds Strip Airport.

The pilot reported that his flight to the Todds Strip Airport, a 1,500-feet-long by 60-feet-wide turf and gravel-covered airstrip was uneventful. He added that while landing, during touchdown in a wheel landing configuration, the airplane veered sharply to the left as the tailwheel touched down. The pilot said he applied a "full deflection" of the right rudder and right brake controls in an attempt to correct the veer, but the airplane continued to veer left. The right wing and right horizontal stabilizer subsequently struck the ground, sustaining substantial damage. 

After the airplane stopped, the pilot unsuccessfully attempted to turn the airplane to the right using rudder, brake, and engine power. The pilot stated that he was unable to straighten the tail wheel with the rudder control inputs because the tail wheel was bent. The pilot reported that the airplane had a total time in service of 96 hours. The airplane tailwheel spring had been replaced with a coil spring assembly that was not part of the original airplane kit manufacturer's assembly, which was a leaf spring. The coil spring was an Alaskan Bushwheels T3 Heavy Duty tailwheel suspension system with a reported total time in service of 30 hours. 

Alaskan Bushwheels T3 Heavy Duty tailwheel suspension system underwent a post-accident metallurgical examination by the National Transportation Safety Board Materials Laboratory. The examination revealed that the underside of the flange used to mount the suspension to the airframe exhibited compression deformation under the faying surfaces of the bolt head washer.

Cracks were observed in weld deposits and at the edge of the bolt hole for mounting the tail wheel. The crack fracture surfaces exhibited orange and dark red corrosion product. The dark red coloration of the corrosion deposits suggested that these cracks were pre-existing to the accident. After removal of the corrosion product, the fracture surfaces were examined with a 5X to 50X stereo-zoom microscope. The fractographic features were consistent with overstress fracture.

The crack emanating from the edge of the bolt hole was examined. After removal of superficial orange-colored corrosion product, the fracture surface of the bolt hole crack was examined with a 5X to 50X stereo-zoom microscope. The fracture initiated and propagated due to fatigue before transitioning to overstress.

The edges of the main side plates of the suspension assembly exhibited deformation and impact wear scars. The location of the scars on the side plates is consistent with contact with the airplane's tail wheel steering arms.

The pivot bracket exhibited permanent deformation and edge cracking in the region of the welded-in-place bushing as indicated in the close views. A though-thickness crack on the right side was opened for examination of the fracture surface. After removal of orange-colored corrosion product, the fracture surface of the crack was examined with a 5X to 50X stereo-zoom microscope. The fractographic features were consistent with overstress fracture.

A weld along the radiused bend in the pivot bracket exhibited a partial thickness crack in the base metal adjacent to the toe of the weld.

The suspension assembly exhibited general twisting deformation, primarily as the result of deformation in the pivot bracket.

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