Aircraft: Kolb Mark III. Injuries: 1 Minor. Location: Stetson, Maine. Aircraft damage: Substantial.
What reportedly happened: The pilot was in cruise flight at 1,500 feet over a heavily wooded area when the airplane began to vibrate violently, then lost all engine power. He made a forced landing in the trees.
The pilot noted that, about 30 flight hours before the accident, the airplane experienced a similar severe vibration in flight. At that time he shut down the engine and made a successful forced landing to a nearby airport.
Examination of the airplane at that time revealed that a triangle-shaped aluminum plate on the rear of the propeller hub had cracked. The front and rear propeller hubs were replaced by Warp propellers. The pilot installed the propeller hub and same propeller blades on July 10, 2010, and began flying again without any vibrations. The total hours on the propeller blades are unknown.
The post-accident examination at the accident site revealed that both carburetors had separated from the engine assembly and were hanging by their fuel lines and throttle cables. One composite propeller blade was delaminated with a large section missing. The remaining two propeller blades appeared to be intact without comparable or crash-related damage. The carburetor detachment likely caused the final engine failure and this separation was likely a consequence of the severe vibration.
Metallurgical examination of the propeller blades identified that separation of a portion of blade No. 1 initiated as a transverse fracture, originating near the trailing edge of the blade. The transverse fracture was progressive in nature, with the crack propagating in some combination of fatigue and/or stress rupture under continually applied loads. Blade No. 2 also exhibited a pattern of parallel transverse cracks on its aft side. Blade No. 3 was also visibly damaged, with significant de-lamination or disbonding of the woven carbon-fiber composite skin from the forward side of the blade, along with de-lamination at the trailing edge. Blade No. 3 also exhibited a pattern of parallel transverse cracks on its aft side. This damage was consistent with bending of the tip of the blade forward under air loading.
Probable cause: The in-flight failure of the No. 1 composite propeller blade due to fatigue, which resulted in a violent vibration, loss of engine power, and a subsequent forced landing to unsuitable terrain.
NTSB Identification: ERA11LA150
This February 2011 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.