On June 29, 2011, approximately 1945 central daylight time, a Klemp Challenger II experimental amateur-built airplane, N12911, impacted trees and terrain during a forced landing following a loss of engine power near Neshkoro, Wisconsin. The private pilot received serious injuries and the non-pilot rated passenger was fatally injuried. The airplane sustained substantial damage during the forced landing. The airplane was registered to and operated by a private individual. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 personal flight. The local flight departed a private grass airstrip approximately 1930.

According to the pilot, he planned on performing a local flight for the passenger who intended on purchasing and building a Challenger airplane. Prior to takeoff, the pilot, who was seated in the rear seat, completed a pre-flight check and engine run-up, and he noted the cylinder head temperature was approximately 250 degrees Fahrenheit. The pilot observed no anomalies during the pre-flight check, engine run-up or takeoff.

During the initial climb at 1,200 feet above ground level (agl) while crossing over the runway, the engine experienced a partial loss of engine power. Seconds later, the engine lost total power, and the pilot initiated a forced landing to the private grass airstrip. The controls to restart the engine were located in the front seat position and could not be accessed by the pilot to attempt an engine restart.

While maneuvering the airplane for the forced landing to the airstrip, the pilot felt his left rudder pedal pressed to the floor (The front and aft seat rudder pedals are interconnected). The pilot felt the passenger may have been frightened and "froze on the rudder pedal." The pilot instructed to the passenger to release the rudder pedal as the airplane entered into a slip and was losing altitude; however, the passenger did not respond to the pilot. Due to the loss of altitude, the airplane was no longer going to make it to the airstrip, and the pilot then attempted to land off the airstrip with the jammed left rudder. During the landing, the airplane impacted trees and terrain. The airplane came to rest on its right side in tall grass adjacent to trees.


The pilot held a private pilot certificate with single engine land and sea airplane ratings. The pilot's most recent third class medical certificate was issued on November 26, 2010. The pilot estimated his total flight time was 3,000 hours with 2,000 hours in the accident airplane make and model.


The experimental amateur-built Klemp Challenger II airplane was a two-seat, high-wing, pusher configuration with a primary structure comprised of fabric-covered aluminum tubing, and fixed landing gear. The airplane was assembled by the owner, who is also a dealer for the Challenger model airplanes. The airplane was powered by a Rotax 503 UL DCDI engine and equipped with a two-blade wooded propeller. The engine was mounted above and aft of the cockpit.

The Rotax engine was a 2-stroke, 2-cylinder, in-line engine, which utilized an integrated water pump and fan-cooled cylinder heads and cylinders. Review of the engine's installation and operations manuals revealed information indicating the following:

"The engine, by design, is subject to sudden stoppage...This is not a certificated aircraft engine. It has not received any safety or durability testing, and conforms to no aircraft standards. It is for use in experimental, uncertificated aircraft and vehicles only in which an engine failure will not compromise safety...User assumes all risk of use, and acknowledges by his use that he knows this engine is subject to sudden stoppage...You should be aware that any engine may seize or stall at any time."

According to the maintenance manual, the cooling fan V-belt tensions should be checked on the first 10 hours of engine operation, and then every 25 hours after, until the engine reaches the time before overhaul (TBO) of 300 hours. The operator's manual states that during the pre-flight check, the operator should verify that the cooling fan turns when the engine is rotated. The V-belt should be replaced if frayed or if it can no longer be tensioned to specification.

In addition, the periodical maintenance seasonal checks recommend the V-belt should be replaced new every 5 years or as required by condition.

The airplane was issued an experimental amateur-built airworthiness certificate on March 5, 2010. The most recent conditional inspection was completed on March 5, 2011. At the time of the accident, the airframe had accumulated 172 total hours, and the owner estimated the engine had approximately 300 total hours since new. The engine logbook, dating back to 1993, did not indicate whether the V-belt had been changed or replaced.


The pilot reported the following weather conditions at the time of the accident: wind calm, visibility 10 miles, and few clouds.


An examination of the airplane revealed flight control continuity to all flight control surfaces. The two-blade wooden propeller showed no evidence of damage. Approximately 6 gallons of fuel was noted in the fuel tank and fuel was found in the fuel lines and both carburetors.

The airplane was recovered upright onto a flat-bed trailer. The carburetor bowls were removed and fuel was noted in each bowl. The bowls were reinstalled and an engine test run was performed. The engine was test run on the airframe at various power settings for approximately 5 minutes utilizing the cockpit engine controls with no anomalies noted.


An autopsy was performed on the passenger by the Fond du Lac County Medical Examiner, Fond du Lac, Wisconsin, on June 30, 2011. The autopsy listed the cause of death as multiple traumatic injuries.

The Federal Aviation Administration's (FAA) Civil Aerospace Medical Institute performed toxicology on specimens from the passenger. The report noted that Carvedilol was detected in the urine and blood. No carbon monoxide, cyanide, or ethanol was detected.

The pilot was treated for serious non-life threatening injuries and released within hours after the accident. No FAA toxicological tests were completed on the pilot.

The pilot and passenger were not wearing helmets during the accident flight, nor were they required to per Federal Aviation Regulations.


On July 6, 2011, at the pilot's hangar, the engine was further examined by the NTSB investigator-in-charge and the pilot. Examination of the engine showed the cooling fan V-belt was broken. The carburetors and exhaust manifold were removed and scoring was noted on the sides of both pistons, consistent with a cold seizure event. The pilot attributed the loss of engine power due to a cold seizure of the engine when the fan V-belt broke during flight.


According to the engine manufacturer's representative, all seizures are caused by heat/friction and there is a thermo-imbalance of the piston and cylinder, caused by a lack of warm-up or excessive temperature of the engine. A cold seizure is where the piston expands faster than the bore it is traveling in and contacts the sides of that bore. These are also known as four corner seizures.

A cold seizure usually occurs after a full throttle run when the engine is powered back to a cruise throttle setting. If the engine has experienced some previous mini seizures, the stoppage can occur anytime in flight as there is already some aluminum (off the piston) attached to the cylinder wall and galling (unwanted removal of aluminum from the piston to cylinder wall) will be occurring at a variable rate.

Contributing factors to the piston seizure are:

1. The imbalance of the carburetors allowing the loading of one piston to be greater than the other.

2. Incorrect carburetor venting that could negatively affect carburetor fuel mixture control.

3. Inadequate cooling of the cylinders. I.E. broken fan belt or poor engine cowling

4. Lack of lubrication (Poor oil quality or not enough oil in premixed fuel)

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