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On September 18, 2010, about 0910 mountain daylight time, an experimental amateur-built Boam, Rans S-6S, N4427M, experienced a loss of engine power during takeoff from a private dirt airstrip near Roberts, Idaho. The pilot/owner was operating the airplane under the provisions of Title 14 Code of Federal Regulations Part 91. The commercial pilot was not injured, and the airplane sustained substantial damage. The local flight departed Roberts about 0910, with a planned destination of Rigby-Jefferson County Airport, Rigby, Idaho. Visual meteorological conditions prevailed, and no flight plan had been filed.
The pilot reported that while turning crosswind after the initial takeoff climb, the engine experienced a partial loss of power. He initiated a return to the runway during which time the engine lost all power. He unsuccessfully attempted to restart the engine, and elected to perform an off-airport landing. During the descent he maneuvered around trees and obstacles, and was subsequently unable to level the airplane for the landing flare. The right main landing gear collapsed during the landing roll, resulting in substantial damage to the lower firewall and both wings.
A deputy from the local sheriff's department responded to the accident site and reported that the fuel tank contained about 10 gallons of fuel. The airplane was subsequently recovered for examination.
TESTS AND RESEARCH
The high-wing, experimental amateur-built airplane was equipped with a Rotax 912 ULS engine, serial number 5643038. The engine was equipped with a two-blade composite propeller, and an overload clutch, also known as a 'slipper clutch'. Maintenance records indicated that the owner performed a conditional inspection on July 13, 2010, at an engine tachometer time of 428.8 hours. The owner reported a total engine time of 444.9 hours at the time of the accident.
The airplane was initially examined by an inspector from the Federal Aviation Administration (FAA), and subsequently examined by a mechanic under the auspices of the NTSB investigator-in-charge. Examination revealed that the engine controls, linkages, cables, and hoses appeared intact and free of damage. No fluid leaks were noted, oil remained within the oil sump canister, and the engine rotated freely by hand via the propeller. Examination of the left carburetor revealed that it had become detached from the inlet manifold flange. The flange remained attached to the inlet manifold, with the carburetor clamp still in place over the rubber flange. The carburetor was observed about 1/2 inch below its nominal mounting position. The carburetor location was adjacent to the firewall; however, no damage to the firewall was noted that would indicate it made contact with the carburetor during the accident sequence.
The engine was equipped with a carburetor support system, consisting of a support spring and bracket as specified in the Rotax Technical Bulletin 912-10. The bulletin stated in part that the support is used to reduce stress on the carburetor flange, incurred due to, 'Increased vibrations, unfavorable engine suspension, too heavy intake filter or intake silencer, erratic idling of engine and bad carburetor synchronization.'
The airplane owner reported that on prior engine startups he had encountered excessive initial vibration and kickback, and as such, was considering the installation of a Soft Start Module (SSM). The SSM, according to its manufacturer, reduces engine kickback during startup by adjusting the initial ignition timing. The manufacturer further stated that engine kickback is often the result of slow cranking speed or an inadequately adjusted slipper clutch. The slow cranking speed can be caused by reduced electrical power available to the starter motor due to either a weak battery, or excessive battery cable length.
The owner stated that he could have prevented the loss of power by more thoroughly inspecting the carburetor mounting clamp tension, installing an SSM, and by inspecting and adjusting the slipper clutch.