MIA02LA138

According to the pilot, he had to clear a ridgeline on his approach path to a 2,000 foot grass strip, necessitating a short field approach that was steep, using idle power. When he rapidly applied power in the flare, the engine hesitated, and the touchdown was firm. The left main wheel fractured at the weld joint between the axle and the hub spacer , causing the wheel to separate, the left landing strut to dig into the sod, and the aircraft to ground loop. The ground loop resulted in a collapsed nose gear strut and a propeller strike. Postcrash examination of the engine revealed metal shavings contamination in the gascolator. Postcrash run of the engine revealed an idle rpm of less than 500 rpm. The normal idle rpm range should be 600 to 800 rpm. The left main wheel/axle was sent to the NTSB Materials Laboratory for analysis of the fracture. The examination showed the axle to spacer weld had only achieved about 30 percent fusion of the surface at the end of the axle. The remaining fracture surface was consistent with overstress separation.

On July 23, 2002, about 1700 eastern daylight time, a Maule MXT-7-180A , N10550, registered to Wellfound Air Inc., operated by a private individual as a Title 14 CFR Part 91 personal flight, crashed while landing at the Lookout Mountain Ultralight Airport, near Trenton, Georgia. Visual meteorological conditions prevailed and no flight plan was filed. The airplane received substantial damage, and the private-rated pilot, the sole occupant aboard, was not injured. The flight departed the Cobb County-McCollum Field Airport near Marietta, Georgia, about 50 minutes before the accident.

The pilot stated that during his short field approach to a 2,000-foot grass strip, the engine was slow to respond to his throttle advancement during his landing flare and the airplane landed hard. The pilot stated that the left axle broke, the left landing gear strut dug into the ground, and during the subsequent ground loop, the nose gear collapsed, the propeller struck the ground, and the airframe sustained some twisting deformation.

On August 8, 2002, the aircraft's left wheel axle assembly, consisting of a steel tube of 1.5-inch length and .12-inch wall thickness welded to a 3-inch diameter low carbon steel hub spacer, was shipped to the NTSB Materials Laboratory, Washington, DC, for failure mode examination. According to the Laboratory Report, "Visual and optical microscopic examination of the fracture surfaces revealed that the welding process had achieved incomplete fusion with the axle." The portion of the weld between the axle and hub spacer that exhibited proper fusion, (about 30-per cent of the total weldment) revealed overstress fracture. There was no evidence of fatigue, and the corrosion that was observed on the fracture surfaces occurred after the parts were separated during the hard landing. The Materials Laboratory Report is an attachment to this report.

On September 17, 2002, during a telephone call to the NTSB, the pilot confirmed that fuel system repairs had been recently performed on the aircraft. Due to a fuel leak in the left wing, the left wing fuel tank had been removed and the left fuel strainer had been replaced. Additionally, the fuel selector valve had been replaced. On October 8, 2002, during a subsequent telephone call to the NTSB, the pilot stated that, if the aircraft's engine had a low idle condition, that the low idle combined with the use of carburetor heat, may have led to the power hesitation he experienced. He mentioned this had been a theory of his all along. He could not recall any recent adjustment made to the throttle linkage or carburetor. He further stated he could not recall what rpm the engine idled at, only that it idled "OK" during his runup. A data sheet provided by Maule Air, Inc., revealed the minimum idle rpm for the MXT-7-180A should be no less than 600 rpm and no more than 800 rpm. Copies of "Record of Phone Conversation" forms and factory data sheets are attachments to this report.

On October 2, 2002, under NTSB oversight, the engine underwent a static test run. Prerun examination by NTSB personnel revealed that the gascolator and its filter screen contained both ferrous and non-ferrous metal shavings of a silver color. The carburetor filter screen was found clean and clear of contaminates. The engine was run using a test propeller and the engine's own electrical and ignition system. The engine started easily and idled for about 2 minutes indicating normal fuel pressure. While insuring that the carburetor linkage adjustment screw was hard against its low speed stop, the idle rpm was less than 500 rpm. When the throttle was jam accelerated from its idle setting, the engine died. When the idle rpm was raised to 700 rpm and carburetor heat was applied, the carburetor easily accepted the jam acceleration. Throttle range of motion was abruptly actioned in both directions with no adverse operation.