CEN18LA059

The flight instructor and private pilot were conducting an instructional flight when the private pilot made a hard landing and the airplane bounced. The flight instructor heard a "snapping" sound during the landing and subsequently took over the flight controls. He increased engine power to compensate for the bounced landing then landed the airplane on the runway. Upon touchdown, the left landing gear collapsed up and aft. The airplane continued forward on its nose and left wing, then departed the left side of the runway. The left landing gear through bolt was found fractured. A metallurgical examination of the landing gear through bolt fracture revealed two opposing regions of fatigue comprising 80-90% of the cross-sectional area of the bolt. These were separated by a narrow region of overstress, consistent with reverse bending fatigue crack propagation, which likely grew under low stress as evidenced by the length of the fatigue cracks. The airplane was subject to a manufacturer service letter for inspection of the landing gear through bolts due to previous instances of cracking and failure. The service letter recommended that the inspections be performed on or before the next 100-hour inspection and at 100-hour intervals thereafter, and at more frequent intervals if the aircraft is used in soft or rough runway operations. The owner of the airplane was unaware of the service letter and stated that the airplane had not received the recommended inspection. Although not required, it is likely that had the service letter been complied with and the through bolt been regularly inspected, the fatigued bolt would have been replaced and the accident would have been prevented.

On December 20, 2017, about 0850 mountain standard time, an American Champion Aircraft 7GCAA, N519MA, landed hard and departed the left side runway at Vance Brand Airport (LMO), Longmont, Colorado. The flight instructor and private pilot were not injured and the airplane sustained substantial damage during the runway excursion. The airplane was registered to CAG International Inc., and operated by Fly Elite Aviation, under the provisions of 14 Code of Federal Regulations Part 91 as an instructional flight. Visual meteorological conditions prevailed at the time of the accident and no flight pan had been filed. The local flight departed about 0845. The flight instructor reported that private pilot made a hard landing during an instructional flight. The flight instructor heard a "snapping" sound during the landing so he took over the flight controls. He increased the throttle to compensate for the bounced landing and then landed the airplane on the runway. The left landing gear collapsed upward and aft damaging the left wing strut. The airplane continued forward on its nose and left wing as it departed the left side of the runway near taxiway A2 (figure 1). Figure 1 – Accident airplane The responding Federal Aviation Administration (FAA) inspector reported that the left landing gear thru-bolt was fractured. The left wing was bent upward, and the bottom and left side of the fuselage was damaged. A metallurgical examination of the landing gear thru-bolt fracture revealed a primary fatigue fracture that initiated at the shank surface and propagated through about 60% of the bolt diameter (figure 2). A second fatigue crack initiated on the shank surface about 180° from the primary crack and propagated through 20% to 30% of the bolt diameter. The fracture surface exhibited a narrow region of overstress fracture between the two fatigue crack regions. Figure 2 – Bolt fracture surface with notation On December 13, 1978, Bellanca Aircraft Corporation issued FAA approved service letter C-135 for the purposes of inspecting landing gear thru-bolts and U-bolts. The service letter applied to the accident airplane and was issued due to reports of cracked and failed thru-bolts and U-bolts which were used to attach the landing gear to the fuselage frame. The service letter stated that these problems were due to one or more of the following: (1) excessive loads during soft or rough runway operations; (2) improper torque; (3) corrosion. The service letter recommended the inspections be performed on or before the next 100-hour inspection and at 100-hour intervals thereafter, and at more frequent intervals if the aircraft is used in soft or rough runway operations. The airplane owner stated that the Bellanca service letter C-135 had not been complied with, nor was it required under the FAA regulations. The owner provided documentation that on September 7, 2017, an airframe annual inspection and engine 100-hour inspection were completed at tachometer time 1,817.5 hours.