Accident Details
On September 11, 2018, at approximately 1637 central daylight time, a Maule M-7-235C, registration N969AW, experienced a collapse of the left main landing gear during landing at a private grass airstrip near Floresville, Texas. The aircraft was operated under 14 Code of Federal Regulations Part 91 as an instructional flight. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which had originated from Floresville about 1600.
The flight instructor and a private pilot-rated student were not injured. The airplane sustained substantial damage, including structural damage to the left wing and propeller strike.
Sequence of Events
According to an inspector with the Federal Aviation Administration (FAA), the flight instructor and student were practicing approaches. Upon landing, the pilots heard a loud popping noise, and the instructor performed a go-around. The student, seated in the left seat, felt vibration under the floorboard. Suspecting a blown tire, the pilots discussed how the airplane might handle during the next landing. The instructor flew the subsequent approach and landing. After a normal 3-point landing, and just before the airplane came to a stop, the left main landing gear collapsed.
Post-Accident Examination
The FAA inspector examined the wreckage and reported that the left main landing gear inboard attachment bolt was missing. The bolt was later found along the path of the landing rollout; its threads were undamaged. The attaching nut was not located. The right inboard attachment bolt and nut were still attached but were untorqued and loose, with only 2-3 threads visible past the nut.
Maintenance Context
The 100-hour/annual inspection procedures in the M7-235C Maintenance Manual (MM) specified that the torque on the inboard attach bolts should be 240-300 inch-pounds and on outboard clamp bolts 160-190 inch-pounds. The airplane was fitted with upgraded high-strength attach bolts. Maule Service Letter (SL) No. 66, revision B, dated May 2, 2006, addressed the installation of these high-strength bolts. According to the SL, the torque for the outboard clamp bolts remained the same as in the MM (160-190 inch-pounds), but the torque for the inboard attach bolts increased to 450-500 inch-pounds. SL No. 66 also stated, "Retorque bolts after 25 hours and per Maintenance Manual thereafter." The MM did not incorporate the new torque requirements for the inboard bolts, allowing them to be torqued to 240-300 inch-pounds, about 200 inch-pounds lower than required by SL No. 66.
A review of the aircraft maintenance records showed that the most recent annual inspection was completed in April 2018. The mechanic who performed the inspection reported that he did not retorque the attachment bolts; he checked them only by feel with his hands. He did not reference SL No. 66 during the annual inspection.