Maule M7 N969AW accident near Floresville, Texas

Probable cause

The separation of the left inboard main landing gear (MLG) attachment bolt due to insufficient torque, which resulted in the collapse of the left MLG.

NTSB analysis

The flight instructor and the private pilot-rated student, who was in the left seat and was also the airplane owner, were practicing approaches. Upon landing, the pilots heard a loud popping noise, and the instructor then initiated a go-around. Subsequently, the pilot felt vibration under the floorboard. The pilots thought that a tire might have blown. The instructor flew the approach and conducted a three-point landing, and just before the airplane came to a stop, the left main landing gear (MLG) collapsed, which resulted in substantial damage to the left wing.

An examination of the left MLG revealed that its inboard attachment bolt and nut were missing. After the accident, the bolt was found along the path of the landing rollout. Examination of the attachment bolt revealed that the bolt threads were undamaged. The attachment nut was not found. The right MLG's inboard attachment bolt and nut were found still attached; however, they were loose, with only two to three threads visible past the nut.

The inboard attachment bolts were upgraded high-strength bolts. The airframe manufacturer had issued a service letter (SL) that stated that, when using the high-strength bolts, the outboard attachment bolts required the same amount of torque as indicated in the airplane maintenance manual but that the inboard attachment bolts required a higher torque amount than the standard bolts. However, a review of the maintenance manual revealed that it did not include the new, higher torque requirements specified in the SL. Therefore, mechanics who only follow the maintenance manual may not be aware that higher torque values are required.

A review of the airplane maintenance records revealed that its last annual inspection was completed about 6 months before the accident. The mechanic who performed the inspection reported that he did not verify the torque on the bolts, that he only used his hands to confirm that they were secure, and that he did not reference the SL during the inspection. It could not be determined whether the mechanic's failure to verify the torque on the bolts or the insufficient torque values in the MM led to the torque on the left inboard MLG attachment bolt being insufficient and the resultant collapse of the left MLG.

Text from the NTSB. Open the NTSB docket for ERA18LA270 for the full record.