N225CKOmf OMF-100-160

On June 2, 2020, about 1930 mountain standard time, a OMF 100-160 airplane, N225CK, was substantially damaged when it was involved in an accident near Glendale Municipal Airport (GEU), Glendale, Arizona. The pilot and pilot-rated passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that neither occupant observed any anomalies during their preflight inspection. They departed uneventfully and remained within the airport traffic pattern while they took turns performing touch-and-go maneuvers. After the pilot-rated passenger completed five normal landings, the pilot took the controls in the airport traffic pattern and configured the airplane for his approach. Following a normal touchdown, the pilot heard a loud thud, and the airplane veered to the right. He applied left aileron and left rudder to return the airplane to the runway center. However, after the pilot applied brake pressure, the airplane rapidly departed the right runway edge, the right main landing gear collapsed, and the airplane came to rest. According to the pilot and pilot-rated passenger, none of their previous touchdowns were hard.

Photographs furnished by the airplane operator revealed substantial damage to the right wing and right horizontal stabilizer. Further, the right main landing gear had separated at the landing gear leg and fuselage, which exposed fractures in the support tubes and structure of the right main landing gear.

As assembled in the airplane, the main landing gear strut is sandwiched between a trapezoid-shaped upper clamp and a flat-plate lower clamp at the outboard attachment (see figure 1). The lower clamp is attached with bolts extending through vertical frame tubes forward and aft of the landing gear strut. The inboard portion of the landing gear strut is held in place by an inboard attachment block and bolt. Both the right main landing gear upper and lower clamps, the inboard attachment block hardware, and longitudinal frame tube were damaged during the accident flight. An illustration of the damaged area is shown in figure 2. A segment of frame tube, the attachment block and hardware, and upper clamp are not shown.

Figure 1: Left main landing gear internal assembly from accident airplane (intact and undamaged)

Figure 2: Right main landing gear internal assembly from accident airplane

An NTSB materials laboratory examination of the damaged right main landing gear components revealed that the attachment block bolt exhibited features consistent with fatigue from relatively high peak stresses. Further, the longitudinal frame tube displayed fracture surfaces consistent with overstress and showed evidence of deformation associated with contact with the upper clamp prior to the fracture. Contact marks were observed on the longitudinal tube that corresponded to contact signatures observed on the upper clamp. The thickness of the longitudinal frame tube measured 0.0357 inches. According to OMF Alert Service Bulletin (ASB) 1107/0002, published September 16, 2003, and which applied to the accident airplane, the specified wall thickness of the frame tube should be 0.058 inch.

OMF ASB 1107/0002 was issued after the company received reports of “cracks in the outside tube of the cage [tube frame], which supports the main landing gear leg. Further investigation revealed that one manufacturer used out-of-design dimensions for the tube elements.” The alert service bulletin required that the main landing gear leg support be inspected in accordance with chapter 05-20-00 of the maintenance manual and to be repeated at 50-hour intervals. If no cracks were found, a special inspection would be required at the next 50-hour interval inspection to determine the wall thickness of the tube. In the event the wall thickness was lower than what was prescribed by the ASB, the tube would have to be reinforced in accordance with OMF repair instructions. Any cracks observed were to be repaired in accordance with instructions from the manufacturer. No evidence of any reinforcement was found on the submitted landing gear components.

The Federal Aviation Administration released Airworthiness Directive (AD) 2004-24-02 on December 28, 2004, which was published with the intent to detect, correct, and prevent future cracks in the tubing for the main landing gear leg, which could result in a failure of the fuselage tubing assembly [tube frame] and potential loss of control of the airplane. This AD affected the same airplanes listed in OMF ASB 1107/0002 and required the owner/operator to “inspect the main landing gear leg for cracks” within 50 hours’ time-in-service and to “inspect following procedures in OMF Alert Service Bulletin No 1107/0002, dated September 16, 2013.” The AD did not include the ASB, wall thickness of the tube, or any procedures outlined in the ASB.

The aircraft logbook contained numerous entries to show that AD 2004-24-02 had been complied with at 50-hour intervals. The most recent compliance inspection was completed on April 20, 2020, at 651 total flight hours, about 32 flight hours prior to the accident. According to the certified airframe and powerplant mechanic who performed these inspections, he did not use the ASB to comply with the AD, which included the wall thickness of the tube.