Incident Overview
The pilot landed the aircraft with the landing gear in the retracted position after both the main and auxiliary hydraulic systems failed to extend the gear. As the aircraft skidded down the runway, a fire erupted.
Investigation Findings
Examination revealed that the left inboard main tire had blown during takeoff, leaving a 30-inch section of tread loose. Black marks extended along the length of the landing gear strut and into the wheel well directly above the left inboard wheel. Both the normal and emergency hydraulic systems connect to a common valve body on the landing gear actuator, which also bore black marks. A gap of 0.035 inch was measured between the valve body and actuator; when either hydraulic system was pressurized, red fluid leaked from this gap.
Further examination disclosed that one of two bolts holding the hydraulic control valve in place had fractured and separated. The fractured bolt experienced a shear load oriented along the longitudinal axis of the actuator, consistent with impact forces from the flapping tire tread section. Separation of only one bolt allowed the control valve to twist about the remaining bolt in response to the load, leading to a loss of clamping force. Hydraulic line pressure then lifted the control valve, rupturing an o-ring that sealed the hydraulic fluid passage.
Design and Certification Issues
14 CFR 25.739 requires protection of equipment in wheel wells from the effects of tire debris. The revision in effect at the time of the aircraft's type design approval mandates that essential equipment and systems located in wheel wells be shielded unless it can be shown that a loose tire tread cannot cause damage. Examination of the aircraft and the FAA-approved production drawings revealed that no shields were installed to protect hydraulic system components in the wheel well.
Official Probable Cause
The probable cause was the complete failure of all hydraulic systems due to the effects of a main gear tire disintegration on takeoff. Also causal was the manufacturer's inadequate design of the wheel wells, which did not comply with applicable certification regulations, and the FAA's failure to ensure that the airplane's design complied with standards mandated in certification regulations.