Accident Overview
During landing, the pilot touched down with the landing gear in the retracted position after both the main and auxiliary hydraulic systems failed to extend the gear. The airplane skidded along the runway, and a fire erupted during the slide.
Examination Findings
Investigators discovered that the left inboard main tire had blown during takeoff, leaving a 30-inch section of tread loose. Black marks extended along the landing gear strut and into the wheel well above the left inboard wheel. The normal and emergency hydraulic systems share a common valve body on the landing gear actuator, and this component also exhibited black marks. A gap of 0.035 inches was measured between the valve body and the actuator. When either hydraulic system was pressurized, red fluid leaked from this gap.
Further inspection revealed that one of two bolts securing the hydraulic control valve 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. The separation of a single bolt allowed the control valve to twist about the remaining bolt, leading to loss of clamping force on that side of the actuator. Hydraulic line pressure then lifted the control valve, causing an o-ring that sealed the hydraulic fluid passage to rupture.
Regulatory Context
Title 14 Code of Federal Regulations Section 25.739 outlines requirements for protecting equipment in wheel wells from tire debris. The revision in effect at the time of the airplane's type design approval mandates that systems essential to safe operation must be shielded against damage from loose tire tread unless it can be shown that no damage is possible. Examination of the airplane and FAA-approved production drawings revealed that no shields were installed to protect the hydraulic system components in the wheel well.