Incident Summary
The pilot reported that during flight, the nose gear dropped from the wheel well. He observed that the hydraulic pressure had dropped to 600 psi and the landing gear in-transit light had illuminated. After accomplishing emergency procedures, he was unable to get the main landing gear to lock in the down position. During touchdown, the main landing gear collapsed and the belly of the airplane was damaged.
System Description
The brake and landing gear systems were powered by two engine-driven pumps—one on each engine—and an auxiliary (electrical) pump. For backup operation, gas pressure from an emergency nitrogen storage cylinder was continuously provided to a second set of main gear actuators to provide pressure for main gear extension. During gear retraction, nitrogen was compressed back into the nitrogen cylinder. During extension, hydraulic pressure and nitrogen pressure work together to extend the main gear.
Post-Accident Examination
After the accident, a hydraulic fluid leak was found at a hydraulic line fitting on the right main landing gear brake system, but no other leak was found. The leak was repaired, and the landing gear was functionally checked; the retraction and extension cycle functioned normally, and no system fault was found. No reason was found to explain why gas pressure from the emergency nitrogen storage cylinder did not lock the main gear in the down position.