What happened
On May 15, 2006, a Boeing 747-400 was undergoing preparations for a scheduled flight to Delhi at Frankfurt/Main Airport. During maintenance at gate A23, technicians identified a hydraulic leak in the left body gear bay and replaced a hydraulic line. Following the repair, a functional and leak test was required according to the Aircraft Maintenance Manual, involving two cycles of gear retraction/extension.
While the first test cycle was completed without issue, the nose gear unexpectedly retracted during the second cycle. As the nose gear door opened, the Ground Lock Pin (GLP) and its associated extension rod fell out of the nose gear bay. This caused the nose gear to immediately and rapidly retract, causing the nose of the aircraft to drop onto a nearby tug/tow vehicle. The impact caused heavy damage to the lower forward fuselage of the aircraft, with the tug also sustaining light damage. There were no injuries among the 2 passenger, 18 crew, or ground personnel present.
The investigation
The BFU examined the Ground Lock Pin, the 2-meter extension rod, and the aircraft's nose gear components. The investigation focused on why the locking pin failed to prevent the gear from retracting. Laboratory analysis at Lufthansa Technik AG revealed that the extension rod, which had been custom-developed by the airline's maintenance department in the 1970s, was significantly worn. Specifically, the control lever used to engage/disengage the pin's locking balls was bent and stuck in the unlocked position.
Findings
- The primary cause of the nose gear retraction was a malfunction in the lever mechanism of the custom extension rod, which left the Ground Lock Pin in an unlocked state.
- The extension rod's control lever had been bent for a long period, causing it to jam in the unlocked position, meaning the locking balls could not properly secure the pin.
- The custom-made extension rod lacked official maintenance or handling instructions, leading to its continued use despite significant wear and damage to its plastic coating.
- During the second test cycle, the movement of the gear doors caused the rod to become caught and bent, allowing the unsecured pin to pull out of the assembly easily.