Event Details
On May 15, 2006, a Boeing 747-400 was being prepared for a scheduled flight at Gate A23 of Frankfurt/Main Airport. During a pre-flight inspection, a hydraulic leak was discovered in the left body gear wheel well. A hydraulic line required replacement, which necessitated a functional and leak test according to Aircraft Maintenance Manual (AMM) TASK 32-33-06024-001. The test was to be performed twice.
The first test cycle passed without any issues. During the second cycle, when the landing gear lever was moved to the retract position, the nose gear doors opened and the ground lock pin (GLP), a safety device intended to prevent nose gear retraction on the ground, fell out of the nose gear well with its rod. The nose gear immediately retracted, causing the aircraft's nose to settle onto a tow tractor that was positioned underneath.
A loadmaster observing the test saw the nose gear doors strike the safety pin rod violently during the first cycle. When the doors opened a second time, the rod and pin fell out. The tow tractor driver, who was standing by with the tractor under the nose, noticed the nose gear doors opening and a red rod falling out.
The aircraft sustained heavy damage to the forward lower fuselage, with the tow tractor penetrating the skin and destroying frames and stringers over an area of approximately 2.5 meters. The tow tractor was lightly damaged. None of the 238 passengers, 18 crew members, or ground personnel were injured.
Investigation Findings
The ground lock pin with its 2-meter-long rod was found lying crosswise under the nose. The rod was made of a square steel tube and covered with a red shrink tube showing heavy wear. The operating lever at the lower end of the rod was slightly bent and jammed in the unlocked position, preventing the locking balls of the GLP from engaging.
The rod had been developed by the airline's maintenance organization in the 1970s and had been in use since then. No maintenance or inspection procedures existed for this tool. A subsequent attempt to insert an original GLP into the nose gear torque link hole showed that the original pin could only be inserted halfway, indicating a misalignment or obstruction.
Laboratory examination by Lufthansa Technik AG revealed that the GLP itself functioned properly and showed normal wear. However, the safety rod showed significant wear and deformation. The lever mechanism that operates the GLP's locking balls was bent and jammed in the unlock position, a condition that had existed for some time based on wear patterns. When the nose gear doors struck the rod during the test, the rod bent further, allowing the unlocked GLP to be pulled out without resistance.
Probable Cause
The investigation determined that the cause of the nose gear retraction was a malfunction of the lever mechanism within the safety rod in conjunction with the ground lock pin. This malfunction left the GLP in the inserted but unlocked state, allowing it to be dislodged when the nose gear doors contacted the rod.