What happened
On 25 November 1998, a Lockheed L1 .11 Tristar, registration 9Y-THA, was preparing for a scheduled passenger flight from London Heathrow to Antigua. After receiving clearance to line up on Runway 27L, the aircraft began a right turn to align with the centreline. During this manoeuvre, the crew felt a heavy thud, suggesting the aircraft had struck an object. The commander noted that the nosewheel steering felt unusually heavy, leading to the suspicion of a tyre failure.
Following reports from another aircraft at the holding point regarding a problem with the nosewheel, the commander halted the aircraft at the runway threshold. The engines were shut down, and the APU was started to maintain electrical power and air conditioning. There were no fatalities and no injuries among the 215 passengers or 11 crew members. Emergency services attended the scene, and passengers were evacuated via ground equipment to waiting coaches. The aircraft was eventually defuelled and towed from the runway, which was cleared for use later that evening.
The investigation
Investigators examined the nose landing gear assembly, specifically the right nosewheel, which had separated from the aircraft due to a broken stub axle. The component was sent to Lockheed Martin and DERA for metallurgical analysis. The examination of the broken axle revealed multi-origin cracking distributed around the bottom half of its circumference, with approximately 80 initiation sites identified.
Analysis showed that the crack development began in an intergranular mode, likely caused by stress corrosion or hydrogen embrittlement. This was followed by a transgranular fatigue phase that eventually led to the final rupture. The investigation also identified a "mud-cracking" pattern in the chrome plating, a feature characteristic of damage caused by an abusive grinding process during manufacturing or overhaul.
Findings
- The right nosewheel separated because the stub axle failed.
- The failure originated from multiple cracks caused by stress corrosion or hydrogen embrittlement.
- The presence of "mud-cracking" in the chrome plating indicated that the underlying steel had been subjected to stress concentrations from previous grinding damage.
- The turning manoeuvre performed during line-up created a high-load case that triggered the rupture of the already compromised axle.
- The specific area of the axle prone to such cracking was not directly accessible for routine inspection while the component was assembled.