What happened
On 15 September 2017, a Boeing 737-800, registration EI-DLV, was performing a commercial passenger flight from London Stansted Airport. As the aircraft was lining up on the runway for departure, the flight crew noted a noise similar to the nosewheels passing over a runway light, though they did not initially find the sound unusual. During the takeoff roll, a nosewheel detached from the aircraft and was blown off the runway.
While the aircraft continued its climb, another aircraft holding nearby witnessed the separation and alerted Air Traffic Control. The crew of EI-DVL was subsequently notified of the incident while at altitude. Following consultation with the company engineering base and the decision to divert to avoid runway closures at Stansted, the crew declared a PAN and diverted to East Midlands Airport. The aircraft landed safely at East Midlands with no fatalities and no injuries to the 184 passengers or 6 crew members.
The investigation
The AAIB investigation focused on the structural failure of the nose landing gear. Examination of the aircraft revealed that the left nose landing gear axle had failed at the left inboard journal. Metallurgical analysis of the fracture face showed evidence of fatigue cracking and intergranular fracture.
Investigators discovered that the steel microstructure had undergone a change to overtempered martensite, a condition known as a re-tempering burn. This damage was caused by abusive grinding of the chrome plating during the component's last overhaul in December 2014. The investigation also noted that the maintenance organization used non-conforming anodes during the plating process, which resulted in uneven chrome thickness and increased the risk of high spots during grinding.
Findings
- The nosewheel separated because the left nose landing gear axle failed at the inboard journal.
- The failure originated from heat-induced cracking and changes in material properties caused by abusive grinding of the chrome plate during the previous overhaul.
- The crack likely propagated through a combination of fatigue and stress corrosion cracking over time.
- There was no effective mechanism in place at the time to detect the heat damage caused by the grinding process.
Safety action
- The Maintenance and Repair Organisation has implemented a new inspection process using Barkhausen testing after grinding to detect any heat damage.
- A Barkhausen inspection has also been introduced earlier in the overhaul sequence, prior to the Nital etch test.
- The organisation is developing a standardized internal protocol for grinding to ensure consistency among operators.