What happened
On 1 September 2004, a Boeing 737-33A, registration 9H-ADH, was performing a scheduled passenger flight from Glasgow Airport to Malta. During the takeoff roll on Runway 2/3, the tread of the left inboard No 2 tyre detached from the wheel. As the aircraft climbed, the crew observed a rapid reduction in the A system hydraulic quantity. Upon attempting to retract the landing gear, the crew found the gear failed to retract, and several warning lights illuminated in the cockpit, including a failure of the left main landing gear green 'down and locked' light.
Ground controllers observed a white stream of fluid emanating from the aircraft's underside shortly after departure. To manage the aircraft's weight and ensure a safe landing, the crew entered a holding pattern for approximately three hours to burn off fuel. During this period, the crew utilized a gear viewer to inspect the gear from the cabin and requested a low-level flypast so ground engineers could visually confirm the aircraft's condition. The engineer confirmed the left main landing gear was down but noted the damaged tyre. The aircraft eventually landed safely at Glasgow.
The investigation
The AAIB investigation focused on the mechanical failure of the tyre and the subsequent damage to the aircraft's systems. Examination of 9H-ADH revealed that the detached tyre tread had been flung upward and rearward with enough force to damage the left main landing gear sidestay, hydraulic control lines, and several flaps. Crucially, debris had become lodged near the aileron and spoiler control cables, posing a risk of flight control restriction. The damage to the hydraulic lines caused the loss of the A system fluid and the failure of the No 1 engine hydraulic pump.
Investigators analyzed the history of the tyre, which was at its sixth retread (R-6) and nearing its wear limit. While recent pressure checks were within acceptable limits, the tyre had completed 230 cycles since its last fitting. The investigation also looked into the inspection methods used during the retreading process, specifically the use of shoulder-to-shoulder Shearography.
Findings
- The primary cause of the incident was fatigue in the sidewall of the tyre.
- The tyre tread loss occurred during the high-stress period of rotation.
- The failure of the A hydraulic system and the landing gear indication error were directly caused by debris from the tyre hitting hydraulic lines and the gear lock link.
- The tyre was at its maximum allowable retread limit (R-6) and was close to its wear limit.
- The previous inspection method (shoulder-to-shoulder Shearography) was unable to detect the impending fatigue failure within the tyre carcass.
Safety action
Following the accident, the operator implemented several measures, including:
- Reducing the retread limit to R-3 for all Boeing 737-300 aircraft in their fleet.
- Implementing full bead-to-bead Shearography inspections after retreading.
- Issuing new maintenance instructions regarding tyre pressure management.
- Removing all tyres manufactured in 2000 or those with a retread level of R-4 or higher from service.