What happened
On June 22, 2009, a Finnair Airbus A340-313, registration OH-LQE, operating flight AY58 from Shanghai to Helsinki-Vantaa, experienced a serious incident following a tire failure during takeoff. At 02:17 UTC, the tread of the inner rear wheel on the left main landing gear detached and disintegrated. The high-velocity rubber debris struck hydraulic lines and brake system components within the landing gear bay.
During the flight, the crew received an amber caution for "BRAKES HOT" and subsequently observed a leak in the green hydraulic system. As the flight progressed, the green system fluid level dropped to its minimum. The crew also received reports that tire fragments bearing Finnair markings had been found on the runway in Shanghai, which increased uncertainty regarding the extent of the damage. Due to the potential for a landing emergency, the captain ordered the cabin to be prepared for an emergency situation approximately 40 minutes before arrival.
Upon landing at Helsinki-Vantaa, the aircraft's blue hydraulic system also began to leak due to damage to a brake line fitting. Because the nosewheel steering was inoperable, the aircraft could not vacate the runway under its own power and had to be towed to the apron, where the 195 passengers and 12 crew members disembarked. There were no injuries.
The investigation
The investigation focused on the sequence of events leading to the tread separation and the subsequent hydraulic failures. Investigators examined the damaged tire, which had previously undergone retreading. It was discovered that the tire had a pre-existing puncture that was not communicated to or detected by the retreading facility. This allowed pressure to build between the tread and the carcass, eventually causing the separation. The investigation also analyzed the impact of the rubber debris on the aircraft's hydraulic and electrical systems.
Findings
- The primary cause of the incident was the separation of the tire tread during takeoff, caused by a hidden puncture that allowed air to enter the space between the tread and the carcass.
- The high-energy impact of the flying rubber fragments caused the leaks in both the green and blue hydraulic systems by damaging lines and fittings in the landing gear bay.
- Evidence of heat-induced discoloration in the rubber suggested that thermal stress may have further weakened the bond between the tread and the carcass.
- The retreading process failed to identify the underlying puncture, which led to the structural failure of the tire.