Burst tyre and ensuing damage
Burst tyre and ensuing damage
Keywords: Aircraft, Safety
During a Boeing 737-33A takeoff from Glasgow Airport, the left inboard (No 2)tyre shed its tread. This led to the loss of the A system hydraulic contents,failure of the landing gear to retract and failure of the left main landing gear(MLG) green “down and locked” light to illuminate in the cockpit. After holding for three hours to burn off fuel, the aircraft landed safely. The tyre failure was most probably due to fatigue in the sidewall. The tyre was at its sixth retread and close to its wear limit and may have reached there its ultimate fatigue life prematurely for an undetermined reason; the retread limit for this tyre was R-6. The operator has since put in place several safety actions to prevent reocurrence. No recommendations are made in this report.
The intention was to fly to Malta. The takeoff flown by the commander,appeared to the crew to be normal but, in the first stages of the climb out, the first officer (FO) noticed that the A system hydraulic quantity was rapidly reducing. At the same time the commander requested the landing gear to be raised. On selecting the landing gear lever to UP, the landing gear failed to retract, the three red gear unsafe warning lights for each landing gear illuminated, two green gear “down and locked” lights also illuminated (the left MLG light was not illuminated) and the HYD master caution light also came on. The commander leveled the aircraft at 3,000 ft and informed ATC that they had a technical problem which needed to be observed. ATC offered an immediate return to Glasgow.
Some five seconds after take off the ground movement controller at Glasgow noticed a white stream coming from the underside of the aircraft and informed the tower. This information was passed to the crew of the aircraft who then confirmed that they had lost their A hydraulic system. They calculated that they would have been overweight for an immediate landing and so they decided to hold in the vicinity to burn off fuel. During this time a fly past and assessment from the tower at Glasgow established that the left inboard main wheel (No 2)tyre was either missing or damaged and the left MLG was down. The aircraft then re-entered the holding pattern to burn off more fuel to reduce its weight in preparation for landing. Some three hours later the aircraft landed safely at Glasgow.
Aircraft examination
Subsequently it was revealed that the No 2 tyre had shed its tread, deflated and become detached from the rim of the wheel. The debris had been flung upward and rearward, becoming lodged against the left rear spar close to aileron and spoiler control cables; debris was also found wrapped around the left MLG actuator. Hydraulic lines feeding the ML transfer unit, which is supplied by A hydraulic system, were damaged and this allowed the leakage of hydraulic fluid. It also led to a subsequent failure of the No 1 engine hydraulic engine driven pump (EDP). Structurally, the left inboard trailing edge flap, mid flap and fore flap also suffered damage from the debris. There was also evidence of tyre debris striking the fuselage above the wing and the lower surface of the left horizontal stabiliser. The left MLG side stay lock link had been hit by debris and this caused it to bend to the extent that the ground lock pin could not be inserted. This also meant that the gear down and locked safety proximity sensor,mounted on the lock link, was out of proximity, preventing the illumination of the left MLG green down and locked light in the cockpit.
Tyre history and examination
The tyre was manufactured in 2000 and had been retreaded on 5 February 2004 with its sixth retread (R-6), which would have been its last retread as the limit is R-6. The shoulder to shoulder Shearography, following the retread, did not show any anomalies. The tyre was fitted to the aircraft on 22 July 2004 and had completed 250 cycles prior to the accident which occurred on 1 September 2004. The operator carries out tyre pressure checks every 24 hours and, in the days leading up to the accident, the values were all within 5 percent of the required tyre pressure. The aircraft manufacturer defines the tyre pressures in the maintenance manual (MM) and states: “a) if the measured tyre pressure is below the necessary pressure by no more than 5 percent, inflate the tyre to the necessary pressure”.
The aircraft operator sent the tyre to the aircraft manufacturer for a detailed examination. The tyre had suffered a full shoulder to shoulder tread loss with the separation occurring at the outermost fabric layer. From the limited amount of retrieved tread pieces, this examination showed that the tyre was close to its fully worn condition. There were two ruptures to the sidewall of the tyre and severe damage had occurred to the inner liner. This was considered to be consistent with a tyre running with little or no tyre pressure. From the splits in the inner liner and sidewall of the tyre, it was evident that the nylon cords of several plys were broken. The damage was considered to be due to fatigue of the sidewall. There were no signs of damage from foreign objects or evidence of cuts. Additionally, there were no signs of manufacturing defects.
The conclusion of the manufacturers examination was that the probable cause of the tyre tread loss was fatigue in the nylon cords of the lower sidewall. This allowed the tyre inner liner to split, air to then pressurize the carcass,which then led to the rupture of the sidewall and tyre deflation. It was not possible to ascertain if the tread loss occurred prior to or following the loss of pressure in the tyre. The wheel assembly was tested with a new tyre installed, and this did not show any signs of leakage. Also removed and tested was the companion wheel and tyre (No. 1), again with no signs of tyre pressure leakage, but it did exhibit signs of damage consistent with running overloaded which was an expected condition brought about due to the failure of the No. 2 tyre.
The loss of the tyre tread on the aircraft was assessed as being due to fatigue of the sidewall of the tyre. Every tyre has an ultimate fatigue life,which is determined by the type of operation and the maintenance of the tyre during its life. Fatigue life is reduced by a tyre which is run under-inflated or run over-load at any time in the past, tyre damage or by having a lower natural tolerance due to manufacturing imperfections. Unfortunately, the only way to determine the fatigue life is to destructively test the tyre and carry out tensile tests of sections of the tyre. So that a tyre never reaches its fatigue life in service, a retread limit is set on the tyre which should never be exceeded. The retread limit is determined through destructive test of sample tyres at various lives, and a determination made as to whether the tyre type would survive another retread level or not. In this case, the tyres used on the aircraft had a retread limit of R-6; the tyre that failed was at this retread level. The tyre that failed was also close to its wear limit, meaning that it would soon have been removed and scrapped. Therefore, the tyre had probably reached its ultimate fatigue limit at an earlier age than predicted by the sample testing on other tyres.
Actions taken by the operator
The operator has put in place various measures to prevent a recurrence of this accident, including:
- 1.
A new retread limit of R-3 on all tyres fitted to Boeing 737-300 aircraft.
- 2.
Full bead to bead Shearography to be carried out following a retread, as opposed to the previous shoulder to shoulder Shearography.
- 3.
A maintenance instruction was issued to clarify the correct tyre pressures for every aircraft in the operators fleet and engineers were briefed on the importance of the management of tyre pressure.
- 4.
Any tyres manufactured in 2000, or with a retread of level 4 and above,have been removed from service.
- 5.
For flights from Malta, take-offs to be carried out with flaps 5 only.
- 6.
Flight crews briefed on vigilance of the tyre condition during the pre-flight inspection.
In consideration of the above and the fact that the aircraft manufacturer is aware of the damage caused to this aircraft as a result of this tyre failure(loss of the A hydraulic system and the potential for tyre debris to cause a control restriction), it was not considered necessary to make any safety recommendations.
