History of the Flight
On 23 August 2008 at 2120 hrs, a Boeing 747-436, registration G-CIVK, was taxiing at Heathrow Airport, London, for a commercial passenger flight to Bangkok. The aircraft had been pushed back from stand 407 at 2105 hrs with 19 crew and 293 passengers on board. After engine start, the aircraft taxied and crossed Runway 27L. During taxi, the crew of another aircraft reported a wheel fire on the Boeing 747 to air traffic control. The commander of G-CIVK acknowledged the report, stopped the aircraft at Link 23, and requested the airfield fire service. The commander observed on the EICAS gear synoptic page that the No. 7 wheel had no tyre pressure indication and a slightly elevated brake temperature (level 2 on a 0-9 scale). The cabin crew were briefed for a possible evacuation.
Damage and Recovery
The fire service arrived and observed smoke emanating from the rear outboard wheel of the left body landing gear (No. 7 wheel) but no fire. The wheel was left to cool while one fire vehicle monitored. Operator engineers subsequently found that the No. 7 wheel bearings were missing and there was extensive damage to the wheel hub and axle. The left body landing gear was jacked, the damaged wheel removed, and the aircraft was towed to a stand where passengers disembarked.
Examination of Wheel and Bearing
Ground inspection by the airport operator revealed the wheel's hub cap and bearing rollers near the N4E Runway 27 intersection, with additional rollers and swarf along taxiway Link 29. The recovered debris included the hub cap, part of the Tyre Pressure Indication Sensor (TPIS) assembly with a severed cable, rollers, retaining ring parts, and broken pieces of outer bearing cone. The axle nut was seized to the axle; both lockbolts were in place (one subsequently lost, the other bent with a sheared shank). The washer was flattened. The axle sleeve upper surface was in good condition but the lower surface was partially abraded with high-temperature discolouration and evidence of molten metal. Large deposits of molten aluminium alloy from the wheel hub were found inside the brake-liner heat shield. The wheel hub was extensively damaged, with loss of material inside and around the outer hub circumference. The section of inner hub supporting the inner bearing cone had been completely removed. No inner bearing rollers or cage material were found inside the hub, though the inner bearing cone was intact around the axle. The outboard bearing cup remained in place but exhibited significant surface erosion.
Flight data recorder (FDR) data showed that the No. 7 wheel tyre pressure was normal (200 psi) during pushback but the signal was lost about 1 minute 15 seconds after taxi start, during the first turn. This loss was attributed to severing of the TPIS cable, indicating the failure occurred during or before the first turn, prior to crossing Runway 27L. The peak lateral acceleration during that turn was -0.1g at about 10 kt. Five and a half minutes later, the No. 7 wheel brake temperature began rising from level 0 to level 2. Analysis of the previous 12 landings from the Quick Access Recorder showed all touchdown accelerations and sink rates were within normal parameters and would not have contributed to a bearing failure.
Maintenance History
The No. 7 wheel was first installed on aircraft G-CIVO in April 1998 and had been in service for about 11 years at the time of the incident. It had undergone 26 tyre changes, two overhauls, and one IRAN (Inspect and Repair As Necessary) occurrence. The last tyre change was on 17 May 2008, three days before installation on G-CIVK. The only subsequent maintenance was replacement of the TPIS sensor, which did not disturb the axle nut or wheel installation. The bearings had likely been in service since 1997, as the records showed no bearing cup or cone changes. Bearing parts are 'on condition' and have no life limit.
Probable Cause
The cause of the fire was a failure of the outer bearing in the wheel, resulting in loss of support and causing the rotating wheel hub to rub against the axle. The reason for the bearing failure could not be conclusively established, but improper wheel installation or an inadequate bearing inspection during the last tyre change were possible factors.