On 29 January 2008, a Bombardier BD700 Global Express on a private passenger flight from Van Nuys, California to Luton experienced a single tyre failure when landing at destination in normal day visibility which caused significant secondary damage to the flight control system and localised structural damage to the wing. The aircraft was stopped on the runway and there were no injuries to the four occupants.
An Investigation was carried out by the UK AAIB. It was established that soon after a normal touchdown, the flight crew became aware of a rumbling noise which they identified as a burst tyre which was confirmed by another aircraft. The aircraft commander began to apply normal braking and shortly afterwards, EICAS messages advising low hydraulic pressure in both No 2 and No 3 hydraulic system were annunciated. The aircraft was brought to a stop on the runway using normal brakes and, as RFFS vehicles approached, the engines were shut down.
Available evidence indicated that the aircraft had touched down with the left inboard wheel locked but that it became free to rotate shortly after the tyre ruptured. The left inboard main-wheel tyre was found to have had suffered a slide-through type failure caused by the locked wheel. This failure had developed into a larger disruption of the tyre carcass which in turn resulted in flailing of a substantial section of both tread and carcass when the wheel then began to rotate. This flailing material was found to have struck the spray guard at the rear of the auxiliary spar below the inboard ground spoiler a number of times and destroyed it as well as causing significant damage to the local auxiliary wing spar structure and fracturing hydraulic pipes which led to the failure of the two hydraulic systems. The same flailing material was also found to have fractured the flap drive torque tube, damaged a major wiring loom and caused metallic debris to be forced between and into contact with the two cables driving the left aileron.
The locked condition of the wheel did not appear to have been the result of high hydraulic pressure being supplied to the associated brake. There was also no evidence of brake unit damage or malfunction and the locked wheel was subsequently attributed to the vulnerability of the aircraft carbon brakes to water ingress and retention when parked.
The Investigating Agency was concerned at the extent and potential effects of the secondary damage that had occurred to flying controls, hydraulic services and electrical conductors which it was considered were sufficient to cause reduction or total loss of control either before or after takeoff as well as during landing. This view was noted to be contrary to the corresponding analysis of this type of failure which indicated that the aircraft would remain controllable. The concern was present in the light of the catastrophic failure in a cross-ply tyre which initiated the fatal accident to a Concorde aircraft departing Paris CDG on 25 July 2000 (see: CONC, vicinity Paris Charles de Gaulle France, 2000). That investigation drew attention to considerable differences between possible tyre failure modes in practice and those assumed for aircraft certification purposes. It was additionally noted that, since the kinetic energy imparted by flailing tyre carcass sections to any aircraft components within the radius of flail is a function of speed, then should such tyre damage occur at the higher runway speeds associated with take off, the resulting secondary damage could be very much greater than that actually experienced in the accident being investigated. In respect of the applicable aspects of tyre certification, it was observed that:
"The European Aviation Safety Agency (EASA) certification rules dealing with consequences of tyre failure apply to a small section of the thin, relatively low strength tread material dis-bonding from an otherwise intact carcass. Failures arising from slide through tyre ruptures and from lateral cutting inflicted by debris can involve partial or complete separation of large sections of total carcass thickness, incorporating substantial portions of sidewall. The flailing section therefore has considerable mass and is reinforced by the chords of the tyre carcass. It will thus inflict greater damage at a given speed than that considered in the certification assumptions. The failure on (the accident aircraft) also demonstrates the greater vertical distance into the wing structure to which damage can be inflicted in practice, compared with the situation assumed by the certification rules.”
It was concluded that the absence of a runway excursion outcome was largely attributable to the generous length of the landing runway for the aircraft type and to the fact that it was dry.
In the light of the findings and analysis of the Investigation, four Safety Recommendations were made:
- It is recommended that Bombardier introduce modifications to the BD700 to reduce the extent of concentrations of water pouring onto the outboard faces of the inboard main-wheel tyres and then onto the brakes when the aircraft is parked in rain.
- It is recommended that Bombardier either:
(a) Develop and implement modifications to the BD700 to effectively shield vulnerable flight critical hydraulic, electrical and mechanical systems in the vicinity of the main-wheel tyres against damage inflicted by items of large, full thickness, high velocity flailing tyre material and / or re-route some systems to minimise vulnerability to such events, or alternatively:
(b) Develop and require fitment to the BD700 and other Bombardier aircraft with similar features, a type of tyre that does not have such a flailing failure mode.
- It is recommended that the Federal Aviation Administration, the European Aviation Safety Agency and Transport Canada raise awareness of the vulnerability of carbon brakes to freezing in flight following exposure to moisture on the ground, emphasising the significance of the slow drying rate of saturated brakes even in warm, low humidity conditions.
- It is recommended that the Federal Aviation Administration and the European Aviation Safety Agency review the certification requirements for automatically stopping flight recorders within 10 minutes after a crash impact, with a view to including a specific reference prohibiting the use of ‘g’ switches as a means of compliance as recommended in ED112 issued by EUROCAE Working Group 50.
The Final Report was published on 4 December 2008 and may be seen in full at SKYbrary bookshelf: AAIB Bulletin: 12/2008 EW/C2008/01/03