Aircraft tyres are designed to withstand extremely heavy loads for short durations, with the number of tyres required increasing with the weight of the plane in order to better distribute the weight.
Aircraft tyre tread patterns are designed to facilitate stability in high crosswind conditions, to channel water away to prevent aquaplaning, and for braking effect. Some types of nose wheel tyres include one (or two) chine moulded into the rubber on the shoulder buttresses that deflects water away during aircraft movement on a wet runway. Aircraft tyres also include heat fuses (sometimes called fusible plugs) which are designed to melt at a certain temperature in order to reduce the risk of an explosive deflation due to overheating.
There are 2 primary hazards associated with tyres:
- Deflation; the tyre deflates in a controlled manner with minimal direct consequence to other systems.
- Explosive break-up; the tyre (and sometimes the wheel holding the tyre) deflates or breaks-up in an uncontrolled manner with a significant probability of secondary damage to other unrelated systems.
These hazards are associated with 4 distinct periods of aircraft operation:
- Ground operations e.g. taxiing
- Take-off (up to gear retraction)
- Post take-off (gear stowed)
- Landing (to the end of the roll-out)
The various combinations of hazard and flight period can have markedly different influences, but all can affect operations to some degree.
- Aircraft Maintenance & Ground Manoeuvre: operating with the correct tyre pressures and maximising turn radii during ground manoeuvres can mitigate against a number of contributing factors associated with overheating and wear issues which can lead to tyre failure.
- Procedures: managing taxi patterns, reduced taxi speeds and allowing sufficient cooling time when necessary can also obviate the issues associated with overheating. In-cockpit procedures for landing can also influence the occurrence of damage to the tyres.
- Inspections: aircraft tyre inspections can identify a worn or damaged tyre that can subsequently be changed before it may explosively deflate or breaks-up, whilst airfield Foreign Object Debris inspections can reduce the possibility of undetected damage occurring after the inspection.
Taxiing: Long taxi patterns at heavy weights with tight turns will generate a lot of heat in the tyres, even if the brakes are used sparingly. This could cause the heat fuses to melt, resulting in a controlled deflation of the tyre. Dependent upon a number of factors, including the remaining number of tyres on the aircraft, there may be restrictions on further aircraft movement prior to the wheel being changed.
Take-off: High speed aborts generate a great deal of heat in both the brakes and the tyres and restrictions may need to be placed on the degree of ground movement that can be undertaken after an abort. The potential consequences of a high speed abort are, therefore, the melting of the heat fuses and the consequential impact as above.
Take-off: Explosive deflation/break-up due to e.g. FOD can have catastrophic consequences. There is a great deal of potential energy stored in the tyre/wheel assemblies and multiple, unrelated system damage should be anticipated. An immediate landing will be the priority. Post take-off: Rapid retraction of the undercarriage following a long, high speed, heavy weight taxi with immediate take-off, or multiple touch-and-goes during crew training, can lead to the tyres overheating in the wheel well. The heat fuses should prevent an explosive deflation, but it is not guaranteed. The main problem with this event is that the crew are potentially unaware that they have an issue with the undercarriage until secondary effects start to occur during the landing run.
Landing: A number of issues can arise from landing with a deflated tyre:
- Handling issues may arise during the landing roll-out phase, and in severe cases may cause the aircraft to depart the prepared surface.
- Landing with a deflated tyre will put additional strain on the remaining tyres, with an increased potential for one or more to subsequently suffer an explosive deflation due to overstress. The implication for secondary damage to the aircraft is high and the potential for rapid aircraft arrest and evacuation is enhanced.
- Foreign Object Damage (FOD) around the operating areas
- Minimal time between push-back/start-up and take-off slot time
- Lighting conditions for inspections
Plan for slower taxi speeds with gentler turning radii. If it is known that the tyres (and brakes) may be hot then the following precautions may be prudent in order to allow the components time to cool:
- Leave the gear down for a protracted period after take-off.
- If at all possible, avoid committing to a landing very soon after take-off.
- Follow the Flight Manual guidelines on cooling periods, for example after an emergency stop at whatever speed.
Ground controllers should consider where they would place an aircraft that requires a prolonged cooling period on the ground so as to minimise disruption to other traffic. Ensure adequate FOD control measures are in place and adhered to.