Aircraft Tyres

Preamble
Prior to the ‘3-part series on F1 machines‘, we had a look at the philosophies behind F1 and Stock Cars’ tyres. So it occurred to me that a peep into the world of ‘Aircraft Tyres’ could be as interesting. I thus spoke at length with my ‘Tyre Guru’ – Mr. Sudershan S Gusain – and this is what transpired…

Today’s Aircrafts are a culmination of the state of art engineering know how of all disciplines – be it mechanical, structural, electrical or electronics. They more than ably fulfill mankind’s eternal desire to fly – literally around the world and in much less than 80-hours!

Present day commercial Jets have some unique features when it comes to their Tyres. For without their required ‘functionality’ AND ‘reliability’, they may perish at the drop of a hat – not to mention carrying hundreds of lives alongwith. Therefore, these two requirements take precedence over aesthetics.

Tread design of ACTs is ‘plain/rib’ type, since such patterns result in great ‘straight-line’ stability, smooth rolling, lower noise levels, less prone to irregular wear, along with very good ability in channeling out water on wet runways to eliminate loss of control while landing/take-off due to ‘aqua-planing’.

You may recall that this is similar to ‘wet’ F1 tyres. Besides, since Aircrafts do not zigzag or are called upon to traverse through soft ground, they don’t need to have Stock Car like tread patterns.

Tread rubbers of ACTs (as well as F1) tyres have to have great braking efficiency and high temperature withstand abilities. As a result, unlike Stock Cars, durability takes a back seat here. Besides, ACTs have also to face extreme operating conditions, such as:

1. They have to withstand very wide range of temperatures and that too within a short span of time. For example, at a cruising altitude of 10,000 meters, their ‘hold’ temperature can be low as (-) 45°C. Within half an hour or so upon ‘approach’ to landing, it changes from this to near ambient at ground level. And soon after ‘touch down’, their tread surface can rise to (+) 250°C or more! And nearly the same is true in reverse after a take off!
2. Load on an ACT is around 25-30 tons ‘per tyre’, as against a typical SUV @ ~800 kg per tyre and a typical truck @ ~ 4-6 tons per tyre.
3. Consequently, ‘inflation pressure’ in an ACT is very high. For commercial jets, it’s around 220 psi, as against 30~35 Psi of a stock car. For such reasons, ACTs (and F1’s) are filled with pure nitrogen out of necessity and this philosophy is trickling down to demanding stock car users as well.
4. The takeoff speed of an Aircraft depends on many parameters such as its ‘laden-weight’, the ambient air temp/density, wind direction/velocity etc. The take off speed of common jetliners these days is around 300 km/h and landing around 240 km/h.

Apart from the above/extreme operating conditions that ACTs have to withstand, their manufacturers have to keep their weight to minimum possible so as to minimize the aircraft’s fuel consumption. This is far from easy and one of the reasons why so few Companies around the world successfully produce ACTs to today’s demands. Because of such extreme requirements and limited tread depths, ACT tread wear’s are much faster and surprising as it may seem, it’s viable to ‘re-tread’ them today, as opposed to ‘Not Recommended’ for Stock Cars.

Accordingly, ACTs are sent for retreading after around 350 landings for Radial and 200 landings for Bias Ply tyres. On an average, a Radial ACT can be retreaded upto 3 times (350×3=1050 landings) and a Bias type upto 6 times (6×200=1200 landings). Quite like Stock Cars, Radial ACTs are preferred due to better rolling resistance, traction and the fact that they offer almost 150% higher number of landings between retreads.

However, unlike conventional retreading techniques, retreading an ACT is a highly evolved task. For example, after a retread, their balance and uniformity in all the domains has to be checked such that it conforms to the ‘original’ spec again. It’s for such reasons that leading ACT mfrs have plants dedicated just to retreads.

To conclude for present, it may interest you to know that the worlds most advanced commercial jetliner as on date, the ‘Airbus-A380’ has 20 nos tyres of size 1400×530R23/40PR in its rear under carriages and 2 nos 1270×455R22/32PR for the nose. Bridgestone Japan is the chosen vendor for these, producing them at their Plant in Kurume, Japan.
And, for a change, I’ll ask readers a question this time. Most of you may have read of ACTs bursting at take off, also. WHY? If you know the answer, e-mail it to expert@indiaautomobile.com ALONGWITH your full name and address. The correct ones received by Sunday the 16th March ’08 will be published in these columns next week i.e. Friday the 21st March ’08.