Slicks vs. Snow Tires -- maybe even Studs

[OverRun]

RatherBeFlying
The concept of using studded tyres for aircraft raises some issues that would need to be addressed before they could be introduced. I don’t want to appear to be negative to new concepts, so let me express these as design challenges to be solved rather than a rebuttal of a new idea.

With studded tyre design, the rubber tread blocks simply serve to hold in place the studs that are entrusted with the job of ensuring grip. The studs work on the same principle as an ice axe - they bite into a very small surface and anchor themselves in thus transmitting acceleration/braking/sideways forces to the ground. To do this, they generate as much pressure on the contact patch as possible. The two issues that follow from this high pressure/small contact area concept that are worrisome for aircraft operations are foreign object damage (FOD) implications, and dry weather skid resistance.

On the FOD side, the mechanism of road surfacing wear from studded tyres can be applied to asphalt (black) runways (and partially applies to concrete or white runways):
1. The scraping action of the stud produces marks of wear on the mastic (which is the bitumen + sand that is the black stuff holding the stones together in the asphalt/asphaltic concrete/bituminous concrete),
2. The aggregate (stone) works loose from the pavement surface as a result of the scraping by studs leading to FOD,
3. Also a stone can be smashed by the impact of a stud and the pieces loosened by the scraping action of the stud leading to FOD.

The FOD problem would be random in its generation and it might be difficult to ensure that FOD is picked up by runway vacuum sweepers rather than engines. FOD is a difficult thing to deal with in risk management because the probability of occurrence is small and the consequences can be extreme. If the seriousness of risk is simplistically take as probability X consequence, even a small increase in the probability of occurrence can escalate the seriousness of the risk dramatically. Most airport engineers would be very nervous about allowing studded tyres because of known increase in FOD.

Edited and stuck into italics since the assumption in this paragraph is incorrect - see the comment by RatherBeFlying below On the dry weather skid resistance side, the studs offer much less friction than the rubber tyre tread. The friction coefficient for a steel wheel roller/asphalt is 0.14 compared to the usual aircraft rubber/dry asphalt friction coefficient of 0.5-0.8. Not so That effectively gives studs on a dry asphalt runway the same sort of friction coefficient that tyres have on a wet runway, or even worse, tyres have on a contaminated runway. At the very least, the use of studded tyres would need the usual wet runway factor of 15% added to the landing distances, and I suspect it the penalty may even need to be more. Plus I vaguely recollect that contaminated runway ops have other limitations such as crosswinds.

So at this stage the bag of engineering solutions to overruns contains:
- aircraft performance certification with factors for slippery runways,
- minimum runway surface texture (leading to grooving or porous asphalt/ friction course),
- runway friction testing which at last (thanks to Tom Yager) is coming of age and starting to yield more accurate reproducible results. However as busy as the engineers are in making friction testing better, the lawyers are busier making it worthless by limiting the friction information that is given out operationally (and the lawyers get paid more),
- the bit of the runway strip past the end of the runway (usually 60 metres) is the first extra bit of overrun distance and has been there for decades,
- RESAs (anything from 30m to 90m to 240m to 300m) are a more recent worldwide/ICAO requirement. It is likely that the actual RESA length at airports will be extended in length in the years to come, but resistance by existing airports with not enough space will mean that this is a slow process.
- EMAS (very expensive, rarely used, and not a good solution to the 50-odd percent of overruns that inconveniently leave the runway other than along the centreline).

The bag of engineering solutions to overruns is clearly not enough, and the problem and solutions will remain topical for years to come. However the solutions to overruns are not all engineering, and we need to move forward on several fronts. Using a benefit/cost philosophy, we are at the point in engineering where the next incremental benefit is going to require a huge cost. I think we can get an equal or higher benefit at a lower cost by widening the focus to include ATC, weather radar, telling the truth about the actual runway conditions, cleaning the blo&dy runway which it gets rubber contaminated, and putting airline accountants/management under the spotlight (with especial attention to low cost airline operating philosophies).

We might even find a way of using studded tyres or runway heaters.