Slicks vs. Snow Tires -- maybe even Studs

[RatherBeFlying]

All these recent overruns have turned [slid] on where the rubber [did not] meet the road [runway].

It seems the closest tread pattern to a/c tires on the automotive side are F1 racing slicks, but they're not used in wet conditions.

Maybe it's time to see if an automotive snow/rain tread would help. The smaller regional a/c without reverse would be a good place to start.

However tread wear could be $eriou$ly expen$ive.

Studs would last longer provided they stay on the tire and don't make holes in the airframe, but they do take a bit away from braking on dry pavement.

Also many flights go between snowy and tropic locales; so, which tire fit would be right in that case?

Or would we be better off providing decent overrun areas with EMAS where space is limited?

[Cardinal]

Or would we be better off providing decent overrun areas with EMAS where space is limited?


We have a winner!

[cdb]

Studs? You must be kidding, right. Unless you want your landing fees to treble because we'll have to get the runway resurfaced every year!

And as runway overrun areas, UK runways have whats' called a RESA, a runway end safety area used to minimise the damage due to overruns. I don't know about other countries. The limiting distance tends to be space available, so if the choice is between, say:

2000m runway and 500m RESA or
2400m runway and 100m RESA

which would you rather have? The runway can be used every day, and will still help you stop in bad conditions.

[green granite]

Stupid question; why dont we have under runway heating? I believe some
motorways have it (I know it would mean shuting the runway to do it and
cause a lot of disruption)

[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.

[RatherBeFlying]

OverRun, I much appreciate the contribution of your thoughtful comments. However 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. does not fit the studded tire case.

A Washington State review http://www.wsdot.wa.gov/winter/studtire/Studded_Tire_Report_Final_Nov_2002.pdf shows little degradation of braking distances on dry pavement.

The interesting part in this study is that ice near the freezing point is considerably more
slippery than ice well below freezing.

Airport authorities would do well to be scrupulous about snow/ice removal when the temperature is near freezing -- also easier to remove.

Pilots should be especially wary of contamination in the +3/-3C temperature band.

I recall a CP flight YUL-YYZ where we had several delays because of runway conditions at YYZ. It eventually turned out that the 737 we were in was the friction tester for the following DC-8. The main gear was thumped on and aggressive braking (with a few swerves) followed quickly.

If the runway had not passed approval, we would have spent the night in YQT.

I can't say the YYZ runway maintenance folks are more conscientious than those at MDW as they do have something like twice the runway length to absorb shortfalls.

[OverRun]

Ooops - I had wrongly considered just the steel/rubber coefficient without allowing for rubber/rubber contact. As the [excellent link to the] Washington State (WS) review shows, studded and unstudded tyres are not dissimilar in braking in dry weather on asphalt.

Differences between the two appear to have reduced over the years because, the report says, regulations around the world have limited the aggressiveness of the studs and because tyre technology has improved the frictional characteristics of newer, studless winter tires.
WS reports that studded tires on dry/wet concrete provide less traction than non-studded tires. This is apparently likely because the studs cannot penetrate the harder roadway surface, which actually lowers the effective coefficient of friction, in much the same way as studded tires lose effectiveness on ice at lower temperatures.

WS report on the 1998 Norwegian meta-analysis on the effect of studded tyres on accidents (meta-analysis is a fancy word for intelligently pulling together the results of a bunch of earlier studies) which concluded that the use of studded tires improves road safety by reducing the accident rate, but the effect is quite small, on the order of 1 to 10 percent. The benefit undoubtedly comes in snow/ice, but modern tyre technology for winter tyres is catching up. Maybe there will be winter tyres for aircraft one day?

I think that some more differences between runways and roads emerge as the discussion progresses, which add to the investigations which would be needed for aircraft studded tyres. The Alaska trials reported in WS were very low speed trials, and I don't know what the difference in friction between studded and unstudded tyres might be at higher speeds. At higher speeds, the time for the stud to "penetrate" the surfacing and grip is less. Perhaps, in the interest of science, someone can run their studded tyre car up to 120 knots (216 kph) and slam on the brakes.

The aircraft tyre load is much higher too. Car loading is typically 0.5-1 tonne per tyre and an aircraft loading is typically 18 tonnes per tyre (737-800) to 24 tonnes per tyre (B747). That might dramatically accelerate the wear on the surfacing and the studs.

Finally there were some new hazards identified in the WS report that result from pavement rutting caused by accelerated wear from studded tire use. First, rutting can cause “tramlining” which adversely affects the directional controllability of a car. However my experience with tramlining on construction joints of asphalt overlays on runways and freeways suggests that this is less of a problem for aircraft than cars (due to more mass and larger tyres). Secondly, when water is present, the rutting allows standing water to accumulate in wheel troughs, thereby raising the potential for hydroplaning. Research shows us that a shallow rut in a runway will allow water to pond. The depth of rut at which ponding starts depends on the runway cross-fall and longitudinal gradient. Since the rut created by studded tyre wear will be quite narrow, the depth of rut for the onset of ponding might be as little as 3-4mm for a 1.5% crossfall runway. This is not particularly high wear. Get another few mm of wear, and the pond standing water can get up to 3mm.

The FOD issue I noted in my first comment still remains, and the WS report notes the problem is a general one. With the extra weight on aircraft tyres noted above, the FOD from surfacing wear on runways remains an important aspect to be considered.

Having said all that, the gap between thought and possibility is closing. Over to any tyre engineers for comments on the tread, compound, life-cycle, repair, weight/speed limit, and cost.