My company Ops Manual defines dry, wet and damp runway conditions, yet there is only performance data for dry or wet.
1) So what is the purpose of defining damp?
2) Legally, which data should we use if damp? After all, damp is clearly not wet, but then it is clearly not dry either!
3) Further, I thought that if a runway was grooved it could not be defined as wet, only dry or contaminated. So are we really justified in using dry performance data on a soaked grooved runway?
Although I doubt the company would object, would we be justified in declining a departure on a soaked grooved runway on the basis that it was actually wet (as in reality it is) instead of using dry data cos it is grooved, and thus cannot be "wet"?
Damp runway A runway is considered damp when the surface is not dry, but when the water does not give it a shiny appearance. There are no performance corrections required for a damp runway. Therefore u use dry runway date.
I guess it menstions the definition of damp, so that u can distinguish it fom being wet.
Grooved is considered dry.. but what crosswind limitations do u use ?? i believe u use dry crosswind limitaions, though some might argue that the grooves primarily give grip in the direction of the runway only. Therefore when side slipping (appologies cant think of the proper term), the runway is acting as if it is wet. i dont know about that, since if u are aborting, and the surface in the direction of the runway acts as if its dry, then with the bettter traction you wont be susceptiple to the side slippage of wet runway.....
Let's approach the issue of dry, wet and damp runway conditions from the pavement engineering perspective. Dry runway means that the dry weather friction can be expected (which is usually good). Damp runway means that the wet weather friction can be expected (which can range from fair to poor; it is not usually poor and would only be so on greasy or polished stone surfaces which are very rare on runways). Wet runway means that the wet weather friction can be expected plus there can be standing water (friction can range from fair to very poor; it can easily be poor or very poor because standing water can lead to aquaplaning).
Now the normal runway micro surface (that is the surface seen from very close-up) should be a little bit rough, just as it should be on any road. ICAO requires a minimum texture for a new surface of 1mm. In fact, as I tell my students of pavement engineering, if you fall off your bicycle, you should skin your knee. This means that the surface has some texture. The texture enables the stones imbedded in the surface to stick up above any water, and small rain showers or very thin sheets of standing water can dissipate between the stones as the tyre rolls over them, and this ensures that no aquaplaning occurs unless there is lots of standing water.
Most [black coloured] runways are surfaced with asphalt (asphaltic concrete for the Americans and bituminous concrete for the British). This typically has a very smooth surface, and the ungrooved texture is low/poor. I measured a pretty smooth asphalt surfaced carpark the other day to test a new texture measuring machine, and got 0.4mm. Normal asphalt texture varies but an average could be 0.6mm. If you fall off your bicycle, you won't skin your knee. Small rain showers or very thin sheets of standing water cannot dissipate between the stones because the surface is so smooth that it doesn't have any stones sticking up. Asphalt needs grooving. The few runways that are "sealed" or "chip sealed" are also black and have a rough texture (typically 1.5mm) and you will skin your knee if you fall off your bicycle. Sometimes the airport will put a chip seal (aka surface dressing) on top of the asphalt to get the same effect.
Grooving is an artificial way of restoring texture to asphalt, and it is a Good Thing. It gives the gaps that the water can dissipate into as the tyre rolls over the asphalt. A grooved runway is restored to being equivalent in surface texture to a surface that is a little bit rough. The ICAO groove spacings of 3mm x 3mm @ 25mm centres adds 0.4mm to the asphalt texture of say 0.6mm, which gives 1.0mm. The "big tropical" grooves of 6mm x 6mm @ 31mm centres add 1.1mm texture, giving typically 1.1+0.6=1.7mm. An asphalt runway that is not grooved is a) a lot cheaper to build, and b) plain dangerous. You'll sometimes see a few around the place, disappearing rapidly under your wing as you frantically try and stop in the wet. The other treatment you may see, which is equivalent in terms of restoring texture to grooving, is "porous", and it is also a Good Thing.
For example, look at Perth, Australia - the 06/24 and 03/21 runways are respectively porous and grooved (well done Torb). This simply means that they are now up to normal surfacing standards in terms of texture. Look at Cairns, Australia - the 12/30 and 15/33 runways are respectively sealed and grooved. This means that 12/30 is already at normal texture surfacing standards, and 15/33 is up to normal surfacing standards (well done Andy).
Grooving won't cope with standing water due to ruts and birdbaths in the runway (common in worn-out runways). Grooving won't cope with deep standing water due to heavy rainstorms (hint: equatorial). Grooving won't cope with standing water due to all the grooves and texture being filled up with rubber (hint: Bangkok).
If you can see standing water (puddles or sheets of water glistening) as you line up, then you should be concerned regardless of whether the book says the runway has been grooved or not.
In conclusion, grooved asphalt should be treated as a normal runway surface, and normal limits applied (wet/dry etc) without further regard to whether it is grooved or not. Ungrooved asphalt or "not porous" asphalt should be considered as sub-standard and treated with caution. By the way, the groves give no grip at all in themselves, and I cannot see why any crosswind limitations should be changed on grooved runways.
[This message has been edited by OverRun (edited 10 April 2001).]
Just a quick PCN question if I may. I presume that the significance of PCN for pavement strength and life is predicated on the actual landing impact of the aircraft as well as static load bearing - if true, how much lower could the PCN of the taxiways and parking areas be (relatively) before one should become concerned about both short and longer term damage?
4dogs, as you've said, the PCN (pavement strength) is predicated on static bearing capacity alone.
However not so at the time of landing, since the maximum downwards force on landing is desirably close to zero. That is because just at touchdown, almost all the lift should still be taken by the wings, apart from occasional Heavy Landing. Longitudinal and lateral forces due to tyre spinup exist but are not calculated. Instead the type of surfacing (asphalt, concrete, chip seal) is chosen according to the size and frequency of aircraft and based on experience. The effects of dynamic loading are not computed for aircraft pavements yet because we're still learning how to - give it a few years.
The maximum effect of the aircraft on the pavement is when it is stopped. This is mainly because asphalt is responsive to the time of loading - if it is loaded/unloaded quickly (like in the fast lane of a road) it is elastic in nature and rebounds quickly. If it is loaded/unloaded slowly (like on an apron) it is more viscous in nature and it can flow. That is why most of the surfacing damage, ruts and bumps appear on aprons.
Some engineers will take that into account, and use concrete for aprons and runway ends, with asphalt on the rest of the runway and on taxiways.
All engineers look at the expected frequency and weight in choosing pavement thickness. Design wise - pavements for departing aircraft on aprons, holding areas and the centre of taxiways and runways are full design thickness. Pavements for [lighter] arriving aircraft such as high speed taxiway turn-offs are 90% of full depth, and pavements in areas of little traffic such as the outer edges of taxiways and runways are 70% depth.
Lower PCN for parking areas? Well if it is the normal apron, it will get as much traffic as the runway and it should be the same as the runway or higher. It will have as many heavy [full of fuel and pax] aircraft as light [empty] aircraft.
If it is an infrequently used parking area, then some overload should be permitted. Anything from 10% to 30%. If the aircraft is restricted to using the apron when empty/light, it can use a weaker apron. We've parked impossibly big aircraft on near-rubbish by requiring them to park empty and refuel/load on another area.
As an example, a 737-400 is PCN 37 full and PCN 17 when totally empty (flexible pavement, B subgrade strength). So it could park and load on an apron of say PCN 28 or higher if this was a once-off, and it could park on apron of PCN 17-20 if it was required to park empty, and refuel and load elsewhere. Hey, and if it gets stuck, we'd just hook up a couple of the big airport fire engines - they'll pull anything out.
Mr Moto "My company says …grooved is treated as Dry" I think that your company may need to re-examine its treatment of grooved runways, because their texture depth is no better than that of most roads. And if highway authorities treat the wet weather characteristics of a surface differently to the dry weather characteristics, perhaps your company should too.
[This message has been edited by OverRun (edited 13 April 2001).]
as regards grooved... while absolutely not doubting you, JAR ops subpart F, 1.480 (5) states that " A dry runway is one that is ......specially prepared with grooves or porous pavments, and MAINTAINED to retain EFFECTIVE DRY braking. The later line being the key to it i guess.
appologies for resorting to quoting scripture (i mean JAR OPS), but your understanding of these matters far outweighs anyone elses
just as an interesting, yet related aside, the speed at which a tyre will commence aquaplaning is 7 times the square root of the tyre pressure. useful stuff eh? There are also different types of aquaplaning as well, one in which the water forms a "mini bow wave" in front of the approaching tyre, thus slowing it down, and degrading grip, and the other where water is compressed between the tyre surface and the friction surface of the runway, and degrading efficiency. I would imagine that this is where a grooved runway will assist in offering a reduction in the water film, leading to extra grip, and therefore extra retarding effect from the brakes.
The JAR definitions of Snow, and other contaminants is equally arcane!
Just to add to this interesting discussion, did anyone else hear the STN ATIS the other day, giving the runway as "dry, damp, wet". That made us think about what figures to use! It somehow seemed a bit daft but also do you use dry or wet figures? Assuming the point of wet figures is for the engine failure at V1 and stop scenario, then based on the fact that you would be starting to stop on the damp bit then running into the wet bit I reckon that wet figures would be the most prudent. But as we'd never come across it before, any other input would be welcome.