Runway conditions

[Flapping_Madly]

Dry, wet, very wet, icy, snow (various depths) and sleet.

Where do these sort out into order of danger to landing and departing aircraft?

I'm supposed to land tomorrow at an airport that will be subject to many hours of sleet. Is sleet stiff water or soggy ice as far as planes are concerned?

Thanks.:8

[Yellow Sun]

Flapping_Madly

Is sleet stiff water or soggy ice as far as planes are concerned?



That's not bad, I haven't heard that one before! You have in fact asked a quite complex question in a short post. I shall try my best to give a shortish answer. Aircraft are designed to fly, on the ground their handling characteristics are fairly basic, put them on a slippery surface and they rapidly deteriorate. For this reason quite small amounts of "contaminant" can start to cause problems, 3mm of standing water is about where "wet" can start to become a problem.

You have to consider contamination from two perspectives, the types that retard your acceleration on takeoff and the types that reduce your stopping performance. At one extreme you could depart from a completely ice covered runway and providing you did not wish to abandon the takeoff would probably get away with it, but you would not wish to land on it. In either case no one in their right mind would even consider it. At the other end of the spectrum you might try to takeoff from a completely flooded runway, say 300mm of water. You would be unable to accelerate to takeoff speed due to the increased drag and end up in the bushes at the far end. In between these two extremes there will be a combination of conditions at which you could still safely operate, abeit with significant restrictions to weight and crosswind component. For as you have no doubt noticed all of the contaminants will have some effect upon the the runway friction and therefore on the stopping efficiency of the aircraft, so that factor must be established and allowed for.

Maybe the easiest way to "visualise" the effect of the various contaminants is to equate them to an equivalent depth of water. So, say (and these figures are not accurate but only for illustration purposes):

5mm water is equivalent to:

200mm Dry Snow
100mm Wet Snow
25mm Slush

You would then have a measure of the drag effect of the contaminant on takeoff. You would still have to account for the reduction in braking action.

Absolute limits would be dictated by a number of additional factors such as the likelihood of aquaplaning and the effect of spray on the aircarft structure.

All in all, operation from contaminated surfaces is not to be undertaken lightly and relies upon careful calculation using accurate reports of the runway conditions. I would however wish to stress what I said in the opening paragraph, quite small amounts of contam. (depth and extent) can have a very significant significant effect.

I realise that this does not really answer your question, but I hope that I have showed that it is a complex area where the penalty for getting it wrong can be disastrous.

YS

[Flapping_Madly]

Thank you for that info.

I get from what you said that landing after 5 or 6 hours of sleet will be more possible than after 5 or 6 hours of snow especially if the ground crew have fought a loosing battle.

Leaving aside the sleet turning to ice because the ground is so cold.:eek:

Oh damn. Looks like a possible divert to ---where? Who cares --just get me somewhere near and I'll improvise.

Thanks