Runway part dry, part wet

[Final 3 Greens]

Question from a curious PPL, would be most gratefulf for any professional replies.

I went to Malta recently as pax and we landed on dry pavement.

There was a very localised and sharp shower half way down and the runway became very wet.

No I've landed on dry runways and wet runways in a light aeroplane, but never one that went from dry to soaked \.

I just wondered how often this happens when flying the line and if it raises any particular issues?

[chiglet]

Runway state is given in the [D]ATIS in "thirds". eg, wet wet wet
If the runway is dry, then no state is given, but if one [or two] third[s] are affected, then that state is given. eg dry damp wet
Hope this helps
watp,iktch

[TheOddOne]

As Chiglet says, runway surface condition is reported in thirds. The surface condtions are reported as 'dry, damp, wet, water patches, flooded etc...' Anything worse than 'wet' is considered to be a contaminated surface and imposes severe operating limitations, if permitted at all.

Each operator and aircraft manufacturer will have their own rules as to how to respond to this; some will say that any third reported as 'wet' requires an adjustment of braking setting accordingly both for arrival and rejected take-off. It may also impose greater cross-wind limitations, too.

Any respectable length of runway is quite likely to have different conditions along its length in showery conditions; we often report 'dry, damp, wet' or any combination you like! Fortunately our cambered grooved asphalt surface drains extremely well and we never go beyond 'wet, wet, wet' even in a summer downpour.

Some runways are notified as 'liable to be slippery when wet'; quite an embarrasment, especially when you've just spent millions on having it resurfaced.

The Odd One.

[Old Smokey]

I just wondered how often this happens when flying the line and if it raises any particular issues?

It happens quite often.

If a runway surface is a composite of various conditions, the worst condition must be assumed for Landing / Takeoff performance. I haven't seen a chart yet that dealt with Dry/Wet/Icy for example.

Use the worst condition.

[pipertommy]

It is also worth remembering that the runway is normally different material to stands/parking area, ie the stand may well be nearly dry but the runway could wet/damp.This is due to the fact the runway holds its water contents slightly longer,since it allows water to soak in,to reduce standing water.

[Spitoon]

I'm not sure how widely the 'thirds' reporting is - it's done in the UK but, as I recall, ICAO only stipulates a single report for the whole runway.

[Airbus Girl]

I've had a dry/dry/wet runway. The main consideration is if you are taking off, then the wet part is the bit you'll be stopping on if you have a late rejected take-off. So I'd use Wet figures, which are adjusted to allow for the reduced braking in such conditions (eg. lower V1 speed).

[Final 3 Greens]

Thanks for the interesting replies, which are mcuh appreciated.

[VRThomas]

Here is a point worth considering:

If you are landing on a dry/wet/wet then your hydroplaning speed will be different due to the fact that your wheels will have spun up on the dry surface prior to reaching the wet.

Therefore is the wet landing data required?

VRT

[OverRun]

Final 3 Greens has raised a good point, which is not very well covered in our technologies.

There was a very localised and sharp shower half way down and the runway became very wet.

The effect of a short sharp shower flooding the runway can be more severe than designers take into account.

Benedetto has done some very interesting research (PM me for a copy) on the safety of airport runways in the case of heavy rainstorms. The runway design and, I'm guessing most of aircraft/tyre design, assumes a wet runway is a steady-state condition with a film of water on the surface. As an example, the wet runway friction testing is done with a steady state 1mm thick film of the water on the surface. This covers many wet runways, but doesn't address the case of a sharp downburst of rain such as in a thunderstorm, which briefly floods the runway. The depth of water on the runway can rapidly increase for a few minutes until the drainage flow pattern is established. Instead of a tame 1mm film of water, you can have a film of 5mm or more.

Conventional hydrological analysis (the engineering science of water and rain) looks at a short period of rain as being typically 1 hour. The short rainfalls are typically more intense than the longer rainfalls. There is a critical duration Dc for the rainstorm that produces the deepest water film on a runway. In fact for D < Dc, it rains for too short a time to produce a deep wet film, and for D > Dc it does not rain so heavily. Benedotto shows that the very short rainstorms (duration D = about 5 minutes) are the most critical. That ties up with the "short sharp shower" concept.

The ICAO wet runway requirements go a good way to protecting aircraft against this problem (and there have been many years of successful wet weather landings validating their approach). ICAO has two requirements: adequate wet weather friction AND adequate macrotexture. With a short sharp shower, macrotexture is the more important. This is the roughness/voids between the stones of the surfacing that permit water to escape from between tyres and the pavement surface.

The ICAO requirement is for a minimum of 1mm texture for the runway, and, if the runway is up to standard, this still provides protection for deeper water films. As a rough guide, if the runway is asphalt grooved OR open graded friction course asphalt OR chip seal OR treated concrete, then the texture is greater than 1mm. If it is smooth asphalt, then it is less than 1mm.

Benedotto found that provided the macrotexture is greater than 1mm, then even though the water film gets deeper, the skid resistance of a pavement only decreases slowly. His modelling (and this may not be exactly what you get on your runway) shows that the skid resistance of a runway (with a texture greater than 1mm) with a film of water of 5mm, drops to slightly less than half what is was with a film of 1mm. His model gives theoretical dry condition SN = 65, 1mm film SN= 42, and 5mm film SN = 20. Translating that into typical runway braking action ratings for a runway with a texture greater than 1mm is:

runway dry = good
runway wet with normal rain giving 1mm film of water = fair
runway wet with sharp downpour giving 5mm film of water = poor

For a smooth runway, the skid resistance strongly decreases and my interpolation is:

runway dry = good
runway wet with normal rain giving 1mm film of water = poor
runway wet with sharp downpour giving 5mm film of water = nil

So Final 3 Greens, the answer to your question of:
I went to Malta recently as pax and we landed on dry pavement. There was a very localised and sharp shower half way down and the runway became very wet . . . if it raises any particular issues?

If the runway was skid resistant (texture greater than 1mm), the runway braking action in the wet section due to the short sharp shower would drop through fair to poor (even though it was supposedly skid resistant). All of the length penalties associated with smooth (non-grooved) runways should be applied, even though the runway is theoretically skid-resistant.

If the runway was NOT skid resistant, the runway braking action in the wet section due to the short sharp shower could drop to nil. If you have slowed below aquaplaning speed by the time you hit it, then you should be OK. If you are landing hot and long, go around. If you have already landed and are still going fast when you hit the water, then conditions will be rather slippery. If you are lined up for takeoff, then a short hold 'as the storm passes' might be prudent.