Welcome to Bangkok.
Analysis of Qantas QF1 overrun at Bangkok This is a really interesting analysis by pilots
Aquaplaning is a long-standing interest of mine, ever since I was friction testing some of Australia's runways in the 1970s with the venerable Mu-Meter (BTW enough time has passed to allow me to tell some of the tales of that testing, but not in this forum).
The lost science of aquaplaning
But the science of aquaplaning is something of a lost art. In the last decade, there has been something of a spotty record for Australian airports friction testing their runways. At one airport, I regularly perform texture measurements on the runway which has given them an engineering understanding of aquaplaning potential, and then back that up with friction testing when the equipment is available.
This page draws together aquaplaning experiences from airports, roads and pilots. In time, with enough inputs, we will get a better understanding of the phenomenon. But first, who else thinks aquaplaning a bit of a "lost art" ?
From Pprune (12/1999)
This (aquaplaning) was a major topic when jets were first operated in Australia on
relatively short runways by today's standard and before runway grooving was
introduced. My airline promulgated a large amount of info on the subject
and there were some training films showing actual flooded runway tests
which are most probably gathering dust in a training library somewhere. The
formulas were quoted then and were well known by pilots of that era. Great
emphasis was placed on the correct technique for operations on wet runways
and was a required every time question on a line check with particular
reference to correct control during the ground roll. Of course no one
deliberately chooses to land on a flooded runway but it may be unexpectedly
encountered or may be unavoidable. It seems that with the advent of runway
grooving that this knowledge has receded in memory. Infact it is still
valid and should still be emphasised to newer pilots as, when there is
sufficient water on a runway, the grooving is rendered ineffective.
Road engineering approach to aquaplaning
For aquaplaning, there is a gap in theory and practice, and another gap between the pilots and the pavement engineers that build and maintain the runways. Let me start by giving you the pavement engineers viewpoint: there are two components that affect stopping distance: the friction and the macrotexture.
FRICTION Good friction is associated with good microtexture. Good dry friction means that you can stop in dry conditions:
a dry skid pan often has good dry friction, because it is very smooth so there is maximum tyre/surfacing contact area, and the stones in the surfacing have not been polished by lots and lots of traffic.
of course a wet skid pan is slippery because it is so smooth.
cobble stones have poor dry friction because there is only limited tyre/surfacing contact and the cobble stones have poor microtexture (they are smooth). To make matters worse, cobble stones are not just smooth, they are often polished by centuries of traffic as well, which makes them even more slippery. Good wet friction means that you can stop in wet conditions. This has two parts - the microtexture and the macrotexture.
The microtexture (and polishing) are relevant for very thin films of water (say less than 0.1 mm).
For highways, with high traffic volumes and lower speeds, it is an issue.
But for aviation, the type of surfacings used on runways are specified to met various strength standards, and in that process, poor microtexture and polishing issues get dealt with quite effectively. So I would not expect to see microtexture as an issue for aviation.
Macrotexture is relevant for thicker films of water. Anytime someone reports standing water on the runway, or you see a plane throwing gusts of spray, I believe that there are thick films of water present, and macrotexture is the more important. This is the main problem in aviation (particularly for commercial jets), and I will deal with it as a separate section below.
MACROTEXTURE Good macrotexture means that the surface is rough enough, or has enough holes or grooves in it, that water can be easily pushed away by the tyre at low to medium speeds, and then the tyre can contact the surface and can be braked. Poor macrotexture means that the surface is smooth, and above slow speeds, the tyre easily rides up onto the water and aquaplanes.
Concrete or rigid roads and runways (the white ones) inherently have poor macrotexture, and grooving is used extensively on concrete runways to provide macrotexture.
Bitumen or bituminous or flexible or asphalt or asphalt concrete or sealed roads and runways (the black ones) often have fair macrotexture. Grooving is often used on flexible runways for large commercial jets, which have asphalt surfacings..
Macrotexture is gradually lost with time due to stone wear. Grooving wears out in time.
Macrotexture is rapidly lost with rubber build-up in the touchdown areas. Grooving fills up in time.
A very few authorities use special bitumen surfacings (known as porous asphalt) to give good macrotexture without grooving.
A sealed road or runway surface (which is typically only found in Australia, New Zealand and South Africa, common on roads but not common on runways taking larger than 737 aircraft) has good macrotexture.
How do we test for skid resistance on the road? The pavement engineers on roads are concerned mainly with low speed traffic (up to 60 km/h). They are especially concerned about British roundabouts, because that is where the British motorist slides off most.
So they check the dry and wet friction of the road using a little pendulum mounted rubber block which swings down from a height of 300mm and 'skids' over the surface, reporting a friction coefficient. This is the British Pendulum Tester. There is no specific provision for flooding the surface, and the block is too small and far too slow to detect aquaplaning anyway. Make no mistake, it is an effective tool for roads though. The British are good engineers!
The British have one full scale friction testing machine - the SCRIM - which is optimised for testing roundabouts at 40 to 60 km/h. It does a good job. Anytime I see someone driving their 747 round and round in tight circles at 40-60 km/h, I'll recommend the SCRIM for their use. However it is not especially relevant to an aircraft in a RTO (rejected takeoff) at 400 tonnes and 140 knots.
There are other full scale friction testing machines, but many of these operate at the lower speeds (below 80 km/h), where macrotexture is not effectively measured.
There are a very few machines that potentially could measure friction at high speed on wetted pavements using locked wheels (MuMeter, SAAB skiddometer, griptester). In practice, most authorities only run them at 65 km/hr, with the water pumps set to deliver a maximum water depth of 1mm. Unfortunately that doesn't tell enough about macrotexture at high speed and deeper water.
ICAO recommend testing at 65 km/h and 95 km/h. What I know from personal experience with the old DCA MuMeters in Australia is that once you test friction at above 80 km/hr, the test results are quite different to those below 80 km/hr. I believe that it is only above 80km/hr that you measure high speed wet friction with the macrotexture component included.
(sometimes the reason that these machines only test at slow speed in practice is that the airport staff and pavement engineers who drive them have something of a track record of running off the end at high speed and wrecking them, especially in testing during real rain conditions. Such as the fence (Mr U T), the ditch (Mr S E), and the harbour (HK), to name but a few. Having said that, the SAAB machines in particular have a very good safety record. Of course it all goes to show that travelling down a flooded slippery runway at high speed is a dangerous occupation. Perhaps that's why the tests should be done in the first place).
These various machines report various measurements, which are roughly correlated with both each other and with friction coefficient.
The other approach to aquaplaning is taken by the road engineers who are concerned about splash and spray in the wet. The French are particularly keen on this, doubtless from their autoroute experience, and have done some very good work. The latest International Friction Index (IFI) links both friction and texture, and is, I believe, a real step forward. The French and the IFI measure the macrotexture of the surfacing in terms of millimetres of texture depth, and this measurement is directly relevant to aquaplaning. The more the texture depth, the more room there is for water pressure to dissipate away from the tyre, and allow tyre-surfacing contact. There are some standards set, but not many because the road authorities are worried about liability issues which could arise from roads with inadequate texture and skid resistance. The shape of the surfacing (depressions) and any wheel ruts also come into this to an extent, because standing water in puddles can cause aquaplaning problems.
So to sum up road engineers, they measure mainly slow speed friction coefficient. A few measure texture, and the new IFI is a good move forward. Pilots however are concerned with braking coefficient at high speeds. They want to look at high speeds and real world standing water conditions, plus the odd tropical thunderstorm thrown in for good measure.