Welcome to Bangkok.

Lost science of aquaplaning

Road engineering approach to aquaplaning

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. In fact 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.

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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:

Good wet friction means that you can stop in wet conditions. This has two parts - the microtexture and the macrotexture.

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.

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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!

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.
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