A friction tester is towed down a runway it is wetting itself, at sixty five kilometres an hour, and it reports a number between zero and one. Nothing wet reads above about zero point eight. Between the twelve hundred and the thirteen hundred metre markers we read zero point nine four.
I am an airfield operations officer and one of my jobs is the friction survey.
The machine, because nothing in this makes sense without it.
It is a trailer towed behind a van, or on ours a unit built into the van itself. It carries its own water and lays a film one millimetre deep in front of a measuring wheel, and that wheel is geared to turn slightly slower than the road speed so it is always slipping a fixed amount. You measure the force it takes to drag it and you divide by the load on it, and that is your friction coefficient. Mu.
You run it at sixty five and sometimes at ninety five, three metres either side of the centreline, and you get a value every ten metres. The runway is reported in thirds.
There are numbers attached. A new surface gives about zero point eight five dry and somewhere in the seventies wet. There is a maintenance planning level where you start scheduling work and a minimum level where you tell the airlines, and below that the runway is not a runway any more.
I have done this survey every two months for eleven years and I have done about four hundred of them.
Between twelve hundred and thirteen hundred metres on runway 26 the wet run reads zero point nine four.
That is ninety metres of it. Nine consecutive readings, sometimes ten.
Zero point nine four is not a good wet number. It is not a wet number at all. A dry runway in new condition does not give you that.
The dry readings on that stretch are zero point eight five.
I want that in early because it is the part people walk past. The same ninety metres, measured dry, reads what every other part of the runway reads dry.
So it is better wet than dry, and a surface cannot be that. Water between a tyre and a road is a lubricant. That is the whole of it.
What has been checked, and most of it twice.
The machine. Ours has been calibrated annually since it went in, against a reference, and the calibration certificates are in the file. I have also had two contractors bring their own units onto this runway, one in 2019 and one in 2023, on different principles, one a fixed slip and one a side force unit. Both of them found the stretch. The side force unit reported it as a rise of the same size in its own scale.
The tyre. Pressure, wear, and the date on it, because a friction tester runs a specified tyre and nothing else. Changed, and then changed for a brand new one, and run again the same afternoon.
The water. This was my own idea and it took a morning. The nozzle lays one millimetre at speed, and you verify it by catching the output into a drum for a measured time and weighing it, which we did, and it lays what it says.
The surface itself. Texture depth by the sand patch method at six points, which is the old method and the honest one: you spread a known volume of sand into a circle and measure the circle. That stretch has the same texture depth as the rest, within the repeatability of the test. The grooves are cut to the same spacing and they are not closed up. There is no rubber on it, and rubber deposits lower friction anyway, which is the wrong direction.
It is all the same asphalt, laid in the same resurfacing in 2008, by the same paver, in one pass.
The thing I found in 2022, which is worse.
The film.
I had a colleague walk behind the van with a depth gauge, which is a ruler on a foot, twenty seconds after a pass.
Everywhere on that runway, twenty seconds later, there is most of a millimetre of water sitting on it.
Over those ninety metres there is two tenths.
The water is going somewhere in twenty seconds. Not draining to the side, because the crossfall there is one and a half percent like the rest of it and the water would have to travel eleven metres. Not soaking in, because this is asphalt with a sealed surface and I have poured a litre onto it in summer and watched it sit there for six minutes.
What I do about it.
I report the numbers as measured. That is the job and it is not negotiable: the survey is the record, and the airlines and the regulator get the thirds as they come off the machine, and a high reading does not restrict anything so nothing has ever had to be explained.
I have raised it twice in writing. The first time I was told to have the machine calibrated, which I had. The second time, in 2021, to somebody senior, who wrote back that an anomalously high wet reading over a short length was most likely a surface texture variation and that as it posed no safety concern no action was required.
There is no safety concern. That is true. I have read that sentence a great many times.
The one thing I do that is not in any procedure is that I run 26 at night now, in rain, with the van lights off and a hand lamp out of the window.
In rain the runway is a sheet of water with the lights in it. Over those ninety metres it is not. It dries from the middle outwards while you watch it, in a shape that is about forty metres across at the widest and narrows to nothing at both ends, and it stays that way for as long as you stand there, and then you have to move because there is an aircraft on approach and the runway has to be clear.