Good Vibrations

The Stripe Hog SH8000T provides both runway rubber removal and marking removal from a single chassis. This unit pictured at London Gatwick is operated by WJ Roadmarkings of Newcastle-under-Lyme.

A runway surface is one of the most overlooked yet vital components of a successful airport.  Words by Stephen Vaughan.

The Stripe Hog SH8000T provides both runway rubber removal and marking removal from a single chassis. This unit pictured at London Gatwick is operated by WJ Roadmarkings of Newcastle-under-Lyme.

The importance of maintaining the surface quality of a runway cannot be overstated.  If dirty, worn or contaminated it may cause difficulties for the pilot.  If in addition it is covered in ice, snow or rain it could prove lethal.  Low friction can result in aquaplaning and aircraft overshoots with potentially catastrophic results.  Maintaining and monitoring its quality is therefore of paramount importance.
London’s Gatwick is Europe’s leading airport for point-to-point flights.  It also possesses the world’s busiest single-use runway.  Gatwick is very much attuned to the importance of maintaining a superior surface.  Kevin O’Leary is the airport’s Airfield Engineering Manager:
“With 250,000 movements last year you can imagine how much wear and tear we get,” he says.  “The critical thing for us is to provide a high-performance surface for aircraft.  Regular monitoring of runway friction is essential in order to enable planned maintenance.  A typical Marshall Asphalt surface has large stones along with smaller micro-texture stones and bitumen tying it all together.  Over time the top layer can lose its micro-texture and become smoother, polished.  If this happens it lowers the friction coefficient and gives you poorer braking action.  Proactive preventative maintenance can keep this to a minimum.”
The Civil Aviation Authority’s CAP 683 document is entitled: ‘The Assessment of Runway Surface Friction Characteristics’.  This describes the level of assessment that should be employed for Continuous Friction Measuring Equipment (CFME) to ascertain the quality of the runway surface.
“There are three levels of friction value assessment described in the document,” says Mr O’Leary.  “When you reach the second level you need maintenance intervention.  At the third you reach a position where you may have to issue a NOTAM (Notice to Airmen) saying: ‘May be slippy when wet’.  Thankfully here at Gatwick we do not have to issue such notices because we make sure we understand the surface condition and address it before it gets to those levels.  We build up a body of evidence – data about the runway – and that gives us a view of the state of the surface and the rate of deterioration.  CAP 683 dictates that runways which have greater than 400 movements per day need to be measured every five months.  We do it more frequently than that.”
 
Testing Testing
The CFME machinery Gatwick uses to measure the surface friction of the runway is called the GripTester and is manufactured by Findlay Irvine of Scotland.  The device is like a small trailer and is fixed to one of Gatwick’s airfield operations vehicles.
A ‘before and after’ view of what can be achieved. (Weigel)

“It is equipped with a water tank, there’s a wheel and a tyre that touches the surface and there are transducers that measure the resistance of that measuring wheel on the surface.  That is then processed into a friction coefficient.  Portable but heavy enough not to bounce, the braked wheel device replicates closely what happens to an aircraft tyre under skidding conditions.”
Gatwick carries out this kind of testing in-house, though many airports, especially military ones, will outsource this process to the likes of Cranfield University.
Constant monitoring of the surface can prevent expensive work.  In addition, timely intervention to improve friction through the retexturing of the surface can significantly extend the life of a runway without incurring the high costs associated with laying new material.  Known as slurry sealing, this process is ideal if the texture of the pavement has deteriorated.
 
This Way and That
When measuring its runway friction, Gatwick airport does approximately 20 runs, all in the same direction.
“Many airports will go down in one direction and back up in the other.  This means that they have the runway half measured in one direction and half measured in the other.  They then aggregate the results.  When you have got a very dominant runway direction as we have, that sees most of the traffic, the edge of the 4mm grooves built into the pavement get smoothed or rounded down much more in one direction than the other.  Therefore it is imperative that to provide a proper representation of the surface we do all 20 runs in the same direction.”
Gatwick is currently conducting its friction tests using the GripTester from Findlay Irvine. (Findlay Irvine)

Though Gatwick is currently using the GripTester by Findlay Irvine, it will shortly migrate to an ASFT (aerodrome surface friction testing) system.  The reason for this is that the ASFT is sold as a vehicle with the measurement equipment built in, rather than attached behind.  “We believe it has better commercial and operational benefits for us,” says O’Leary.
 
Come Sleet or Snow
Aside from regular designated friction tests, airports will perform similar tests when the runway is covered with ice, slush, snow or water.  This is to establish whether it is still safe to operate on.
“The better-maintained the surface is at other times, the better it will perform during difficult weather conditions,” says O’Leary.  “If you get rain or snow and your underlying surface is not in optimal condition it could mean avoidable closures.  Extensive maintenance during non-adverse conditions is vital.”
Very often adverse weather friction tests will take place early in the morning when the snow has already fallen and the runway is still closed from the previous night.  The friction equipment is deployed via two runs, 3m either side of the centre line.  Based on these results a SNOWTAM (Snow Notice to Airmen) may be issued.
 
Clean-up
Surface friction may be reduced through overuse, leading to a polished finish and a requirement for retexturing.  There is, though, another reason why friction can become diminished – this is due to the build-up of surface contaminants, especially rubber from aircraft tyres.
“Rubber on the pavement surface leads to a poorer braking action and must be dealt with,” says O’Leary.  “Rubber deposits tend to build up all over runways, especially in the surface grooves.  Many think asphalt runways are grooved to provide friction, but the grooves are in fact there to remove the water.  It is vital to keep these grooves clear and clean otherwise you will see a build-up of surface water which will lead to deterioration in the braking action.”
Rubber detritus left behind on Gatwick’s principal runway is removed using a simple piece of equipment provided and operated by the contractor, TrackJet. “The system is a high pressure water system manufactured by Weigel delivering 40,000lbs per square inch (psi) of power,” says O’Leary.  “It looks like an LGV and drives down the runway dispensing high pressure water at the front of the vehicle and sucking up the debris and liquid at the back.  Although we hire the equipment, the price of this kind of vehicle is coming down.  In the future you will certainly see more airports owning them outright.”
Covering approximately 7,535 sq ft (700m2) per hour, the Weigel tends to be operated at Gatwick between the hours of 23:00 and 05:00.  To clean the entire Gatwick runway takes five full nights of intense operation.
“You must be very measured in the way you do it so that you balance the removal across the runway,” says O’Leary.  “If you only did the south side then had to vacate and went back into service you would get differential braking in different areas.  That is certainly something to be aware of.”
Although the vehicle is hired by Gatwick, the decision-making process of when to use it is entirely internal.
“It’s our runway,” concludes O’Leary.  “We have to protect it and are responsible for the performance.  Nothing in my job is as important as maintaining the high quality of the runway surface.  Maintaining its high level of performance is good for business and helps uphold the reputation of the airport.”