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    -Heard on Southwest Airlines just after a very hard landing in Salt Lake City: The flight attendant came on the intercom and said, "That was quite a bump and I know what ya'll are thinking. I'm here to tell you it wasn't the airline's fault, it wasn't the pilot's fault, it wasn't the flight attendants' fault.....it was the asphalt!"

    Pretty funny! But so often it's true. And sometimes, where those tires touch down is particularly rough because the runway centerline markings are like speed bumps. Many busy airports lose their centerline markings within days of having it freshly painted because of rubber from the aircraft tires. So the airfield maintenance crews diligently apply more paint to freshen it up. And the more paint that's applied, the worse the build up becomes, and if it's asphalt, it may start to look like this!

    There are differences between highway and airfield markings; and yet airfield markings are treated in much the same way in terms of maintenance. Customarily, state DOTs send paint crews out once a year (or more) to repaint road lines, depending upon average daily traffic (ADT). Painted markings on airfields don't experience the same kind of traffic wear, and yet airports all over the world maintain their markings by repainting every one to three years; except in the case of the runway centerline which often gets painted a lot more frequently than that. Unfortunately, little, if any, surface preparation is performed prior to applying more paint, just like highway markings. (I had to say it!) On most of the markings, this frequent repainting is often not necessary and can lead to other problems that are seldom recognized by airfield maintenance crews and operations staff.

    Standard centerline marking width on a highway is 4 inches vs. 18-36 inches on a runway. A 4-inch line on a highway does not have the same surface tension of a 36-inch wide line on a runway. The wider the painted marking, the higher the surface tension. Paint on a runway sometimes bonds better to the asphalt than the asphalt does to itself, causing premature cracking of the asphalt surface. This gives water easy access to the paint and the pavement, undermining both.

    A median or two solid yellow center lines dividing opposing lanes of traffic often split a four-lane highway compared to a typical runway. Each highway lane ranges from 10-12 feet wide. The markings are 4 to 6 inches wide. But an airfield has a wide expanse of pavement where markings are not nearly as close together. On a typical commercial runway, the centerline consists of a 3-foot wide by 120-foot long bar, separated by 80-feet of space. Most commercial runways are 150-foot wide, so the nearest marking to the centerline might be an edgeline 75-feet away. The size and distance of the lines are understandably larger due to the size and speed of the vehicles using them; and they are quite visible from long distances, as long as the markings are well maintained.

    Traffic flow blows water from highways; runways are crowned and often grooved, allowing water to flow off the pavement to prevent hydroplaning. However, runways sometimes have high grass adjacent to the pavement, causing water to pond at the edges. Water accelerates paint wear and contributes to pavement deterioration.

    With a high volume of traffic, highway lines take a beating and must be remarked at least annually. On runways, a high volume of traffic buries the runway centerline and other markings under black, carbonized rubber deposits. But aside from the centerline, runway markings should have a much longer wear cycle. When the airfield markings are repainted, they should first be surface blasted, not just painted over. Over-painting on airfields causes the paint to build up, crack and peel, because the markings are not being worn down with traffic. Snowplows and sweepers can sometimes dislodge some of the loose paint, but pieces can be found on pavement surfaces. These chunks containing layers of paint and glass beads constitute FOD (foreign object debris) and, like nuts and bolts, can be ingested in jet engines, possibly causing premature wear of engine components.

    Blue-green algae grow in warm, damp environments. On highways, traffic generally keeps the algae at bay. But on airport surfaces with traffic on a small portion of the paved surface, algae can spread across the pavement, often obscuring the markings to the point that they appear worn or gone. Often water blasting alone will clean the algae from the marking, negating the need for repainting. The picture to the right demonstrates just how much algae can 'take over' an untrafficked area of an airfield.

    Another problem detected in some parts of the world, predominantly on runways, is rust staining due to high iron sulfide content in the ground and/or aggregate. When due to rain water contamination, possibly picked up from the runway surface, the leading edge of the marking and low areas where the water flows receive the worst stain. When due to ground contamination, the entire marking becomes rust colored. The color standard for white is no longer within tolerance.

    Markings on runways should be viewed and maintained differently than on highways. Much greater emphasis is placed on highway markings for the safety of the traveling public. However, even though there are only one or two people maneuvering an aircraft on a runway, the passengers may number into the hundreds; and they are relying on the ability of the pilots to get them safely to their destination. It would seem prudent for airfield operators to focus more attention on the integrity of their markings, look at them in terms of visibility (both day and night), as well as how they are affecting the underlying pavement. The pavement, after all, is one of an airport?s biggest assets; but asphalt pavements on airfields continue to show signs of cracking and spalling, partially due to repeated repainting of the markings. Maintenance of runway markings should be on an "as needed" basis. We at Sightline can help your staff determine when it?s needed.

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