Pilots recognize the dangers of flying in inclement weather. Standing water, slush, or wet snow can contaminate a runway surface producing a dangerous landing situation. Hydroplaning, the result of these weather-induced conditions, can have serious adverse effects on ground controllability and braking efficiency and can render an airplane partially or totally uncontrollable anytime during the landing.
The Federal Aviation Administration (FAA) realizes the dangers a pilot faces when losing braking effectiveness on a wet runway. Avoiding a dangerous or out-of-control landing situation is the goal of each and every landing.
Despite the weather, runways, and taxiways must always deliver the best possible overall ground handling and stopping characteristics that today’s state-of-the-art technology can provide.
To better ensure that airplanes and their passengers are safe, FAA has issued a safety tip recommending that when confronted with the possibility of hydroplaning, it is best to land on a grooved runway. This is consistent with what pilots have found in the field. Pilots have observed that transverse-grooved surfaces drastically reduce all types of skids on wet or flooded runways and provide positive nosewheel steering during landing rollout. Grooved surfaces also prevent the onset of drift and weathervaning. Pilots find overall ground handling and stopping characteristics on grooved surfaces are a dramatic improvement over ungrooved surfaces.
The FAA Safety Team addresses how to plan for hydroplaning in the December 2010 “Approach and Landing Safety Tip, Notice Number: NOTC2669” (http://www.faasafety.gov/spans/noticeView.aspx?nid=2669). In this publication, grooved runways are recommended by the FAA as the preferred surface to avoid weather-induced problems.
Grooved runways are a win-win situation. Not only does grooving make runways safer, but it’s also a time- and cost-effective solution for rehabilitating older runways. Because it can be targeted specifically to the areas that need additional traction, grooving does not require massive shutdowns that can elevate costs. It’s also easy to perform during off-peak hours, which minimizes logistical issues. Further, grooving does not have an adverse affect on pavement fatigue life.
From the pilot’s point of view, the overall airplane ground handling and stopping characteristics on grooved surfaces show remarkable improvement over other surfaces.
Putting it to the test
Flight test programs conducted by NASA have confirmed the effectiveness of grooved pavement. The tests investigated the differences in wet runway braking effectiveness on a grooved pavement surface. In addition, a test on different types of grooving determined that sharp edge grooving ? diamond saw cut grooves ? was the most effective.
In one of the tests, the landing research runway was divided into test sections and bound by two-inch rubber dams to provide an even water depth. Braking was done for ground speeds ranging from some 50 to 150 knots. The tests involved accelerating to the desired speed from a standstill to the takeoff position, or landing short of the test section and adjusting speed. This speed, or a slightly higher speed, was held until about 100 yards before the appropriate test sections. At this point, power was reduced to idle and the spoilers extended. The timing allowed the engines to spin down to idle thrust before entering the test area. Brakes were abruptly applied to the maximum deflection while the wheels were still on the dry reference section and were maintained through two wet or flooded test surfaces, one grooved, and one ungrooved.
In another test, eight concrete test surfaces were used to determine which was most effective. The surfaces included one ungrooved and the rest grooved with different pitches and using different methods, either combing or sawing. The combing technique involved raking or combing the grooved shape into the surface while the concrete was in a plastic state. With the saw technique, the groove was cut into the cured concrete surface with a diamond-tipped circular blade. These surfaces were all flooded for the test purpose.
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