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.
Effective Solution at FLL
At the Fort Lauderdale Hollywood International Airport (FLL), the asphalt-paved Runway 9L/27R was experiencing distress and lost friction. The Broward County Aviation Department decided to form a plan to rehabilitate its runway.
According to Gasser Douge, airport engineer for the department, a loss of up to 40 percent of the transverse grooves triggered an investigation into the pavement condition, with concern centering on the pavement’s loss of friction. The design engineer recommended grinding and grooving as the solution to the problem.
The scope of work included patching areas of deteriorated pavement, diamond grinding approximately 110,000 square yards of asphalt runway deep enough to remove the existing grooves, then re-grooving the runway in accordance with standard FAA specifications, and finally re-striping. During the project, high levels of production were required and maintained by the crews during short work windows, seven nights per week. An added challenge was the inconvenience caused by having to evacuate the runway intersection when planes were arriving.
Diamond grinding a major runway is a first for Broward County and possibly for the entire region. The technique is a viable and cost-effective method for rehabilitating asphalt-paved runways and highways. While the total project cost was $1.5 million, the diamond grinding and grooving cost was only $500,000, or $5 per square yard. The cost savings were substantial when compared to an alternate repair method such as milling and filling. The project was completed in August, 2010, leaving the facility with a like-new surface and the friction necessary to ensure the continued safety that modern day airports require.
Results at Dyess Air Force Base
Located outside of Abilene, TX, the Dyess Air Force Base is home to the 7th Bomb Wing and 317th Airlift Group. Years of wear and rubber deposits from landings by B1-B Lancer bombers and the C-130 Hercules aircraft had made the runways significantly polished due to the lack of texture.
An aggressive two week runway closure window was negotiated with the Air Force to complete the grinding and grooving of the 125,000-square yard main concrete runway as well as the 25,000-square yard asphalt auxiliary runway.
“Grinding and grooving definitely helps reduce the potential for hydroplaning on a wet runway,” says Barry Mines, Chief of CE Technical Support for the Dyess Air Force Base. “The grinding helps provide a “rougher” surface for better traction (frictional resistance) and the grooving provides channels for the water to run laterally off the runway. If you get the water off of the runway, you reduce your hydroplaning potential.”
Grooving was chosen since it is a surface treatment recommended by the Air Force. “… Our Unified Facilities Criteria (UFC) 3-260-2 titled “Pavement Design for Airfields” requires that new, reconstructed, or resurfaced runways must be grooved,” says Mines.
The production settled into a relentless pattern with two 12-hour shifts working around the clock. Six water trucks were in use nearly full time, cooling down the diamond blades. The job was completed on December 23rd, six days ahead of schedule and came in within budget and without accident or injury.
The Air Force continues to employee this technique on additional runways at Dyess Air Force Base. “We are currently completing the resurfacing of a 3,500-foot-long landing zone which is being grooved,” says Mines.
A transverse grooved saw-cut surface provides impressive braking and generates a side force that can prevent lateral drift. Pilots find that deceleration on a wet, saw-cut grooved surface is almost the same as on the dry surface, and any nosewheel steering lost on the ungrooved surface returns as soon as braking resumes on the grooved surface.
Equally important is the finding that grooves with sharp edges are more effective than rounded groove edges. This vital NASA research has led to the current FAA recommended runway grooving configuration of ¼x¼-inch (groove depth and width), spaced 1-½ inches center to center, which is used on most major runways in the U.S.
With the Approach and Landing Safety Tip, the FAA is insisting on a safer surface for all runways by using a grooved surface. A saw-cut transverse-grooved concrete landing surface will increase the potential that all aircraft will have a safe, uneventful landing every time, despite the weather.
The International Grooving & Grinding Association (IGGA) is a non-profit trade association founded in 1972 that is committed to the development of the diamond grinding and grooving. For more information, visit www.igga.net.