Why do some US airport runways have grooves cut across the pavement? They help stop aircraft tires from hydroplaning when the runway is wet

Rain strikes and trickles down the window as your plane lands. The wheels touch the runway, and yet the landing feels smooth. You probably never think about the pavement under you. There are tiny, thin cuts in that pavement that might be working quietly to make sure you have a safe ride. The Federal Aviation Administration (FAA) explains how airports build them in Advisory Circular 150/5370-10H, its standard specifications for airport construction, issued on December 21, 2018.

Those lines are not random

If you stood on a grooved runway, you would see fine lines running across it. They look like notebook lines and are called saw-cut grooves. The FAA's construction standard has a full item on them, called P-621. AC 150/5370-10H states that the item consists of constructing saw-cut grooves to minimize hydroplaning during wet weather and provide a skid-resistant surface to the pavement. The size is specified by the standard. Each groove is 1/4 inch wide and 1/4 inch deep, with a spacing of 1.5 inches between the centers of adjacent grooves. That's a very small cut, but the item says the grooves must run continuously for the full length of the runway. Because the grooves are cut across the width of the runway, crews stop them no closer than 10 feet from the edge of the pavement, or where the plans say, to allow space for the equipment to work.


Hydroplaning

You probably think hydroplaning is something only boats do, but tires can hydroplane too. Imagine walking on a wet tile floor in your socks. You slip because water is between you and the ground. A tire can have the same problem at high speed. In Phenomena of Pneumatic Tire Hydroplaning, NASA researchers Walter Horne and Robert Dreher of Langley Research Center describe what happens. They list one effect as the tire footprint lifting off the ground. In simple terms, the tire is not entirely on the runway anymore. The report, published in November 1963, goes further. In experiments, the effects observed were loss of braking traction and loss of directional stability. It also provides formulas for estimating the ground speed at which hydroplaning occurs. This is important on a runway. When a plane lands, it has to slow down and stay straight, and both jobs are a matter of grip.

How grooves help

Rain has to go somewhere, and grooves offer it a place to go. Think of them as small drains running across the runway. The FAA supports this approach. Its skid-resistance circular, Advisory Circular 150/5320-12C (issued in 1997 and updated in 2004), states that cutting or forming grooves is a proven and effective method for skid resistance and for preventing hydroplaning in wet weather. That guidance is also clear about shortcuts. It says that wire tining, a method of texturing the pavement, is not a replacement for grooves because it does not stop planes from hydroplaning. A grooved runway is not the same as a rough runway.

How crews make them

The method depends on the paving. The same circular explains that grooves in existing pavement, whether asphalt or concrete, must be saw-cut. For new concrete pavement, the grooves can be formed while the concrete is still plastic. In asphalt, they must be saw-cut whether the pavement is new or existing. So the grooves are not decoration or an afterthought. They follow a process outlined in national guidance.

Why doesn’t every runway look the same

The FAA's approach varies with the runway, and not all runways are grooved. AC 150/5320-12C considers grooving of all runways serving, or expected to serve, turbojet aircraft to be high-priority safety work to be done during initial construction. For other runways, it says to consider factors such as rainfall and any history of hydroplaning incidents. The grooves are not randomly distributed. They are cut at the locations indicated on the project plans or as directed by the engineer, per Item P-621 of AC 150/5370-10H. The circular also spells out how far it reaches. It states the document is not a regulation by itself, and that it is not mandatory in general.

However, the FAA recommends the use of its specifications for pavements serving aircraft in excess of 30,000 pounds. The FAA also says the circular must be used for projects funded by federal grant programs such as the Airport Improvement Program, and for projects funded with Passenger Facility Charges. The circular cites the programs’ own assurances (Grant Assurance No. 34 and PFC Assurance No. 9) as the basis for the requirement, thus keeping the document non-regulatory but binding on funded projects. Many federal-funded airport projects have this same requirement.

What this means for your next flight

Grooves are just one layer of safety; they are not magic. And one feature just isn't enough on a rainy day. The NASA report describes procedures to minimize the effects of hydroplaning. Grooves fit that idea well. They're small cuts with a big job to do. Next time your plane lands in rain, look out the window if you can. The grooves can be hard to see, but these quarter-inch cuts, done again and again, are part of what makes the landing feel so calm.