What Is Runway Grooving?

What grooving is

Runway grooving is the cutting of narrow transverse channels into a runway surface to carry water out from under aircraft tyres. A transverse groove runs across the direction of landing and take-off. The grooves give water under a rolling tyre a short path to escape. The ICAO Annex 14 guidance says the primary purpose of grooving is to improve surface drainage, and that grooves shorten the drainage path. The cross slope of the runway does the main drainage work. The grooves add to it.

Annex 14 and EASA also use the word “scored”. A grooved or scored surface has channels that are cut or formed in it. This page uses the word “grooved”.

Why runways are grooved

Since the first turbojets, braking on a wet runway has been a safety concern. The FAA says that heavy aircraft and high landing speeds made wet braking a significant safety consideration. Two studies led to grooving. NASA at the Langley Research Center completed one in 1968. The FAA Technical Center completed the other in 1983, at Lakehurst. Both found that closely spaced transverse grooves give high friction on wet pavement, because rain water escapes from beneath the tyres of a landing aircraft.

Hydroplaning is the loss of contact between a tyre and a wet surface. NASA described the effect in 1963. Water pressure under the tyre grows with the square of the ground speed. At a critical speed, the water lifts the tyre off the surface. For a tyre with an inflation pressure of p psi, NASA gives a hydroplaning speed of 9 times the square root of p, in knots. A tyre at 100 psi gives 90 knots. The formula assumes that the water is deeper than the tread grooves. ICAO says that the objective of surface drainage is to minimise the water depth on the surface, and that grooves shorten the drainage path.

The FAA names two properties of a wet runway surface. Microtexture is the fine roughness of the individual aggregate particles. It helps the tyre break through the thin water film that remains. Macrotexture is the visible roughness of the surface as a whole. Its primary function is to give water paths to escape from beneath the tyres, and it matters more as speed rises. The FAA says that both properties are essential.

Texture and grooves

ICAO describes the drainage of a surface as a function of its texture and its grooving. It classes macrotexture from A to E, after the ESDU 71026 scale:

Grooving any of these surfaces improves the dynamic drainage. The effect depends on the size of the grooves and on the space between them. ICAO notes that aeroplane performance credit goes to grooved or porous friction course runways whose design, construction and maintenance meet criteria that the State accepts. For aerodromes with heavy or torrential rain, ICAO says to consider grooved pavements in class E.

The FAA standard groove

The FAA standard groove is 1/4 inch (6 mm) deep and 1/4 inch (6 mm) wide. The spacing is 1-1/2 inches (38 mm) centre to centre. The grooves are continuous along the full runway length and perpendicular to the direction of aircraft operations. They stop about 3 m (10 ft) from the pavement edge, to leave room for the machine.

Item P-621 sets the acceptance test. At least 90 percent of the grooves must be 3/16 inch (5 mm) or more in depth and in width. At least 60 percent must be 1/4 inch or more. No more than 10 percent may exceed 5/16 inch (8 mm). In concrete, grooves stay 3 to 9 inches (8 to 23 cm) from a transverse joint. They stay 6 to 18 inches (150 mm to 0.5 m) from an in-pavement light fixture. Where lighting cables lie in the pavement, the machine does not cut through longitudinal or diagonal saw kerfs.

At the intersection of two runways, the whole length of the primary runway is grooved. The secondary runway is saw cut in a step pattern. High-speed and angled exit taxiways also get a step pattern. It starts at the projecting pavement edge and is not wider than 40 inches (102 cm).

Groove direction

On a runway, grooves run across the direction of travel. Annex 14 and CS ADR-DSN.B.090 recommend that grooves lie perpendicular to the runway centre line, or parallel to non-perpendicular transverse joints where they exist. The FAA asks for grooves that are continuous for the entire runway length and perpendicular to the direction of landing and take-off.

These runway rules cover transverse grooves only. Longitudinal grooves appear in highway practice (see Grooving on roads).

Where it is required

ICAO and EASA do not give groove dimensions. Annex 14 and CS ADR-DSN.B.090 recommend that a new surface has an average texture depth of at least 1.0 mm. They recommend that grooves lie perpendicular to the runway centre line.

The FAA is more direct. AC 150/5300-13B says to provide grooving or another friction treatment on primary and secondary runways at commercial service airports and on runways that serve turbojets. It gives the same advice for high-speed taxiways as a recommended practice. AC 150/5320-12C calls the grooving of turbojet runways high priority safety work. The boldface items in its paragraph 2-21 are mandatory on federally funded projects.

For other runways, the AC lists the factors to weigh:

• the history of accidents and incidents with hydroplaning
• how often the runway is wet, from annual rainfall rates and intensity
• grades, flat areas, depressions and other features that stop water from running off
• the quality of the surface texture, including polished aggregate and contaminant build-up
• the strength and condition of the runway pavement

How grooves are made

Grooves in asphalt are saw cut with diamond blades. The FAA says that grooves in hot mix asphalt must be saw cut, in new and in existing pavement. Grooves in concrete can be saw cut or formed. In new concrete, a vibrating ribbed plate or a ribbed roller forms the grooves while the concrete is plastic. The roller does not always reach the full depth of 1/4 inch.

The pavement must be fit for grooving. Bumps, depressed areas, faulted joints and badly cracked or spalled areas are repaired first. If the surface or the strength is not suitable, an overlay comes first. For asphalt, the FAA lists more factors that set how long grooves stay effective. They are the maximum surface temperature, the tyre pressure, how often aircraft brake in an area, the mix and the aggregate.

New pavement cures for 30 days before grooving, unless the grooves are shown to be stable. The grooving machine is self-propelled and has a slurry vacuum. The contractor grooves a control strip first. Waste must not enter the storm sewer. The FAA asks for clean-up all through the work, because waste left on the runway can be hazardous to aircraft.

Grooving, wire combing and porous friction course

Grooving is not the only way to give a runway a drainage texture. The FAA separates it from two others.

• Wire combing and wire tining. Steel wires form a deep texture in plastic concrete. The FAA calls these texturing methods. They cannot replace saw cut or plastic grooves, because they do not prevent aircraft from hydroplaning.
• Porous friction course (PFC). This is an open graded hot mix asphalt layer, 3/4 inch to 1-1/2 inches (25 to 40 mm) thick. Rain water drains through it to the side of the runway. Research in the UK and the US found that it also gives good friction. Rubber can plug its voids. The FAA recommends against PFC on runways with over 91 turbojet arrivals per day per runway end.

The EASA inspection list covers both surfaces. It asks the inspector to check that grooves are open and within their design limits, and that a porous friction course drains as designed.

Wear and rubber

Rubber deposits in grooves reduce their effect and make hydroplaning more likely (AC 150/5380-6C). Rubber from landing jets is the most persistent contaminant. It builds up in the touchdown zone. ICAO adds that rubber can fill the texture and cut the drainage capacity.

Traffic also wears the grooves. AC 150/5320-12C says the operator should measure groove depth and width periodically. When 40 percent of the grooves are 1/8 inch (3 mm) or less over 1,500 feet (457 m), the operator should restore them at once.

The FAA gives four methods to clean rubber and other contaminants from a runway: high pressure water, chemicals, high velocity impact, and mechanical grinding. It says that nobody can judge the result of rubber removal by eye. After the work, the operator should measure friction. The Mu values should return to within 10 percent of those on the clean centre part of the runway. The AC suggests a removal frequency from the number of turbojet landings per day per runway end:

If more than 20 percent of the landings are by wide body aircraft, the operator picks the next higher row.

Inspecting grooves

Annex 14 says to inspect grooves regularly. EASA asks that an inspection checks that grooves are open and within their design limits.

A visual inspection can note groove deterioration and ponding. AC 150/5320-12C says that visual evaluations of friction are not reliable, and that it cannot judge texture depth. Texture depth needs direct measurement. TarmacView does not measure groove depth, width or friction.

Grooving on roads

Highway agencies also cut grooves, to raise skid resistance and cut wet-weather crashes. The FHWA literature review (FHWA-HRT-14-065) defines pavement grooving as longitudinal or transverse cuts in a surface. Its primary objective is to improve drainage and reduce hydroplaning.

The two directions have different uses on roads:

• Longitudinal grooves run parallel to travel. The review says they work best for directional control of the vehicle.
• Transverse grooves run across travel. They are less common, partly because grooving machines cut parallel grooves more easily. They work best where vehicles stop often, such as at intersections and toll booths.

The review lists reported drops in wet-weather crashes between 55 and 72 percent. It warns that the studies did not control for regression to the mean, so the figures need caution. It also names possible costs: more noise, faster wear of the pavement, and an effect on steering. On asphalt, grooves can flow back together in hot weather and on young pavement.