What is Runway Friction Testing?

A runway friction test is a survey that measures the friction of a runway surface with a self-wetting continuous friction measuring device (CFME). The FAA writes the name as continuous friction measuring equipment. The device runs along the runway and records a friction coefficient, written Mu (μ), along the runway length. It can be self-contained, or a vehicle can tow it.

What the test measures

The device wets the pavement ahead of the test tyre. The nozzles give a uniform water depth of 1 mm (0.04 inch). The FAA says that wet pavement always gives the lowest friction values, so the test shows the worst case. EASA describes the value as an indication of the texture, contaminant and drainage state of the surface under the tyre. The value is a dynamic friction coefficient, because the device forces some relative motion between the tyre and the pavement. How much motion depends on the device.

Devices

The Mu Meter measures side force friction. It has two freely rotating test wheels, set at an angle to the direction of travel. A vehicle pulls it over the pavement at constant speed under a constant load. ASTM E670 is its test method. Other devices brake the test tyre at a fixed slip. The Norsemeter RUNAR, for example, runs at a fixed 16 percent slip.

EASA lists nine devices in its table of friction levels, below. An operator can use a device that is not in the table. The device must correlate with at least one device that is in the table. The FAA qualified its devices in correlation tests at NASA’s Wallops Flight Facility.

The FAA performance specification for a device asks for these features:

• A continuous record of friction along the full runway, less the run-up and stopping distances.
• Averages for each 150 m (500 ft) segment and for each third of the runway.
• Test speeds of 65 and 95 km/h, within 5 km/h.
• A self-wetting system with a water depth of 1 mm and a flow rate within 10 percent.
• Repeat results for each 150 m segment within two standard deviations of ±0.06 Mu.
• A smooth test tyre without tread, so that tread wear does not affect the result.

Texture and what lowers friction

Two scales of texture set the friction of a wet runway. Annex 14 describes both.

• Microtexture is the texture of the individual stones. The eye can hardly see it. It is the main part of skid resistance at slow speed.
• Macrotexture is the texture among the stones. The eye can judge it approximately. It has the major effect on drainage at high speed.

Rubber deposits in the touchdown area mask the microtexture, and they do not need to reduce the macrotexture. Aggregate polishing removes microtexture. EASA names three causes of lost friction: rubber deposits, surface polishing and poor drainage. A pavement that feels smooth to the touch lacks microtexture. Annex 14 recommends an average texture depth of at least 1.0 mm for a new surface.

Requirements

ICAO Annex 14 requires the State to set a minimum friction level. The aerodrome must keep a paved runway at or above this level. It must measure friction periodically with a self-wetting CFME and document the results. The frequency must show the trend of the surface. Personnel who measure friction must be trained. If a runway falls below the minimum friction level, the aerodrome must take corrective action and must notify users.

EASA Part-ADR.OPS gives the same duties. Its AMC gives values for each device and test speed:

Values differ by device, so a value from one device does not compare with a value from another. FAA Table 3-2 adds a third level, a design level for new surfaces. For a Mu Meter it is 0.72 at 65 km/h and 0.66 at 95 km/h.

How a survey runs

• A complete survey uses two speeds, 65 km/h and 95 km/h.
• Recording starts 150 m from the threshold at 65 km/h and 300 m at 95 km/h. This gives the vehicle room to accelerate. It ends about the same distance from the opposite end, so the vehicle can stop.
• For runways with narrow-body traffic only, the run is 3 m to 5 m from the centre line. For mixed traffic, the runs are at 3 m and 6 m.
• The runway surface is dry, and the self-wetting system adds the water.
• The operator compares each result with earlier surveys of the same runway and the same device.

The runway is not usable by aircraft while the test runs. The FAA tells the operator to choose a time that disturbs air traffic least.

How to read the result

A single value does not name a cause. EASA says that no single capability can be extracted. The operator can deduce certain qualities when the same device runs on the same surface. EASA links the two speeds to causes:

• Low values at 65 km/h indicate less exposed microtexture, from rubber cover or polished aggregate.
• Low values at 95 km/h indicate rubber build-up together with weak drainage. At high speed the water film may become so thick that the microtexture cannot reach the tyre.
• Low values at high speed also indicate less macrotexture. Macrotexture makes escape channels for bulk water.

EASA states that a survey on a dry runway cannot find ponding or rutting. The self-wetting film does not show them. Pools as shallow as about 3 mm can cause aquaplaning where landing aeroplanes meet them at high speed. The FAA says that a self-wetting device cannot show the potential for hydroplaning. It tells the operator to check the surface during rain and to note the position and depth of ponded areas. The FAA says that an average water depth of more than 3 mm over 152 m calls for correction. The operator brings the depressed area back to the standard transverse slope.

How often to test

ICAO and EASA give no fixed interval. EASA tells the operator to consider the jet movements per runway end, the aircraft weight, the type and age of the surface, and the climate. FAA guidance gives a minimum for each runway end:

The FAA says to evaluate each runway end separately. If 20 percent or more of the aircraft mix is wide-body, the operator should use the next higher level. Every runway for jet aircraft should be evaluated at least once each year.

After a low result

EASA says that a measurement at or below the maintenance planning level triggers a complete survey. The survey covers the texture, contaminant and drainage state of the affected runway third. The survey checks that exposed texture can indent the tyre rubber and that water drains from the pavement. The inspector looks for these points:

• exposed microtexture, by touch, if polishing or rubber covers 100 m of the zone that aeroplanes use
• macrotexture
• open grooves, within the limits of their design
• porous friction course that drains as designed
• slopes above the minimum design values

Annex 14 requires corrective maintenance, so that friction does not fall below the minimum. A portion of runway about 100 m long may be significant for maintenance or reporting. The FAA gives these guidelines, which it bases on 500 ft (152 m) segments:

• Below the planning level, but above the minimum, for 500 ft with neighbouring segments at or above the planning level: no corrective action. Monitor with periodic surveys.
• Below the planning level for 1,000 ft (305 m) or more: investigate the cause and extent, and take corrective action.
• Below the minimum for 500 ft with neighbouring segments below the planning level: take corrective action immediately after the cause is known.

The FAA adds a rule for a newly constructed turbojet runway with saw cut grooves or a PFC overlay. The average value of each 500 ft segment should not be below the design level.

Rubber removal

Rubber is the most persistent contaminant. It occurs at the touchdown areas and can cover the whole surface texture. The FAA lists four removal methods: high pressure water, chemicals, high velocity impact, and mechanical grinding. After removal, the operator should measure friction again. The values should be within 10 percent of the values on the uncontaminated centre of the runway. A visual inspection cannot judge the result. The FAA suggests that the removal contract pays on the final CFME test. It also gives a suggested removal frequency:

EASA also lists the other mitigations. Retexturing or resurfacing handles polishing. Reshaping handles poor drainage.

Grooves and porous friction course

A groove cut across the runway gives rain water a short path out from under the tyre. Annex 14 states that grooving speeds up the drainage. The FAA found that a grooved surface keeps a high level of friction when wet. The operator must measure groove depth and width from time to time. When 40 percent of the grooves are 3 mm deep or less, over 457 m, the grooves no longer prevent hydroplaning well. The FAA asks the operator to restore a groove depth and width of 6 mm.

A porous friction course (PFC) is a thin overlay of open-graded asphalt, 25 mm to 40 mm thick. Rain water soaks through it and drains to the side. The FAA found that a high level of friction stays on a PFC for the whole runway length. Rubber deposits can plug the voids, and then the water cannot drain through the layer. When rubber plugs the voids, the FAA says that it is impossible to remove the rubber without serious damage to the overlay. The FAA therefore recommends no PFC on runways with more than 91 turbojet arrivals per day per runway end. EASA tells the operator to check that a PFC drains as designed.

Friction is not braking action

Friction values are not used to report surface conditions. A runway condition report states the runway condition code from the assessment matrix. EASA allows friction values only to support an upgrade or downgrade of that code, in a comparative way. Annex 14 recommends that the aerodrome does not report friction measurements on contaminants other than compacted snow and ice. Measurements on loose snow and slush are unreliable.

A runway is slippery wet when it is wet and tests show degraded friction on a significant portion. A friction survey is one of the methods that can show this. Others are observations by maintenance staff, repeated pilot reports and analysis of stopping performance. Annex 14 requires that information about a slippery wet runway is available.

Visual inspection

A visual inspection cannot replace a friction test. It does find rubber build-up and ponding. The FAA adds that visual inspections are essential for drainage problems, groove wear and structural faults. When an operator does not use a device, EASA tells the operator to evaluate slopes, texture and drainage over the full pavement. The operator also looks for rubber deposits, aggregate polishing and the amount of exposed texture in the area that the aeroplane wheels use. TarmacView does not measure friction. A drone survey of cracks and distress complements the test and does not replace it.