This article has the following parts:

1.0 What is Runway Friction?

2.0 Regulatory Requirements of Runway Friction Testing

3.0 The Principles of Surface Friction

4.0 Sources of runway contamination

5.0 Surface Friction Measurement

6.0 Conclusion

1.0 What is Runway Friction?

Runway surface friction characteristics defines the roughness of the runway surface that will provide for the braking action of the aircraft and will decelerate the aircraft after touch down and also help maintain its directional control during landing, or after a decision to reject a take-off.

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Hard surface runways are constructed using concrete or asphalt but the principles which affect their operational characteristics in respect of aircraft braking response are the same. The surface is ultimately formed by aggregate (microstructure and macrostructure – discussed further) which singularly and collectively are responsible for providing braking action to the aircraft.

1.1 What would happen if optimal runway friction characteristic is ignored?

To understand this we need to consider the following situations where a runway has insufficient friction characteristic which results in the following:

  • Loss of directional control which results in asymmetric aeronautical forces further resulting in symmetric engine power (e.g. engine failure on take-off leading to abandoning take-off).
  • Loss of aircraft stability because of asymmetric wheel brake application on landing or during abandoned take-off.
  • Loss of aircraft directional control due to significant cross-wind component.
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As it is understood that the above events may critically affect aircraft stability so in each case the runway surface friction plays a vital role in counteracting these forces or movements.

2.0 Regulatory Requirements of Runway Friction Testing

In Annex 14, ICAO sets only the principles which cover the provision of paved runway surfaces with acceptable friction characteristics. Contracting States (countries) are given the authority to develop detailed schemes to provide acceptable levels of safety, both in respect of the objective and operational determination of surface friction. As a result, the methods of determination and availability of information differ widely between Contracting States (countries).

However, in India we follow CAR section 4, Aerodrome Standard and Licensing series B, Part 1, which requires all aerodrome operator to undertake friction testing “periodically in order to identify runways with low friction when wet” and also to define and publish in their AIPs the Minimum Friction Level (MFL) which will require Notice To Airmen advice, if reached, for any given runway. States must also establish a ‘Maintenance Planning Level’ (MPL) of runway friction below which prompt corrective action is required. ADAC 1 0f 2019 contains the guidance material on materials that are to be included in the maintenance program by aerodrome operators to ensure continuous monitoring of runway friction characteristic.

3.0 The Principles of Surface Friction

Aircraft braking coefficient is dependent upon the surface friction between the tyres on the aircraft wheels and the pavement surface. Less friction means less aircraft braking coefficient and less aircraft braking response.

Friction is expressed as the coefficient of friction; this is the ratio of the friction force (F) between two surfaces in contact and the normal force (N) which exists between the object resting on the surface and the surface i.e. F/N. This ratio is particularly, but not exclusively dependent, upon:

  • The physical characteristics of the two surfaces.
  • The prevailing temperature at the point of contact.
  • The speed of movement of the object (the tyre) over the surface.

3.1 States should specify three friction levels as follows:

  • Design Objective for New Surface (DONS): It establishes the minimum friction level for a newly constructed or resurfaced runway surface;
  • Maintenance Planning level (MPL): Below this corrective maintenance action should be considered; and
  • Minimum friction level (MFL): Below this the information that a runway may be slippery when wet should be made available and corrective action initiated.

3.2 Runway Surface Texture

The precise texture of a pavement has a considerable effect upon friction, especially when the surface is wet.

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Macrotexture is the structure made by individual stones and is “visible roughness” that allows water to escape from beneath aircraft tyres. It becomes more important as the factors which can lead to aquaplaning come into play – increasing speed, decreasing tyre tread depth and increasing water depth.

Microtexture is the structure of individual stones which have `fine scale roughness’ which is detectable by touch rather than appearance. It allows the tyre to break through the residual water film that remains when the bulk of water has run off and is especially important at low speeds.

4.0 Sources of runway contamination

The following are the sources of runway contamination:

  • Mechanical wear and tear from aircraft rolling over the runway surface resulting in deposition of rubber on the surface.
  • Local weather conditions like accumulation of snow, slush, ice, algae which may result in reduction of runway friction characteristics.
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  • Failure of structure like gaps in the runway surface, rutting, cracking
  • Contamination due to fuel leakage
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It becomes very important for every aerodrome operator to carry out runway inspections with utmost vigilance to detect any contaminant and take corrective actions accordingly.

5.0 Surface Friction Measurement

Since contamination of runway surface is unavoidable it is important to be vigilant on the deterioration of runway friction characteristics. This can be done my continuous measurement of the runway friction using devices which detect surface friction and are called the ‘Continuous Friction Measuring Equipment’ (CFME). Their primary application is the determination of reference friction levels on dry and artificially wetted surfaces. The latter requirement needs a controllable self-wetting capability which can deliver a 1mm water depth.

These reference friction measurements allow airport operators to monitor the runway friction characteristic and plan corrective action in a systematic manner.

There are currently at least eight different types of CFME of which the ‘Grip Tester’ and ‘Mu Meter’ are in widespread use. Usually, CFME is towed behind a vehicle at a constant speed and a wheel with a smooth tyre is fitted with equipment which can directly measure the friction encountered. Measurements are usually output to an on board processor which, when downloaded, can produce tabulations and charts showing the friction level detected.

The results from measurement using various CFME devices are as follows:

It is important for the aerodrome operators to maintain the resources used for measurement of friction as per manufacturers’ specifications. It must be checked that CFME with self-wetting systems have the correct rate of flow of water and depth of water being maintained evenly below the measuring wheel. It must also be ensured that the measurement is done only by trained and competent staff.

It is recommended by ICAO for aerodrome operators to assess their own frequency for assessment if runway friction characteristic based on the density of aircraft operations and weight of aircraft annually using the runway. The table is as follows:

Further, if it is observed that the friction characteristic of the runway is deteriorating faster than usual than in that case the aerodrome operator may also increase the frequency of friction measurement.

It is also important that surface friction does not differ markedly between painted and unpainted runway surfaces and validating this similarity should be a part of the routine assessments of friction. It is usually achieved by adding a small amount of silica sand or glass beads to the paint mix.

Before friction measurement is commenced it must be ensured that friction measurement is performed only on a surface free from precipitation and water patches. Dampness, fog and other environmental factors like cross winds may affect friction measurement adversely.

Thus, it is important to seek advice from environment team before planning such activities and also take care that there is minimum disruption to air traffic.

The CFME provides surface friction information for each third of a runway. The one third segments are called A, B and C. The lower designation being A. When giving information to a pilot, the segments are referred to as the first, second or third part of the runway. The first part always means the first third of the runway where the aeroplane will land.

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Measurement of friction must be done along two parallel tracks to the runway centre line 3m and 6m respectively and two different speeds i.e. of 65kmph which determines the micro and the macrostructure, contaminants and overall drainage characteristic of the runway and 95kmph which indicates the macro structure alone. The average of these 4 runs can be averaged to find the 100 meters average friction value and average friction value for the runway.

5.1 Action To Be Taken As A Result Of Runway Friction Assessment

  • If the friction level is below the MPL, maintenance should be arranged to restore the friction level.
  • If the friction level indicates a falling trend, increase the frequency of runway friction assessments in order to identify any further or rapid deterioration and take appropriate action.
  • If the friction level is below the MFL, maintenance should be arranged urgently in order to restore the friction level. A NOTAM shall be issued advising that the runway may be slippery when wet.
  • If the friction level is significantly below the MFL, withdrawing the runway from use when wet must be considered and NOTAM should be issued in that respect.
  • Corrective maintenance action shall be taken when the friction characteristics for either the entire runway or a portion thereof are below a minimum specified friction level. Note— A portion of runway of the order of 100 m long may be considered significant for maintenance or reporting action.

5.2 Removal of rubber from the Runway

It is essential to remove rubber from the surface of the runway to restore a good coefficient of friction in wet conditions so as to provide safe operational conditions for all aeroplane.

A change in surface color, can be very misleading, because even a small amount of residual rubber in the pores of the pavement can produce low friction values, while giving an overall clean appearance. It is therefore essential to quantify the friction coefficient by means of a reliable friction-measuring device.

The removal of rubber is carried out by means of:

  •  High-pressure water blasting (Truck mounted system with rotary device using water pressure of 8-10,000 psi.)
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  • Chemical solvents (Chemical solvents are used for removal of rubber deposits)
  •  Mechanical Removal

5.3 Removal of Oil and Grease from the Runway

Free deposits of oil/ grease etc. may be blotted up with rags, sawdust, sand, etc., and the residue then scrubbed with detergent using a rotary power broom.

6.0 Conclusion

The runway friction characteristic of the runway is one of the major deciding factor that determines whether a runway is usable or not. Continuous monitoring and timely maintenance must be assured to always maintain the runway fit for operations.

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