1.04 Aircraft Deceleration: Braking, Hydroplaning, Directional Control and Runway Friction

1. Decelerating Forces During Landing Run and Rejected Take-off

In general an aircraft is decelerated by three forces:

• aerodynamic drag;

• reverse thrust;

• runway/tire friction.

The relative contribution of each force varies with aircraft type. In fig. 1 an illustration is given of how the deceleration forces of a typical aircraft vary with speed.

As deceleration is proportional to deceleration force, the vertical scale in fig. 1 for deceleration force (in 1000 lbs) may be substituted by a scale indicating deceleration rate (in kt/sec).

Deceleration Distance and Braking Friction

The decelerations shown in fig. 1 result in deceleration distances given in the following table:

For short landing distance a good deceleration at the high speed end of the landing run is of paramount importance; it is the most effective way to cut down on landing distance.

In the various AOM’s, information can be found on the effect of the runway friction force and reverse thrust in the Actual Landing Distance data.

Aerodynamic Drag

Aerodynamic drag is highest in the high speed portion of the landing run. Extra drag is primarily provided by full landing flaps and to a certain extent by spoilers.

Reverse Thrust

Reverse thrust decelerating force depends on design and location of the thrust reversers and may reach values up to 40 or 50% of the forward thrust.

On wing pod mounted engines, effective reverse thrust is also dependent on flap setting. This applies to the inboard engines and especially in the flap setting range where the extension virtually consists of angular deflection.

As most jet engines stall below 80-60 kt IAS due to reingestion of hot exhaust gases, reverse thrust should be applied immediately after main gear touchdown to obtain maximum effect.

Runway/Tire Friction

Runway/tire friction force equals the vertical load on the wheels times the runway/tire friction coefficient (μ). Vertical load on the wheels equals aircraft weight minus lift.

For the braking friction force, only the part of the vertical load working on the braked wheels is effective; this depends on the type of aircraft and would be 100% on aircraft where the nose wheel is also braked and approximately 90% on aircraft where main gear wheels only are braked.

For good braking friction forces in the beginning of the landing run, decreasing of lift by early spoiler extension is essential. Flap retraction after touchdown for further lift decrease is not effective as the resulting loss of aerodynamic drag is greater than the possible gain in friction force.

2. Mechanics of a Braked Wheel

The value of the braking friction coefficient μ between tire and runway depends on a great number of factors that will be discussed below.

The braking friction coefficient (μ braking) may be defined as follows: braking friction force between tire of braked wheel and runway divided by vertical load on braked wheel.

On a free rolling wheel four forces are working: the vertical axle load of the aircraft on the wheel; the footprint reaction force which is equal to the tire pressure x footprint area; the horizontal axle force; the footprint friction force which the runway surface yields against the forward movement of the wheel.

If the wheel rotation is steady, i.e. at constant rotational speed, the moment around the wheel axle of the footprint friction force is compensated for by a moment delivered by the vertical footprint reaction force.

The free rolling friction force is normally 2-3% of the vertical load.

Braking and Slip Ratio

What happens now when brakes are applied? Torque is directly applied to the wheel, retarding wheel rotation and generating a runway/tire friction force. This force causes deformation of the tire. The braking friction force is transferred from the ground to the airplane as a deceleration force.

Theoretically, maximum braking is obtained when brake torque is just fully absorbed by the tire elasticity. This means that practically no sliding or skidding may occur between the tire footprint and runway.

Further increase of brake torque starts locking of the wheel and skidding of the tire. Skidding reduces friction, since the rubber may start to melt or is already melting, giving the tire a liquid surface to slide on. Even if the heat is not great enough to melt the rubber, small particles torn loose by the shearing action will act as rollers, thus reducing the friction appreciably.

The slip ratio may be defined as: (free rolling RPM – actual RPM) / free rolling RPM.

Fig. 4 depicts the variations of the braking friction coefficient with slip ratio for a dry runway. It may be concluded that no sliding or skidding occurs up to a slip ratio of approximately 15%.

Skidding must be avoided; for this purpose modern aircraft are equipped with anti-skid devices. Later developments of anti-skid devices also aim at attaining maximum braking friction. Because of the transient nature of the tire-slip phenomenon, some overshooting or undershooting of this point is unavoidable, resulting in an average braking friction as developed between slip ratios of 10 and 50%.

3. Factors Affecting the Braking Friction Coefficient

Runway surface texture or material has relatively little effect. Decreasing ground speed tends to increase braking friction coefficient slightly.

Contaminated runway surface: in this case braking friction coefficient can be affected by hydroplaning.

Hydroplaning can be defined as a condition in which the fluid (water, slush) is not displaced at a rate fast enough from the tire/ground contact area, by a rolling tire or by a moving but non rotating (fully skidding) tire, to allow the tire to make contact with the runway surface over its complete footprint area.

When hydroplaning occurs, the tire rides or skids on a wedge or film of fluid all over or part of its footprint area, depending upon conditions.

Water or contaminated water has extremely low friction capacity and therefore a tire, riding partly or fully on a wedge or film of water, has reduced or no braking friction at all.

Fluid pressure build up below the tire depends on fluid layer thickness, fluid density and viscosity, runway texture, tread pattern and ground speed. Various combinations of these factors can result in three types of hydroplaning: dynamic hydroplaning, viscous hydroplaning and reverted rubber hydroplaning.

Dynamic Hydroplaning

When the tire hits the fluid, momentary fluid stagnation takes place when the tire tread pattern or runway surface profile does not allow the fluid to escape.

Fluid pressure build up is directly proportional to the fluid density and ground speed squared. Therefore the speed at which full hydroplaning occurs can be predicted.

Normal tire pressures for the jets are around 160 psi and thus for these airplanes dynamic hydroplaning may occur whenever the ground speed is over approximately 115 kt.

For wheel spin up, full hydroplaning will occur at a slower speed. This type of hydroplaning with the same tire could occur on touchdown during landing at ground speeds above 97 kt.

Viscous Hydroplaning

Viscous hydroplaning occurs on all wet runways; it is the normal slipperiness due to the lubrication action of the water.

Viscous hydroplaning is enhanced by a thin film of water on the runway contaminated with oil, rubber or dirt deposits.

Slush has a much higher viscosity than water; therefore on slush covered runways at low speed, viscous hydroplaning predominates and tests have shown lower traction and lower braking friction in the lower speed regime in slush than on water covered runways.

Reverted Rubber Hydroplaning

Reverted rubber hydroplaning is very probably responsible for a number of traction loss incidents which could not be explained previously. In reverted rubber hydroplaning the tire footprint is heated to such an extent that a thin layer of rubber loses its elasticity and becomes a sticky, tarry substance as if the rubber were reverted to the state it had before vulcanization.

A possible explanation for the reduced traction down to very low speeds may be the generation of superheated steam in the skidding tire footprint.

Pressure and temperature so developed should be high enough to equal tire/ground contact pressure, lifting the tire footprint from the ground; maintain a steam generating process; and continue the formation of reverted rubber, thus creating an efficient seal for the high pressure steam under the tire footprint.

This condition may persist right down to the stop. As yet, no proven way to avoid formation of reverted rubber is available.

The operational line directly in the hands of the cockpit crew is: prior to any departure, check that no signs of reverted rubber are present; avoid skidding the tires over dry parts of a runway; on a wet or damp surface, touchdown should be firm; and maintain good landing and deceleration techniques, including early application of reverse thrust during any wet landing.

Hydroplaning Mechanics

The technical solution is the cutting of lateral grooves in portions of runways. There are strong indications that such runway treatment could prevent cases of low braking friction. The grooves provide an escape path for water or steam and scrape off reverted rubber if present.

Fig. 5 shows three phenomena: part of the tire contacts fluid instead of runway surface, friction force on the tire decreases and may even be lost almost completely; fluid displacement drag is generated, which only partly compensates for the reduced friction force in partly hydroplaning and reduces to practically zero in full hydroplaning; and the vertical reaction force on the tire acts forward of the wheel axle, implying a spin-down moment.

Maintaining normal dry runway brake application would lock the wheel completely. In order to avoid this, the anti-skid system releases or reduces brake pressure. The effect of such brake systems on net brake force output is closely related to the spin-down/spin-up phenomenon.

In cases of viscous or reverted rubber hydroplaning no fluid displacement drag is generated. As for dynamic hydroplaning, wheel spin-down may occur due to the very low friction force.

1. Cornering Capability of the Tire

Cornering capability of the tire, i.e. side force on the tire resisting tire break-away sideways, is greatest at zero wheel slip ratio, i.e. free rolling.

Situation (1) shows a free rolling tire. In situation (2) the aircraft tire heading is changed, but the track is still the same. The heading change should be not more than a value at which cornering force equals maximum available friction force.

In situation (3) brakes are applied; maximum available friction force is now to be divided between braking force and cornering force, meaning that cornering force is reduced.

To obtain the maximum cornering capability, a wheel must be rotating freely (no braking) at approximately a 15° slip angle on a clean dry runway.

Maximum cornering capability will be reduced by any or all of the following: other than a clean, dry runway; greater than maximum slip angle for existing conditions; application of brakes.

The opposite reasoning also holds true, namely that any cornering will reduce braking capability.

2. Operational Control on the Runway

If, due to any reason, e.g. crosswind, asymmetric reverse thrust, unequal braking or whatsoever, the airplane track deviates from the runway center line, the only way to correct this is by applying on the airplane a cornering force directed towards the required track.

On the ground, this cornering force can only be generated by changing the heading of the airplane. Although this is the first step to do, it will only produce the necessary corrective force for bringing back the airplane to the required track if some conditions are met.

Corrective Forces

The main gear cornering force delivers the major part of the corrective force. It is only existing if the available ground/tire friction is not partly or fully consumed for braking. Therefore brakes should be released when making the heading change.

Reverse thrust is shown to be counteracting the motion of the airplane to the desired track. Therefore reverse should be minimized during the corrective action; it may even be necessary to apply idle forward thrust temporarily.

After an initial change in heading, exceedance of the maximum cornering capability will result in skidding of the main gear tire. The pilot, in a position far ahead of the main gear, may not readily detect that the change in heading is not followed by a change in track. Special attention for this possibility is therefore recommended.

1. Introduction

Dry runways pose little braking problems but wet or otherwise contaminated runways may show serious deterioration in braking friction.

Through the years many attempts have been made to develop suitable equipment and procedures to determine and report braking friction on contaminated runways. In addition, similar equipment is needed for wet runway certification purposes.

Large scale runway friction research programs have been carried out in Sweden and in the USA and the knowledge of the physical phenomena that take place in the contact area between a braked tyre and the contaminated runway surface has improved considerably. However, the correlation of the friction as determined by measuring device and the aircraft, as well as the correlation between the measurements of the various devices, is still not very consistent.

This lack of correlation may be explained by differences of device and aircraft which affect the friction layer on a contaminated runway: wheel load and tyre pressure; tyre pattern and pavement texture; anti-skid characteristics; speed.

Notwithstanding the limited quality of the runway friction measurements, these data will be used and interpreted by the pilot when making operational decisions.

2. Runway Friction Measuring Equipment

Skiddometer (Sweden): a three wheel trailer towed by a car. The centre test wheel is geared to the outer wheels so that it experiences a controlled and continuous slip of 17% when the trailer is towed. The torque in the test wheel axle is measured and recorded.

Mu-meter (UK and USA): a trailer towed by a car with two wheels mounted at the ends of independently moveable arms, pivoted at the towing eye. The wheels are adjusted to a specific tow-out angle and the resulting side load is measured and recorded.

James Brake Decelerometer, JBD (Canada): a device indicating peak horizontal deceleration. The device is placed in a car and at 30 mph brakes are firmly applied to lock all wheels. The deceleration is read in terms of 1 g and referred to as James Brake Index (JBI).

Tapley meter (USA): a horizontal deceleration indicator used in a similar way as the JBD.

Diagonal Braked Vehicle, DBV (USA): a stationwagon with a modified brake system and special tyres allowing two wheels to be locked while the remaining two wheels ensure directional control. Stopping distances from 60 mph are measured on wet and dry runway surfaces and the wet/dry stopping distance ratio is calculated.

Stradographe (France): a stationwagon with a measuring wheel on each side, which can be braked or completely locked. Toe-in angles from 0 to 15 degrees are possible and the vertical load can be varied. All essential parameters are recorded.

Tester: a Saab automobile equipped with a fifth wheel allowing measurements using the principles of the Skiddometer. In addition measurements can be made using the diagonal brake vehicle method.

Miscellaneous: other means include locking brakes of a car or truck at a given speed and determining the coefficient from stopping time and/or stopping distance, using pilot reports, and estimating braking friction on the basis of runway surface state.

3. Reporting of Runway Friction Information

ICAO rules require runway state and braking friction information to be reported.

States are required to include in the Aeronautical Information Publication (AIP) for each international airport at which snow conditions normally occur a SNOW PLAN.

In case of snow or ice conditions a SNOWTAM must be issued containing detailed information.

In Europe SNOWTAM information is added to NOTAMs disseminated through the MOTNE network. In Canada JBI reports are included in voice communications when the JBI falls below 0.4.

In other parts of the world reporting and dissemination of runway state and braking friction information is less advanced.

4. Interpretation of Runway Friction Reports

Wet: the braking friction achieved on a wet runway depends on pavement texture, tyre pressure, brake system efficiency and hydroplaning speed.

An average texture depth of non-grooved runways of 0.5 mm is considered a minimum acceptable value; for new or resurfaced runways the texture depths should be at least 1.0 mm.

The result of wet runway friction tests is not used for day-to-day operational dissemination but is intended to monitor runway friction characteristics, for example determining the need for rubber deposit cleaning or runway resurfacing.

Reports on wet runway characteristics of immediate interest to the pilot will be along the lines of ‘slippery when wet’ or ‘standing water’ warnings.

Snow and ice covered runways: braking friction on compacted snow or ice covered runways, which may be sanded, shows only a small variation with speed. Repeatability and accuracy of measured values is comparatively good except at near zero temperatures.

In near-zero conditions, small temperature changes may cause dramatic changes in braking friction. The runway surface temperature may differ significantly from the reported airport temperature.

On compacted snow and ice covered runways the correlation of the various friction measuring devices is comparatively good and errors when expressing friction values with descriptive terms such as good, medium and poor are rather small.

The terms medium and poor can be considered equivalents of the same terms used in connection with Actual Landing Distance. By good in relation to actual landing distances a dry runway is meant, whereas a good braking friction report on a compacted snow or ice covered runway indicates that the aircraft should not experience directional control or braking problems, but the braking friction is obviously less good than on a clean, dry runway.

Slush covered runways: runway friction reports on slush covered runways must be considered unreliable.