Recommendations for further research

TERMINOLOGY

Terms used in this section have the following meaning: Contaminated runway

A runway is considered to be contaminated when more than 25% of the runway surface area (whether in isolated areas or not) within the required length and width used is covered by the following:

¾ Surface water more than 3 mm (0.125 in) deep, or by slush, or loose snow, equivalent to more than 3 mm (0.125 in) of water;

¾ Snow which has been compressed into a solid mass which resists further compression and holds together or breaks into lumps if picked up (compacted snow); or

¾ Ice, including wet ice.

Chapter 8 Special Operational Procedures and Hazards

Damp runway

A damp runway is where the surface is not dry, but the moisture on it does not give it a shiny appearance.

Dry runway

A dry runway is one which is neither wet nor contaminated, and includes those paved runways, specially prepared with grooves or porous pavement, and maintained to retain effectively dry braking action even when moisture is present.

Wet runway

A wet runway is a water covered surface, or equivalent, less than as defined in ‘Contaminated runway’ above or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.

Contaminant Depth

If exceeding the following limits, then do not attempt a take-off:

¾ Dry snow > 60 mm

¾ Very dry snow > 80 mm ¾ Water, slush or wet snow > 15 mm

AQUAPLANING (HYDROPLANING)

Aquaplaning is the effect of the tyres of an aeroplane riding over water on the surface of a runway. As the speed of the aeroplane on the ground increases a ‘bow wave’ of water builds up in front of the tyre and eventually the tyre is lifted off the surface. This allows the tyre to slow, and creates a friction boundary between the tyre and the surface of the runway. The heat generated by the friction can cause the tyre to scald and the rubber to melt with the possibility of tyre explosion. In any event, when a tyre is aquaplaning there is a loss of adhesion and thus loss of directional control. This is not such a problem for normal take-off except in the case of a rejected take-off. It is a major concern for aircraft landing on to a contaminated runway especially in a cross wind condition. Aquaplaning does not generally begin at a speed less than the critical speed given by the formula:

V = 9√P

Where: V is the groundspeed (kt)

P is the tyre pressure (lb per in2)

However, once hydroplaning starts, it continues to speeds well below the critical speed. There are three types of hydroplaning:

Dynamic Hydroplaning

Dynamic hydroplaning is a condition where the tyre lifts completely above the surface of the runway. As little as 2.5 mm of water is sufficient to produce dynamic hydroplaning. Viscous Hydroplaning

Special Operational Procedures and Hazards Chapter8

Operational Procedures 8-25

Reverted Rubber (Steam) Hydroplaning

Hard braking during a rejected takeoff or long landing may causes the brakes to lock, although the maxaret units should act in the same manner as the ABS systems in cars. If brake locking occurs on a wet runway, the tyre track area heats up due to friction causing some of the rubber to revert back to a gummy state, trapping water. The water turns to steam and steam pressure lifts the tyre from the runway.

STATIONARY TYRE

Studies show that a tyre that is not rotating does hydroplane at a lower speed than a rotating tyre. NASA has evaluated the speed as 7.7 x √P. There is a question in the question bank concerning non-rotating tyres.

RECOMMENDATIONS

It is nearly impossible to land an aeroplane at a speed below the critical speed, but using the following techniques can reduce the effects of hydroplaning:

¾ Approach to land at the slowest airspeed consistent with safety; that is, use the short-field landing technique.

¾ Land firmly, rather than making the smooth, “greaser” type landing.

¾ Lower the nose wheel to the surface as soon as the main wheels are firmly on the surface.

¾ Know the hydroplaning critical speed and avoid heavy braking above this speed. ¾ Retract the flaps immediately after landing to place more weight on the tyres.

¾ Divert to an alternate aerodrome when conditions indicate a potential hydroplaning hazard on runways experiencing a strong crosswind.

Tyre Pressure Vs. Hydroplaning Speed Tyre Pressure lb/Bar Hydroplaning Speed Knots

30 /2.0 49 50/3.45 64 100/5.5 90 150/10.35 110 200/13.8 127 (B737) 225/15.5 135 (B777)

If the surface is covered by a contaminant other than water, then divide P by the specific gravity of the contaminant. Tyre configuration, treading, etc., increase the speed at which aquaplaning begins. Beware: there is a question in the exam where the given tyre pressure is in Bar. (1 Bar = 14.5 psi).

WHEEL BRAKING ON WET RUNWAYS

The retardation effect of an aircraft braking system relies on friction with the surface of the runway. If the surface is not dry then the amount of friction is reduced. The reduction in friction can be given in a factor known as the co-efficient of braking, defined by the value of friction of the runway at an instant in time, determined by measurement, divided by the value of friction for the same runway when dry. If the runway is dry, the coefficient of braking is 1. If not dry, the co- efficient is less than 1.

Chapter 8 Special Operational Procedures and Hazards

All paved runways of 1200 m or longer require calibration for co-efficient of braking. When wet, good braking action is possible to calculate.

RTF reports the presence of water on a runway as follows: Dry The surface is dry.

Damp The surface shows a change of colour due to moisture.

Wet The surface is soaked, but no significant patches of standing water are visible.

Water Patches Significant patches of standing water are visible.

Flooded Extensive standing water is visible.

INTERPRETATION

When a runway is reported as dry, damp, or wet, pilots may assume an acceptable level of braking friction is present. Water patches or flooded means that braking may be affected by hydroplaning and appropriate adjustments should be considered. Water patches will be reported if at least 25% of the runway is affected.

When a runway is notified as slippery when wet, take-offs and landings in wet conditions should only be considered if the distances equal or exceed the distances required for icy runways as defined in the aircraft manual.

SNOW, SLUSH, OR ICE ON A RUNWAY

Whenever a runway is affected by snow, slush, or ice and it has not been possible to clear the precipitant fully, assess the condition of the runway, and the friction coefficient measured.

The table below, with associated descriptive terms, was developed from friction data collected in compacted snow and ice and should not be taken as absolute values applicable in all conditions.

Friction Co-efficient

Measured Coefficient Estimated Braking Action Code

0.40 and above Good 5

0.39 to 0.36 Medium to good 4

0.35 to 0.30 Medium 3

0.29 to 0.26 Medium to poor 2

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Operational Procedures 8-27

If the surface is affected by snow or ice and the braking action reported as “good”, pilots should not expect to find conditions as good as on a clean dry runway (where the available friction may well be greater than that needed in any case). The value “good” is a comparative value and is intended to mean that aeroplanes should not experience directional control or braking difficulties especially when landing.