Meteorological Observations

Meteorological Observations

Meteorological observations in connection with knowledge of previous runway conditions will, in many cases, permit a fair estimate to be made of braking action.

On snow- or ice-covered runways that are not treated, the friction coefficient varies from as low as 0.05 to 0.30. In these conditions, it is very difficult to state exactly how and why the runway conditions vary. Here, the braking action is very much dependent upon the temperature, especially near the freezing point. When the temperature is below freezing, the braking action could be fairly good. It will so remain if the temperature decreases but if the temperature rises to the freezing point or above, the braking action will decrease rapidly. Sometimes very low friction coefficient values occur when humid air is drifting over an icy runway even though the temperature may be well below the freezing point.

Some of the various atmospheric conditions that influence braking action are given below:
• Friction Coefficient between 0.10 and 0.30 (Poor - Medium/Poor)
The following conditions typically produce these friction coefficients:
   - Slush or rain on snow- or ice-covered runway;
   - Runway covered with wet snow or standing water;
   - Change from frost to temperature above freezing point;
   - The type of ice which is formed after long periods of cold;
   - A thin layer of ice formed:
   - By frozen ground having been exposed to humidity or rain at 0°C or above;
   - When due to radiation, e.g. when the sky clears, the runway surface temperature drops below freezing point and below the dew point (this ice formation can take place very suddenly and occur while the reported air temperature may still be quite a few degrees above the freezing point).

• Friction Coefficient between 0.25 and 0.35 (Medium/Poor-Medium)
The following conditions typically produce these friction coefficients:
   - Snow conditions at temperature just below freezing point;
   - Snow-covered runways at temperatures below freezing point, exposed to the sun;
   - Slush-covered runway.

• Friction Coefficient between 0.35 and 0.45 (Medium/Good-Good)
The following conditions typically produce these friction coefficients:
   - Snow-covered runways which have not been exposed to temperatures higher than about –2 ºC to –4 ºC;
   - Damp or wet runway without risk of hydroplaning (less than 3 mm water depth).

Aeroplane Performance on Wet or Contaminated Runways

Take-off and landing performances on wet or contaminated runways are given in OM Part B. The information presented to the flight crew in line operations is usually the mixture of contaminant type, contaminant depth, friction measurements and braking action reports.

The Commander shall consider all available information, including runway surface condition reports, braking action reports, and friction measurements. Given a friction or braking action report, the Commander shall apply sound judgement in considering its applicability in determining aeroplane braking performance. He shall consider the following factors:
• The age of the report;
• Meteorological conditions present since the report was issued;
• Type of aeroplane or device used to obtain the report;
• Whether the runway surface was treated since the report;
• The methods used for that treatment.

Important: Take-off or landing on a runway, contaminated with ice and/or compacted snow, for which the reported FC is less than 0.20 and/or BA is reported as »POOR«, shall be avoided. For other contaminants deposit limits as specified in OM Part B are relevant and shall be respected. When different values of contaminant depth, runway friction or braking action are reported for different portions of the same runway, the most adverse values shall be used in all performance computations.

The cleared/treated runway width should normally not be less than 30 m (100 ft). If, however, the width of the treated runway (sanded, sprayed, etc.) is less than 30 m, the operation on such runway is under Commander's discretion. The criteria for the FC/BA must be met for the treated runway width. If the treated runway width is less than 40 m, the Commander should carefully evaluate the maximum acceptable crosswind.

Guidelines for Operations on Wet and Contaminated Surfaces

• General Consideration
The use of thrust reversers is mandatory for take-off and landing on contaminated runways. The two most important variables confronting the pilot when runway coefficient of friction is low and/or conditions for hydroplaning exist, are the length of runway and crosswind magnitude. The total friction force of the tires is available for two functions – braking and cornering. If there is a crosswind, some friction force is necessary to keep the aircraft on the centreline (cornering). Tyre cornering capability is reduced during braking or when wheels are not fully spun up. Locked wheels eliminate cornering. Therefore in crosswind conditions, a longer distance will be required to stop the aircraft.

• Taxiing
Aeroplane may be taxied at the Commander discretion on ramps and taxiways not cleared of snow and slush. More power than normal may be required to commence and continue taxi so care should be taken to avoid jet blast damage to buildings, equipment and other aircraft. Be aware of the possibility of ridges or ruts of frozen snow that might cause difficulties. The boundaries/edges of manoeuvring areas and taxiway should be clearly discernible. If in doubt, request ‘Follow me’ car guidance.

When executing sharp turns while taxiing or parking at the ramp, remember that braking and steering capabilities are greatly reduced with icy airport conditions; reduce taxi speed accordingly. Slat/flap selection should be delayed until immediately before line up to minimise contamination.

• Take-off
Severe retardation may occur in slush or wet snow. In most cases, this lack of acceleration will be evident early on the take-off run. Maximum permissible thrust/power must be used from the start. Large quantities of snow or slush, usually containing sand or other anti-skid substances may be thrown into the engines, static ports and onto the airframe. Pod and engine clearance must be watched when the runway is cleared and snow is banked at the sides of runways or taxiway.

• Landing
Pilots should be aware that where rain, hail, sleet or snow showers are encountered on the approach or have been reported as having recently crossed the airfield, there is a high probability of the runway being contaminated. The runway state should be checked with ATC before commencing or continuing the approach. Very often a short delay is sufficient to allow the runway to drain or the contaminant to melt.

Use of reverse thrust on landing on dry snow in very low temperatures will blow the dry snow forward especially at low speed. The increase in temperature may melt this snow and form clear ice on re-freezing on static ports.

The shortest stopping distances on wet runways occur when the brakes are fully applied as soon as possible after main wheel spin up with maximum and immediate use of reverse thrust.

Landing on contaminated runways without antiskid should be avoided. It is strongly recommended to use the autobrake (if available) provided that the contaminant is evenly distributed. The factors and considerations involved in landing on a slippery surface are quite complex and depending on the circumstances, the pilot may have to make critical decisions almost instinctively.

The following list of items summarises the key points to be borne in mind (several may have to be acted upon simultaneously):
• Limit crosswind components to values specified in OM Part B;
• Establish and maintain a stabilised approach;
• Consider the many variables involved before landing on a slippery runway:
   - Landing weather forecast;
   - Aircraft weight and approach speed;
   - Landing distance required;
   - Aquaplaning speed;
   - Condition of tires;
   - Brake characteristics (anti-skid, autobrake mode);
   - Wind effects on the directional control;
   - Runway length and slope;
   - Glide path angle;
• Do not exceed VAPP at the threshold. An extended flare is more likely to occur if excess approach speed is present;
• Be prepared to go-around;
• Flare the aircraft firmly at the 1000 ft aiming point. Avoid build up of drift in the flare and runway consuming float. A firm landing, by facilitating a prompt wheel spin up, also ensures efficient antiskid braking;
• Select reverse thrust as soon as possible;
• Get the nose of the aircraft down quickly. Do not attempt to hold the nose off aerodynamic braking. Aim to have the nose wheel on the ground by the time reverse thrust reaches the target level;
• The brakes are the primary means for stopping the aeroplane but if necessary the full reverse thrust may be maintained until the aeroplane is fully stopped. Excessive braking in crosswinds will lead to the aeroplane drifting away from the centreline. Do not de-crab completely as the aeroplane will yaw on the slippery runway due to its weathercock stability;
• Keep the aircraft aligned with the runway centreline. Do not allow large deviations from the runway heading to develop as recovery can become very difficult. Use of the nose wheel steering is not recommended. Under slippery conditions, the nose wheels must be closely aligned with the aeroplane track or they will scrub;
• If directional or lateral control difficulties are experienced, disconnect the autobrake, if necessary, reduce reverse thrust levels symmetrically, and regain directional control with rudder, aileron and differential braking. Once under control, re-apply manual braking and increase symmetrical reverse levels as required while easing the aircraft back towards the runway centreline; •After landing in heavy slush do not retract the slats and flaps. Allow ground personnel to clear ice and slush from slats and flaps before full retraction. Taxi with caution to parking area as flaps extended provides a much reduced ground clearance.