Adverse Runway Conditions and Measuring of Runway Friction Coefficient
Adverse Runway Conditions
Runway Friction Characteristics
The stopping performance of an aeroplane is for the most part dependent on the available friction between the aircraft tires and the runway surface, and the landing and take-off speeds.
Adequate runway friction characteristics/suitable braking action are required for: •The deceleration of the aircraft after landing or a rejected take-off;
• The directional control during the ground roll on take-off or landing, in particular in the presence of crosswind, asymmetric engine power or technical malfunctions;
• Wheel spin-up at touchdown.
To compensate for the reduced stopping and directional control capability for adverse runway conditions (such as wet or slippery conditions) performance corrections are applied in the form of a:
The friction level of a concrete runway is not as good as black top asphalt runway. Asphalt needs grooving to be skid resistant, especially when wet. Grooving is an artificial way of restoring texture to asphalt. It gives the gaps that the water can dissipate into as the tyre rolls over the asphalt. A grooved runway is restored to being equivalent in surface texture to a surface that is a little bit rough.
Another treatment, which is equivalent to grooving in terms of restoring texture, is called Porous, or Porous Friction Course (PFC). Finally, a few runways that are Sealed or Chip Sealed are also black and have a rough texture. Grooved/Porous/Sealed asphalt should be treated as a normal runway surface, and normal limits are applied (wet/dry etc.) without any further performance credit assumed. Un-grooved asphalt or ‘not porous’ asphalt should be considered as sub-standard and treated with caution when the surface is not dry.
However, even grooving may not cope with standing water due to:
Measuring of Runway Friction Coefficient
The friction coefficient (FC) is defined as the ratio of the maximum available tire friction force and the vertical load acting on the tire. This coefficient is named “Mu”. Various systems are used to measure the runway friction coefficient / conditions: 1) Skiddometer High pressure tire (SKH);
2) Skiddometer Low pressure tire (SKL);
3) Surface Friction Tester (SFT);
4) Mu-meter (MUM);
5) Diagonal braked vehicle (DBV);
6) Tapley meter (TAP);
7) James Brake Decelerometer (JBD).
Experience has shown that the presence of contaminants may under certain conditions adversely affect the accuracy of the friction values obtained by any of the above friction measuring devices. The following are conditions under which friction measurements may not relate to actual aeroplane braking performance:
The only perfect way of measuring the friction coefficient for a specific aeroplane is by using that specific aeroplane braking system on the surface concerned.
Friction measurements or braking action estimation may be reported:
When friction measurement are not available but can be only estimated, the flight crew will be informed only of the estimated braking action (BA) reported as »DRY«, »GOOD«, »MEDIUM« (or »FAIR«), »POOR«, »UNRELIABLE/NIL« or a combination of these terms.
The following ICAO ATC runway friction report codes relate to the surface conditions i.e. ‘good’ on a wet runway relates to wet stopping performance which is already degraded with respect to dry runway operations.
• DRY: Maximum energy stops possible with little deterioration in certified (dry) stopping distance.
• GOOD: More braking is available than will be used in an average airline type deceleration.
If a maximum energy stop were attempted (wet), some distance in excess of certified stopping distance would be expected. If the surface is affected by snow or ice and the braking action is reported as ‘good’, pilots should not expect to find conditions as good as on 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.
• MEDIUM: Sufficient braking and cornering force is available for a well-flown approach and landing using light braking. However, excess speed or long touchdown would result in an extremely low safety factor depending on runway length and crosswind component. Careful planning and good judgment are required.
• POOR: Very careful planning, judgment, and execution are absolutely essential. Crosswind becomes a priority consideration. While a safe and successful approach, landing, and stop can be accomplished if all factors are favourable, there is little room for error. Care must be exercised in every aspect of the operation and a very careful evaluation of all conditions is necessary.
• UNRELIABLE: Will be reported when surface conditions do not permit a meaningful braking action value to be determined (i.e., standing water, slush, wet snow with potential hydroplaning).
Combination of reports is possible (MEDIUM/POOR for example) when conditions are between those described above.
Pilots should treat reported braking action reports with caution and interpret them conservatively.
For example, the pilots of two identical aeroplanes landing in the same conditions, on the same runway could give different braking action reports. These differing reports could be the result of differences between the specific aeroplane, aeroplane weight, pilot technique, pilot experience in similar conditions, pilot total experience, and pilot expectations.
Practically the following correlation may be used as a guideline:
Runway Friction Characteristics
The stopping performance of an aeroplane is for the most part dependent on the available friction between the aircraft tires and the runway surface, and the landing and take-off speeds.
Adequate runway friction characteristics/suitable braking action are required for: •The deceleration of the aircraft after landing or a rejected take-off;
• The directional control during the ground roll on take-off or landing, in particular in the presence of crosswind, asymmetric engine power or technical malfunctions;
• Wheel spin-up at touchdown.
To compensate for the reduced stopping and directional control capability for adverse runway conditions (such as wet or slippery conditions) performance corrections are applied in the form of a:
- Runway length increment;
- Reduction in allowable take-off or landing weight;
- Reduction of allowable cross-wind component.
The friction level of a concrete runway is not as good as black top asphalt runway. Asphalt needs grooving to be skid resistant, especially when wet. Grooving is an artificial way of restoring texture to asphalt. It gives the gaps that the water can dissipate into as the tyre rolls over the asphalt. A grooved runway is restored to being equivalent in surface texture to a surface that is a little bit rough.
Another treatment, which is equivalent to grooving in terms of restoring texture, is called Porous, or Porous Friction Course (PFC). Finally, a few runways that are Sealed or Chip Sealed are also black and have a rough texture. Grooved/Porous/Sealed asphalt should be treated as a normal runway surface, and normal limits are applied (wet/dry etc.) without any further performance credit assumed. Un-grooved asphalt or ‘not porous’ asphalt should be considered as sub-standard and treated with caution when the surface is not dry.
However, even grooving may not cope with standing water due to:
- Ruts and birdbaths in the runway (common in worn-out runways);
- Heavy rainstorms;
- All the grooves and texture being filled up with rubber deposits.
Measuring of Runway Friction Coefficient
The friction coefficient (FC) is defined as the ratio of the maximum available tire friction force and the vertical load acting on the tire. This coefficient is named “Mu”. Various systems are used to measure the runway friction coefficient / conditions: 1) Skiddometer High pressure tire (SKH);
2) Skiddometer Low pressure tire (SKL);
3) Surface Friction Tester (SFT);
4) Mu-meter (MUM);
5) Diagonal braked vehicle (DBV);
6) Tapley meter (TAP);
7) James Brake Decelerometer (JBD).
Experience has shown that the presence of contaminants may under certain conditions adversely affect the accuracy of the friction values obtained by any of the above friction measuring devices. The following are conditions under which friction measurements may not relate to actual aeroplane braking performance:
- When the water on a pavement surface exceeds 1 mm;
- When water is present on top of an ice layer on the pavement surface, and its depth exceeds 1 mm);
- When there is presence of slush on the pavement exceeding 3 mm;
- When there is slush or wet snow on ice or compacted snow exceeding 3 mm;
- When the depth of dry snow on the pavement surface exceeds 25 mm.
The only perfect way of measuring the friction coefficient for a specific aeroplane is by using that specific aeroplane braking system on the surface concerned.
Friction measurements or braking action estimation may be reported:
- In plain language by the tower;
- By the routine weather broadcast (METAR);
- By SNOWTAM.
When friction measurement are not available but can be only estimated, the flight crew will be informed only of the estimated braking action (BA) reported as »DRY«, »GOOD«, »MEDIUM« (or »FAIR«), »POOR«, »UNRELIABLE/NIL« or a combination of these terms.
The following ICAO ATC runway friction report codes relate to the surface conditions i.e. ‘good’ on a wet runway relates to wet stopping performance which is already degraded with respect to dry runway operations.
• DRY: Maximum energy stops possible with little deterioration in certified (dry) stopping distance.
• GOOD: More braking is available than will be used in an average airline type deceleration.
If a maximum energy stop were attempted (wet), some distance in excess of certified stopping distance would be expected. If the surface is affected by snow or ice and the braking action is reported as ‘good’, pilots should not expect to find conditions as good as on 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.
• MEDIUM: Sufficient braking and cornering force is available for a well-flown approach and landing using light braking. However, excess speed or long touchdown would result in an extremely low safety factor depending on runway length and crosswind component. Careful planning and good judgment are required.
• POOR: Very careful planning, judgment, and execution are absolutely essential. Crosswind becomes a priority consideration. While a safe and successful approach, landing, and stop can be accomplished if all factors are favourable, there is little room for error. Care must be exercised in every aspect of the operation and a very careful evaluation of all conditions is necessary.
• UNRELIABLE: Will be reported when surface conditions do not permit a meaningful braking action value to be determined (i.e., standing water, slush, wet snow with potential hydroplaning).
Combination of reports is possible (MEDIUM/POOR for example) when conditions are between those described above.
Pilots should treat reported braking action reports with caution and interpret them conservatively.
For example, the pilots of two identical aeroplanes landing in the same conditions, on the same runway could give different braking action reports. These differing reports could be the result of differences between the specific aeroplane, aeroplane weight, pilot technique, pilot experience in similar conditions, pilot total experience, and pilot expectations.
Practically the following correlation may be used as a guideline:
Adverse Runway Conditions and Measuring of Runway Friction Coefficient
Reviewed by Aviation Lesson
on
10:34 AM
Rating:
Reviewed by Aviation Lesson
on
10:34 AM
Rating:
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