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Airbus aircraft are all basically certified for takeoff on runways whose slopes are between -2% and +2%. Nevertheless, these values can be extended to higher limits for operations on particular runways, but it remains marginal as it requires additional certification tests. 

From a performance point of view, an upward slope degrades the aircraft’s acceleration capability and, consequently, increases takeoff distance. On the other hand, the stopping distance is shortened in case of a rejected takeoff. This is why, depending on the takeoff limitation, an upward slope can sometimes improve MTOW and sometimes lower it. 

5.5. Runway Conditions (Dry, Damp, Wet, Contaminated) 

The previously-discussed performance aspects only concerned dry and wet runways. But contaminants also affect takeoff performance, and have to be considered for takeoff weight calculation. The following section aims at defining the different runway states that can be encountered at takeoff. 

5.5.1. Definitions 

“JAR-OPS 1.480

 (4) Dry runway: A dry runway is one which is neither wet nor contaminated, and includes those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain ‘effectively dry’ braking action even when moisture is present” 

“JAR-OPS 1.480 

(3) Damp runway: A runway is considered damp when the surface is not dry, but when the moisture on it does not give it a shiny appearance.” 

The FAA does not make any reference to damp runways, which are considered as wet, whereas JAR-OPS 1.475 states that a damp runway is equivalent to a dry one in terms of takeoff performance. Recently, JAR 25 and JAR-OPS Study Groups came to the conclusion that a damp runway should be considered closer to a wet one than to a dry one in terms of friction coefficient (μ)1. As of today, a JAA Notice for Proposed Amendment (NPA) is under discussion, so that in the future, a damp runway may have to be considered as wet. 

“JAR-OPS 1.480 

(10) Wet runway: A runway is considered wet when the runway surface is covered with water or equivalent, [with a depth less than or equal to 3 mm], or when there is a sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.” 

In other words, a runway is considered to be wet, as soon as it has a shiny appearance, but without risk of hydroplaning due to standing water on one part of its surface. The water depth is assumed to be less than 3 mm. 

For “grooved” or “porous friction course”2 wet runways, specific friction coefficients wet (between μdry and μwet) can be used, if provided in the Aircraft Flight Manual. The resulting ASD improvement can sometimes result in higher takeoff weights than on smooth wet runways.  Nevertheless, Airbus AFMs don’t provide any specific data for these runway types. 

“JAR-OPS 1.480 

(2) Contaminated runway: A runway is considered to be contaminated when more than 25% of the runway surface area within the required length and width being used is covered by the following:” 

Standing water: Caused by heavy rainfall and/or insufficient runway drainage with a depth of more than 3 mm (0.125 in). 

Slush: Water saturated with snow, which spatters when stepping firmly on it. It is encountered at temperature around 5q C, and its density is approximately 0.85 kg/liter ( 7.1 lb / US GAL). 

Wet snow: If compacted by hand, snow will stick together and tend to form a snowball. Its density is approximately 0.4 kg/liter ( 3.35 lb / US GAL). 

Dry snow: Snow can be blown if loose, or if compacted by hand, will fall apart again upon release. Its density is approximately 0.2 kg/liter ( 1.7 lb / US GAL). 

Compacted snow: Snow has been compressed (a typical friction coefficient is 0.2). 

Ice : The friction coefficient is 0.05 or below. 

1 μ = friction coefficient = ratio of maximum available tire friction force and vertical load acting on a tire. 2 Runways specially prepared and treated with a porous friction course (PFC) overlay 

5.5.2. Effect on Performance 

There is a clear distinction of the effect of contaminants on aircraft performance. Contaminants can be divided  into hard and fluid contaminants. 

Hard contaminants are : Compacted snow and ice. They reduce friction forces. 

Fluid contaminants are : Water, slush, and loose snow. They reduce friction forces, and cause precipitation drag and aquaplaning. 

5.5.2.1. Reduction of Friction Forces 

The friction forces on a dry runway vary with aircraft speed. Flight tests help to establish the direct relation between the aircraft’s friction coefficient (μ) and the ground speed (Figure C26).