Until recently, regulations stated that, for a wet runway and for a runway covered with standing water or slush, the aircraft’s friction coefficient could be deduced from the one obtained on a dry runway, as follows: 

μwet = μdry/2 (limited to 0.4) 

μconta = μdry/4 

This concerns A300, A300-600, A310, A320 (except A320-233), A321-100 (JAA certification only), A330-300 (JAA certification only) and A340 basic versions. 

As of today, a new method, known as ESDU, has been developed and introduced by post-amendment 42 in JAR/FAR 25.109. The proposed calculation method of the μwet accounts for the tire pressure, the tire wear state, the type of runway and the anti-skid efficiency demonstrated through flight tests. The μconta (water and slush) results from an amendment based on a flight test campaign. The ESDU model concerns all aircraft types which are not mentioned above. 

For snow-covered or icy runways, the following values are considered, whatever the aircraft type: μsnow = 0.2 μicy = 0.05 

5.5.2.2. Effective μ and Reported μ 

Airport authorities publish contaminated runway information in a document called “SNOWTAM”, which contains: 

The type of contaminant 

 

The mean depth for each third of total runway length 

The reported μ or braking action. 

The reported μ is measured by such friction-measuring vehicles, as: Skidometer, Saab Friction Tester (SFT), MU-Meter, James Brake Decelerometer (JDB), Tapley meter, Diagonal Braked Vehicle (DBV). ICAO Airport Services Manual Part 2 provides information on these measuring vehicles. 

The main problem is that the resulting friction forces of an aircraft (interaction tire/runway) depend on its weight, tire wear, tire pressure, anti-skid system efficiency and… ground speed. The only way to obtain the aircraft’s effective μ would be to use the aircraft itself in the same takeoff conditions, which is of course not realistic in daily operations. 

Another solution is to use one of the above-mentioned vehicles, but these vehicles operate at much lower speeds and weights than an aircraft. Then comes the problem of correlating the figures obtained from these measuring vehicles (reported μ), and the actual braking performance of an aircraft (effective μ). 

To date, scientists have been unsuccessful in providing the industry with reliable and universal values. But tests and studies are still in progress. This is why Airbus publishes contaminated runway information as a function of the type of contaminant and depth of contaminant, and not as a function of the aircraft’s effective μ. Regulation states that: 

“IEM OPS 1.485 

(b) If the performance data has been determined on the basis of measured runway friction coefficient, the operator should use a procedure correlating the measured runway friction coefficient and the effective braking coefficient of friction of the aeroplane type over the required speed range for the existing runway conditions.” 

5.5.2.3. Precipitation Drag 

Precipitation drag is composed of: 

Displacement drag: Produced by the displacement of the contaminant fluid from the path of the tire. 

Spray impingement drag: Produced by the spray thrown up by the wheels (mainly those of the nose gear) onto the fuselage. 

The effect of these additional drags is to : 

Improve the deceleration rate: Positive effect, in case of a rejected takeoff. 

Worsen the acceleration rate: Negative effect for takeoff. 

So, the negative effect on the acceleration rate leads to limit the depth of a fluid contaminant to a maximum value. On the other hand, with a hard contaminant covering the runway surface, only the friction coefficient (effective μ) is affected, and the depth of contaminant therefore has no influence on takeoff performance. 

5.5.2.4. Aquaplaning Phenomenon 

The presence of water on the runway creates an intervening water film between the tire and the runway, leading to a reduction of the dry area (Figure C27). This phenomenon becomes more critical at higher speeds, where the water cannot be squeezed out from between the tire and the runway. Aquaplaning (or hydroplaning) is a situation where the tires of the aircraft are, to a large extent, separated from the runway surface by a thin fluid film. Under these conditions, tire traction drops to almost negligible values along with aircraft wheels’ braking; wheel steering for directional control is, therefore, virtually ineffective. 

 

Aquaplaning speed depends on tire pressure, and on the specific gravity of the contaminant (i.e. how dense the contaminant is).