VAQUAPLANING  (kt) = 34 (PT/σ)0.5 

With 

PT = tire pressure (kg/cm2) 

σ = specific gravity of the contaminant. 

In other words, the aquaplaning speed is a threshold at which friction forces are severely diminished. Performance calculations on contaminated runways take into account the penalizing effect of hydroplaning. 

5.5.3. Aircraft Manufacturer Data 

The aircraft manufacturer has to provide relevant data for operations on runways contaminated by one of the above contaminants, as quoted below: 

 

“JAR 25X1591 (a)(c) Supplementary performance information for runways contaminated with standing water, slush, loose snow, compacted snow or ice must be furnished by the manufacturer in an approved document, in the form of guidance material, to assist operators in developing suitable guidance, recommendations or instructions for use by their flight crews when operating on contaminated runway surface conditions.” 

“JAR 25X1591 

(d) The information [on contaminated runways] may be established by calculation or by testing.” 

As far as performance determination is concerned, Airbus provides guidance material for the following runway contaminants and maximum depths (Table C10): 

Table C10: Wet and Contaminated Runways 

Note that takeoff is not recommended, when conditions are worse than the above-listed. 

5.5.4. Takeoff Performance on Wet and Contaminated Runways 

5.5.4.1. Acceleration Stop Distance 

The ASD definition on a contaminated runway is the same as on a wet runway. Reversers’ effect may be taken into account in the ASD calculation, as soon as the surface is not dry. The distances can either be established by calculation or testing. 

5.5.4.2. Takeoff Distance and Takeoff Run 

The TOD and TOR definitions on a contaminated runway are similar to the ones on a  wet runway. They can either be established by calculation or testing. 

5.5.4.3. Takeoff Flight Path 

“JAR-OPS 1.495 

(a) The net flight path must clear all relevant obstacles by a vertical distance of 35 ft.” 

“JAR 25.115 

(a) The takeoff flight path begins 35 ft above the takeoff surface at the end of the takeoff distance.” 

On a wet or contaminated runway, the screen height (height at the end of the TOD) is 15 feet. The net takeoff flight path starts at 35 feet at the end of the TOD. So, the gross flight path starts at 15 feet while the net flight path starts at 35 feet at the end of the TOD (see Figure C28). 

“IEM-OPS 1.495 When taking off on a wet or a contaminated runway and an engine failure occurs at V1, this implies that the aeroplane can initially be as much as 20 ft below the net takeoff flight path, and therefore may clear close-in obstacles by only 15 ft”. 

While the net flight path clears the obstacles by 35 feet all along the takeoff flight path, the gross flight path can initially be at less than 35 feet above close-in obstacles. 

5.5.4.4. Takeoff Weight 

The TOD and ASD requirements differ between wet and contaminated runways on one side, and dry runways on the other side. 

Indeed, on wet and contaminated runways, the screen height is measured at 15 feet rather than 35 feet on dry runways. Moreover, the use of reverse thrust is allowed for ASD determination on wet and contaminated runways, whereas it is forbidden to take it into account for the ASD determination on dry runways. 

Therefore, it is possible to obtain shorter TODs and ASDs on wet and contaminated runways than on dry runways for the same takeoff conditions. Thus, it is possible to obtain higher takeoff weights on surfaces covered with water, slush, or snow than on dry runways. This is why the regulation indicates that: 

 

“JAR-OPS 1.490 (b)(5) On a wet or contaminated runway, the takeoff mass must not exceed that permitted for a takeoff on a dry runway under the same conditions”. 

6. MAXIMUM TAKEOFF WEIGHT DETERMINATION 

6.1. Speed Optimization Process 

Airbus recommends that the MTOW on a given runway and given conditions be computed by optimizing both the V1/VR ratio and the V2/VS ratio. 

The performance software provided by Airbus automatically carries out this optimized computation, whose aim is to achieve the highest possible MTOW. This optimization process is described in Appendix 2 of this manual. 

6.2. Regulatory Takeoff Weight Chart (RTOW Chart) 

To determine the regulatory takeoff weight for repetitive takeoff planning, it is mandatory to provide pilots with data, which enable quick calculations of the Maximum Allowed Takeoff Weight and its associated speeds. This can be done via ground or onboard computerized systems, such as the LPC (Less Paper Cockpit: see Appendix 3), or through paper documents.