1.5.1. Introduction
01. General
When operating piston engine aircraft, runways contaminated with slush or standing water were not considered as giving very serious problems. Only after the introduction of turbine aircraft, it was realized that a take-off from a slush covered runway may be very critical. During a FAA Research Program carried out in 1961 using a CV-880 aircraft, it became clear that the wheel drag in slush and the drag caused by impingement of the slush spray on the aircraft were much higher than assumed until that time.
There are three operational problems which arise when operating on slush covered runways:
– the increased distance required for take-off due to slush drag;
– the possibility of aircraft damage, thrust loss, or other adverse consequences of slush spray thrown-up by the wheels;
– stopping problems due to reduced braking friction between tires and runway (discussed in guide 1.4.).
02. Terminology
In this discussion the term – slush – is used to cover the whole range of precipitation from snow to standing water. However, some definitions are in order.
– Dry snow, specific gravity up to 0.35.
– Wet snow: snow at or above freezing point that can easily be compacted into snowballs, specific gravity 0.35 – 0.5.
– Slush (also referred to as “clutter”): wet snow with free water that splashes if stepped upon, specific gravity 0.5 – 0.8.
– Standing water: specific gravity 1.0.
1.5.2. Operation on Slush Covered Runways
01. Slush Drag During the Take-off Run
During the high speed take-off on a runway covered with slush or standing water additional drag is generated:
– Displacement Drag; this drag obviously only applies to the front wheels of a bogie landing gear, since the path of the rear tires is cleared by the front tires.
– Impingement drag is caused by slush thrown-up by the wheels against fuselage and wing. (Fig. 1.)
The nose gear – which is responsible for only a small part of the total displacement drag – causes a very large part (up to approx. 40%) of the total drag due to the impingement of slush thrown-up by the nose gear.
An effective, but not necessarily acceptable method to reduce slush drag is to lift the nose gear off the runway early during the take-off run.
Interference drag of the landing gear caused by high energy spray impingement between the bogie wheels.
When reaching the so called “hydroplaning” speed the tires loose contact with the runway and full wheel load is carried by hydrodynamic forces. With further increase of the speed the tires finally plane on the top layer of slush or water, whereby the slush drag is reduced significantly, since both slush displacement and – as a consequence – slush impingement have disappeared to a large extent. (Fig. 2.)
02. Effect on Performance
The performance sections in the AOM’s contain directives to determine take-off weight limits for the presence of slush, snow and standing water. Internationally it is accepted that only the required take-off distance need be compensated for the effect of slush drag, while reduced braking action due to slush need not be taken into account. The take-off corrections for DC-8 and DC-9 are worked-out indeed according to this principle. For 747 and DC-10 the manufacturers have provided slush take-off data that consider the engine failure case. To that end weight corrections as well as V1 reductions are given. It must be added, however that these data have not been flight tested and it is not certain in case of engine failure near V1 (which may have been reduced by some 30 kt) at marginal weight, whether an accelerate stop resp. a continued take-off can safely be made. Less than optimum deceleration can be foreseen when stopping on a slush covered runway and controllability problems may affect the continued take-off.
03. Certification
Most aircraft are certified for take-off and landing runways covered with slush or standing water up to a depth of 0.5 inch. The certification tests are usually limited to verify that the spray patterns are such that no damage occurs, no noticeable power-loss nor any other adverse effect that could be detrimental to the safety. Such tests may lead to the requirement to install a spray-deflector.
04. Operational Directives
For the reasons stated in 1.5.2. 02. and other considerations it will be clear that the safety level of the slush take-off may be significantly less than the safety level of the routine take-offs.
Take-off from runways with slush or standing water should therefore only be considered when waiting for slush-clearance is highly impracticable.
For planning and execution of a take-off from a slush covered runway the applicable AOM directives should be followed.
The determination of the depth of the precipitation may not be accurate and moreover the depth may vary significantly over the runway. The reported depth should therefore be applied conservatively.
This also applies to the use of the NOTAM data reported through the MOTNE system.