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Aquaplaning & Tyre Pressures

 Aquaplaning is a phenomenon that many private pilots will have heard about, but may not completely understand. You may think that it only interests pilots of large aircraft with high-pressure tyres and high touchdown speeds, but an understanding of the cause of aquaplaning and the very low thresholds involved may convince you otherwise. It will also give you a clue as to why we need high-pressure tyres on aircraft that land and take off at high speeds.

 

 Aquaplaning is generally understood to occur when a tyre encounters a layer of water on the runway and then suffers a marked and sudden reduction in retardation. That is only part of the picture. The first surprise is that aquaplaning can occur without any braking force being applied to a wheel. The second surprise is that it can occur at quite low speeds and does not need much depth of water to begin.
      Let's look at what causes aquaplaning. As an UNBRAKED wheel rolls along a water-covered runway it contacts and displaces the stationary water lying on the tarmac. This causes a change in momentum of the water (i.e. the water is moved) that creates hydrodynamic pressure that reacts on both the runway surface and on the tyre surface in contact with the runway. The hydrodynamic pressure increases as the square of the groundspeed of the wheel. As the speed increases, the inertia of the water retards its escape from the tyre/runway contact area and a wedge of water forms that begins to lift the tyre from the runway. Further increase of the groundspeed increases the hydrodynamic lift until it equals the weight being supported by the tyre, lifting the tyre clear of the runway. This condition is known as total aquaplaning.

 

 

 

Vr is the wheel rotational speed.

Dt is the drag force caused by all the tyre drag sources that combine to form the wheel spin-up moment.
Fm is the vertical load due to aircraft mass. ‘A’ is the tyre footprint area.

On a wet runway (Figure 2) the wedge of water described above has started to penetrate the tyre footprint and the wheel is partially supported by the hydrodynamic force produced by this wedge of water. 'B' is the tyre/water contact area. Note that the tyre/runway contact area ('A') is decreased when compared with Figure 1. Because the tyre/runway contact area is decreased, the tyre drag ('Dt') is also decreased and the force Fm is moved forward of the wheel axle line, causing a wheel spin-down moment, reducing the rotational speed of the wheel. The wheel rotates more slowly although the groundspeed of the aircraft has not decreased.

In Figure 3 we have reached total aquaplaning and the tyre has been lifted completely clear of the runway surface. The wheel spin-up moment is approaching zero and the vertical force (Fm) has moved even further forward of the axel line. The wheel spin-down moment is at a maximum and the rotational speed (Vr) will decrease rapidly to zero. Put another way, the wheel will stop rotating. Ground friction is zero, so it is impossible to brake or steer the aircraft. Any crosswind may well drift the aircraft off the side of the runway unless aquaplaning ceases.

There are several obvious factors that can affect the onset of aquaplaning. Aircraft weight may appear to be one of these, but it isn't. As the weight on the tyre changes so the contact area between the tyre and the runway (the footprint) changes. The ratio of weight to area remains constant; this is essentially due to tyre PRESSURE. The hydrodynamic lift pressure over the whole footprint must equal the tyre pressure before total aquaplaning occurs. A simple equation based on hydrodynamic lift theory can be used to predict total aquaplaning speed for any vehicle that uses pneumatic tyres; this is:

V = 8.6√P where V is the total aquaplaning speed in knots and P is the tyre pressure in pounds per square inch.

Aquaplaning is a progressive phenomenon and significant losses in braking ability and in directional control will occur at speeds below total aquaplaning speed. Once aquaplaning has begun, wheels can spin-down and stop fairly rapidly. However, it will take longer for wheels to spin-up again after speed reduces below total aquaplaning speed. Aquaplaning, once begun, will persist to a speed lower than that given by 8.6vP.
      The actual depth of water required to initiate aquaplaning is not well defined and varies considerably with tyre tread design and with runway surface smoothness. It can occur on smooth tarmac with smooth tyres in a depth of only 3 millimetres of water - slightly more than 1/10 of an inch in old money.
      The Manual of Air Traffic Services Part 1 defines the presence of water on runways as 'essential aerodrome information which shall be issued to pilots in sufficient time to ensure the safe operation of the aircraft.' The table below contains the definitions of the terms that Air Traffic may use to describe the runway surface.

 

 

When reported, the presence of surface water on a runway should be assessed over the most significant portion of the runway (i.e. the most likely area to be used by the aircraft taking off or landing). For JAR-OPS performance purposes, runways reported as 'wet patches' or 'flooded' should be considered as contaminated. That gives you the legalistic situation, but you will now know exactly what a controller means when 'wet', 'water patches' or 'flooded' is passed to you during an approach.

 

 

I have referred to an unbraked wheel on a hard runway in this basic explanation of the aquaplaning phenomenon to make the point that it is not necessary to apply the aircraft brakes before aquaplaning will occur. If you attempt to slow the aircraft by the application of wheel brakes in conditions that are conducive to aquaplaning then it is likely that the braked wheels will stop rotating and the braking force between the runway and the tyres will drop to zero. A pneumatic tyre that has stopped rotating because of excessive braking actually produces less retardation that a tyre which is being braked, but is still rotating. Locking the wheels, even in dry conditions, is bad. This problem led to the development of the Dunlop 'Maxaret' braking system in the 1950's. The 'Maxaret' system worked by sensing when the wheel was about to lock. It then released the applied brake pressure for a very short period before reapplying maximum braking effort. This reduced the chance of locking the wheel while trying for maximum retardation. Many other types of ABS (Automatic Braking Systems) have been developed and are now standard equipment on many cars - but not on many General Aviation aircraft!

What does aquaplaning feel like in the cockpit?
My own experience of aquaplaning is, happily, limited but I do recall a very sudden feeling of loss of control of both direction and retardation. The aircraft really does feel as if it is skating along on ice. Aquaplaning in a strong crosswind with a big braking parachute trying to drag you off the side of a runway is quite exciting. Your first reaction will probably be to think 'Brake failure!' If you have a brake pressure gauge you will see that normal pressure is available. If you are on a shortish runway looking down at the brake pressure gauge is wasting valuable time and distance along the runway, and you should have checked the pressures during your pre-landing checks.

 

What should you do about it if aquaplaning begins?
My personal recommendation is to immediately apply full power, adjust your flaps as required, trim and go around. This is particularly important if you have touched down slightly long on a wet runway. If you haven't got enough runway left ahead to go around safely then it's not your day. I hope you and your passengers are firmly strapped in before you all become members of The Hedgers and Ditchers Club. Assuming you have made your decision quickly and have enough runway remaining then go around, fly an accurate approach and touch down close to the threshold. If your aircraft has a nosewheel then apply the brakes immediately after touchdown and move the elevator gently towards the fully up position to ensure that the maximum weight is on the mainwheels. If your aircraft does not have some type of ABS then it may help you use 'cadence' braking by applying and releasing brake pressure in rapid dabs.

 

A secondary problem associated with aquaplaning is localised tyre wear. When a wheel slows or stops rotating because of aquaplaning it is held off the runway surface by a water layer. As the aircraft speed decreases sufficiently below aquaplaning speed the wheel remains locked (especially if brakes are applied) as it comes into contact with the surface, often producing a localised patch of excessive wear on the tyres. Scalding of the tyre surface may also occur during aquaplaning. It's a good idea to examine the whole circumference of your tyres if you think your aircraft may have suffered momentary aquaplaning. A new tyre costs less than a new aircraft.

      Finally, let's look at practicalities. My favourite light aircraft, the Cessna 310R, has mainwheel tyre pressures of 60 psi and a nosewheel tyre pressure of 24 psi. These numbers predict a total aquaplaning speed of 66 knots groundspeed for the mainwheels and 42 knots for the nosewheel. Both figures are certainly within the realms of possibility. Notice that the nosewheel will start aquaplaning before the mainwheels. What significance will this have when trying to steer on a wet runway? It's also worth a thought that if I'm careless enough to operate the 310 with mainwheel tyre pressures reduced to 50 psi then my total aquaplaning speed will drop to 60 knots groundspeed - so that's another reason to regularly check your tyre pressures. What are the mainwheel pressures on your aircraft? What are they supposed to be?
      Next time you are driving down the motorway in pouring rain on your 35 psi tyres at 70 mph (who goes faster?) be aware that you could be well into the total aquaplaning region and find yourself making sudden contact with the barrier or that big truck a few yards ahead.
      Aquaplaning is not an every-day hazard for light GA aircraft but if you do meet it you need to understand what's going on and sort it out quickly before your lovely aircraft drifts off into the surrounding scenery. Plan ahead and live long, as the Vulcans say!

 

PS. Forty years ago we were advised not to try for a 'greaser' landing on a flooded runway, but to whack it on and so reduce the chances of aquaplaning by making sure the wheels spun up at touchdown, but that was on a type that had mainwheels inflated to 330 psi and a minimum touchdown speed of 175 knots IAS - and we seldom achieved more than a couple of landings per set of mainwheel tyres. Perhaps your instructor will believe you if you explain your next firm arrival as an anti-aquaplaning measure?

 

 

 

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