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Performance & Navigation Topic 4.

Operations on wet/contaminated runways


History indicates that:

  • Almost 60% of Rejected Takeoff (RTO) decisions by airline crews are incorrect, often leading to the aircraft overrunning the end of the runway.
  • About 50% of all airliner accidents occur during the approach/landing phase of the flight.
  • About 28% of all airliner accidents occurring on the runway are the result of “overruns”.

A factor in many, but not all overruns, is that the runway was contaminated by water, ice, snow, or slush (wet snow). It is important for you, as the future operator of airline type aircraft, to know that anything other than a perfectly dry, sealed runway, may significantly reduce braking force applied through the tyres.


Definition: This describes a situation when a tyre is lifted clear of the runway due to fluid pressure beneath the tyre. Such a condition happens mostly when the runway is wet, and occurs when fluid (water, slush) is not displaced at a rate fast enough from the tyre/ground contact area (footprint). A melted area of tyre can achieve the same effect, whether on a wet or dry runway (called “Reverted Rubber Hydroplaning”).

The result of the tyre not being in touch with the runway is a total loss of braking ability. When hydroplaning occurs, the tyre rides or skids on a wedge or film of fluid over all or part of it’s footprint area. This wedge is the residual surface fluid that has not been removed by the tyre from the footprint area. As speed increases the wedge moves further back on the footprint area, until at “Full Hydroplaning Speed” it has lifted the tyre clear of the runway surface. At lesser speeds there may be “Partial Dynamic Hydroplaning”.

Water, and contaminated water has extremely low friction capability, and so a tyre riding on a film of water has little or no braking friction at all.

The build up of fluid pressure beneath a tyre depends on:
  • tyre pressure
  • fluid layer thickness
  • fluid density and viscosity
  • runway texture
  • tyre tread pattern and depth
  • groundspeed

Full dynamic hydroplaning. 
Tyre “planes” on standing water.


Dynamic hydroplaning

Really, this can be split into two sub-types, namely “Partial Dynamic Hydroplaning”, and “Full Dynamic Hydroplaning”.

Partial dynamic hydroplaning occurs regularly when operating on contaminated runways, and the antiskid system does a very good job of limiting it’s effect on braking.

This is where PART of the tyre is lifted off the runway due to the effect of the fluid wedge. It can occur to a grooved treaded tyre at water depths of about 2mm or more.







The above diagram shows three wheels at various groundspeeds on the same flooded runway.

At “A” the aircraft is moving forward slowly, and the entire footprint is in contact with the pavement. The tyre is managing to “pump” enough water off the runway as the wall of water built up at the front is small.

At “B” the aircraft is moving at such a speed that the wall of water ahead of the tyre cannot be entirely pumped off the runway surface, and a wedge of water is lifting part of the tyre clear of the runway. This is partial dynamic hydroplaning.

At “C” the aircraft has accelerated to it’s full dynamic hydroplaning speed, as witnessed by the entire tyre being lifted off the runway. This means, in the case of the B747-400, the entire 397, 000 kg weight of the aircraft at takeoff is being supported on a film of water only a few millimetres thick.

Finding the minimum speed for hydroplaning

The major factor involved in determining the hydroplaning speed of a tyre is it’s tyre pressure.

There are other factors, which due to their variability are hard to quantify. They are:

  • Tyre wear (ie: tread depth)
  • Runway surface texture, and whether the runway surface is grooved.
  • Contaminants on the runway such as rubber deposits and runway markings.

For theoretical calculation purposes these can be ignored when finding full hydroplaning speeds.

The Full Hydroplaning speed for a tyre depends on whether it is rotating, and therefore able to pump water off the runway surface, or is not rotating (not able to pump water off the runway).

Remember that tyres may be considered as pumps, and a pump that is not turning cannot move any fluid.

In effect the speed formula for a non-rotating wheel relates to a wheel landing on a puddled runway.

The rotating wheel formula is for a tyre rotating during the takeoff phase, or after the wheel has “spun-up” after landing.

Rotating wheel formula on flooded runway

Minimum FULL Dynamic Hydroplaning speed is 9 x ÖTyre pressure (PSI)

This formula gives the theoretical minimum speed in knots required to lift the entire tyre up off the runway. The tyre pressure is in pounds per square inch (PSI). As an example, the average airliner tyre pressure is about 160 psi. The minimum full dynamic hydroplaning speed of the tyre is 9 x the square root of 160, which is about 114 kt. If you lift off at a speed less than this, you are unlikely to get to the point where the entire patch area is not in contact with the runway surface, but you may encounter “partial hydroplaning” at less than 114 knots.


Formula for stationary wheel landing on flooded runway

Minimum speed for FULL dynamic hydroplaning is 7.7 x Ötyre pressure (PSI)

In the case of a 160 PSI tyre discussed previously, this would give a full dynamic hydroplaning speed of around 97 knots. Notice that this gives a lower speed than for a rotating wheel. Swept wing jet aircraft typically land at around 130 knots, so it follows that a landing onto a flooded runway could cause some anxious moments until the wheels “spin-up”, and even then the wheels may fully hydroplane down to the rotating wheel hydroplaning speed of 114 knots. Below this speed there may be partial hydroplaning, but the antiskid system will handle any loss of traction well.


The twin jet aircraft was being flown by a pilot undergoing assessment for entry into an airline that operates in the tropics. The aircraft was approaching to land on a runway that had been very recently lashed by a severe tropical downpour. A considerable depth of water was still on the sealed runway as the aircraft landed. The pilot being tested pulled off a “greaser” landing and was rather proud of this achievement, thinking he had impressed the Check Captain. After pulling the aircraft up at the terminal, the Check Captain told the pilot to never land the aircraft in such a manner again. The pilot asked, “Why not?”. The Check Captain explained to him that touching down so gently inhibits wheel spin up and increases the chances of Full Dynamic Hydroplaning, and Reverted Rubber Hydroplaning.


It is extremely important to land firmly on a flooded runway, so that the tyres break through the surface water and “bite” into the runway surface and get the wheel rotating. This will reduce the chance of hydroplaning and may significantly reduce the landing distance.


Once the aircraft has landed, you should lower the nose onto the runway without delay. This reduces the angle of attack (and lift) and places more of the aircraft weight on the tyres, which will also reduce the chances of hydroplaning.
As a guide, Full Dynamic Hydroplaning is most likey when water depth on runway exceeds 6mm (1/4 inch).

Viscous Hyroplaning

This usually occurs on very smooth runway surfaces, with as little as 0.025 mm of water depth.

Major danger areas are on the white runway markings such as the piano keys, and the 500ft and 1, 000 ft touchdown markings. Also of danger is the runway touchdown zone, about 1, 000 to 1, 500 ft past the threshold where most aircraft land. This area becomes contaminated with burnt-on rubber deposits, which have the affect of smoothing out the runway surface by filling in the minute gaps between the grit.

These areas of rubber build-up are supposed to be cleaned off by airport operators, but don’t count on it.

Viscous hydroplaning is often associated with contaminated runway surfaces, not just wet ones. Probably the worst scenario for creating a viscous hydroplaning event is when contaminants build up on the runway surface over a long dry spell of weather, followed by a dew, or very light rain. An aircraft touching down on a part of the damp runway that is contaminated with rubber deposits may experience the same degree of traction as it would if landing on an ice covered runway. Rubber, landing on rubber with a little bit of water between them make a slippery and dangerous sandwich combination. There is no set formula for calculating viscous hydroplaning, but bear in mind it can persist down to normal taxi speeds.

The runway contaminant could be ice, for which the minimum speed for hydroplaning is anything faster than a complete stop.

Viscous Hyroplaning.

Wet snow (slush) is often encountered on runways in Europe and North America. Slush has a much higher viscosity than that of water, therefore viscous hydroplaning can occur on slush covered runways to much lower speeds than for water covered runways.
A runway that is contaminated with rubber deposits, oil, or mud, in addition to being wet is called a “Slick Runway”.
Reverted rubber hydroplaning

This occurs when a wheel becomes locked up, and can occur on very slippery or icy runways. It can occur at any speed, and may persist until the aircraft comes to a stop. It normally follows after either viscous or dynamic hydroplaning. With the wheel locked up, high temperatures are formed which result in the tyre rubber REVERTING to it’s natural latex state. This leads to loss of tyre tread, flat spots on tyres, and filling of tyre tread with molten rubber. It also can cause tyre blowouts and tyre fires.

During reverted rubber hydroplaning, the molten rubber forms the slippery surface for the tyre to ride up on.

On water contaminated runways, the friction of the locked wheel can cause steam to develop at the tyre footprint. This steam assists further in lifting the tyre off the runway and melting the rubber.

There is no speed formula for reverted rubber hydroplaning.

Reverted rubber hydroplaning.

End of mini-editorial.

I trust this gives you a new perspective on the dangers of operating on contaminated runways.

The next mini-editorial will be on the related subject of tyre cornering ability on contaminated runways.

Goodbye until then !

Rob Avery


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Marty says ... "Goodbye to GA".

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