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HYDROPLANING:
Keep Your Wheels Rolling

by Bob Norris

Dynamic Hydroplaning | Viscous Hydroplaning | Rubber Reversion Hydroplaning

I received an inquiry at the AIR, Inc. conference in Dallas on the 23rd of February... if you weren't there, you should have been — this is the kind of session where you can receive up-to-date information and have an opportunity to network with the key players... the question was: "What do they expect from me in terms of technical knowledge?" Let me put this question in proper perspective. You are competing with candidates of similar technical backgrounds; I need to evaluate you in comparison with your experience level. To form the basis of the technical questions, I refer to your log book or flight records for three reasons:

1. Does the time in your log book agree with what I see on your application... have you padded the time?

2. I will present questions specific to the type of equipment you are flying, system knowledge, FARs, and weather I would expect you to be able to answer. If you are flying an F-18 or a Cessna 152 heavy, I expect you to be up to speed on the technical aspects of your equipment, rules and environment.

3. Every entry in your log book (out of your control in terms of military flight records) represents an investment you have made or someone has made in you; is the log book torn, coffee-stained, a lot of white-out, incomplete, inaccurate... your log book tells me a lot more about you than your sharp two-piece suit and your red power tie. A log book is a diary, the more detail the better. Yes, we will accept your computer records; however, they are really sterile.

Based upon your responses, I will evaluate you as being below average in the technical skill area (you will not be offered employment), average in the technical skill area (might consider you if all other areas test superior) or technical knowledge is above average (excellent chance you will be hired). So if you are flying a turboprop and you have a good understanding of high altitude meteorology and high speed aerodynamics... welcome aboard!!!

During the technical portion of the Captain Board Interview, I frequently present the following scenario: "You are inbound to the airport, the copilot pulls up the latest METAR report, and Approach advises that a heavy rain shower has just passed over the airport. This is the first rain of the upcoming winter season. You will be landing on a runway that is not grooved. The potential of hydroplaning exists; at what speed will your current aircraft hydroplane?"

Landing on a wet or icy runway, from the pilot's point of view, represents an operationally challenging situation. When there is considerable water or ice on the runway, an increase in landing distances of 40 to 100 percent can be experienced for similar conditions of gross weight, density altitude, wind, etc. There are many complex factors involved when landing on a wet runway that could affect the pilot's ability to properly control and bring the airplane to a stop. The pilot should understand the following:

1. Aerodynamic, propulsive, inertial, and external forces acting on the aircraft
2. Coefficient of friction and friction forces
3. Reverse thrust effects
4. Hydroplaning phenomena
5. Anti-skid system operation

In this article, I will only deal with item number four: hydroplaning phenomena (if I am to keep myself employed as a technical writer, I must save some material for subsequent articles). As a review, during normal landings we (1) retard the throttles to idle, (2) deploy the speed brakes, (3) apply reverse thrust, and (4) apply wheel brakes as you slow the aircraft to exit the runway. On wet runways, we want to get on the brakes as soon as possible so the anti-skid system can function and provide maximum braking.

Question time! What are the three different types of hydroplaning?

There are three forms of hydroplaning: dynamic, viscous, and rubber reversion.

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DYNAMIC HYDROPLANING.
Dynamic hydroplaning occurs when standing water on a wet runway is not displaced from under the tires fast enough to allow the tire to make pavement contact over its total footprint area. What happens is that the tire rides on a wedge of water under part of the tire surface. And it can be partial or total hydroplaning, meaning the tire is no longer in contact with the runway surface area. It is possible that as the tire breaks contact with the runway that the center of pressure in the tire footprint area could move forward. At this point, total spin-down could occur and the wheel stops rotating, which results in total loss of braking action... not a good thing. The speed at which this happens is called minimum total hydroplaning speed.

Question two: At what speed will your current aircraft main and nose wheels begin hydroplaning?

	The formula that is used to compute hydroplaning speed is: Minimum total hydroplaning speed (knots) equals 9 times the square root of tire inflation pressure (psi) or: V = 9 ÆP For the B-757 main wheels, the speed would be: 9 Æ144 = 108 knots
Figure 1: Partial Hydroplaning		Figure 2: Total Hydroplaning

Figure 1 and Figure 2 show partial and total hydroplaning.

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VISCOUS HYDROPLANING.
Viscous hydroplaning can cause complete loss of braking action at a lower speed if the wet runway is contaminated with a film of oil, dust, grease, rubber or the runway is smooth. The contamination combines with the water and creates a more viscous mixture... more slippery. It should be noted that viscous hydroplaning can occur with a water depth less than dynamic hydroplaning, and skidding can occur at lower speeds, like taxiing to the gate during light rain, applying the brakes and rolling over an oil spill... look out, terminal!!!

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RUBBER REVERSION HYDROPLANING.
Rubber reversion hydroplaning is less known and is caused by the friction-generated heat that produces superheated steam at high pressure in the tire footprint area. The high temperature causes the rubber to revert to its uncured state and form a seal around the tire area that traps the high-pressure steam. It is theorized that this condition would occur on damp runways or when touchdown occurs on an isolated damp spot of a dry runway, which results in no spin-up of the tires and a reverted rubber skid.

OK, now that you have some understanding of hydroplaning, summarize some of the piloting techniques that could be employed to minimize the potential of hydroplaning

McDonnell Douglas provided the following information to answer the question:

1. Do not be afraid to delay landing. Under zero wind conditions, most runways have adequate crossfall (rounding of the runway surfaces or crown) to provide drainage under quite high rates of precipitation. It appears that drainage can be seriously affected in crosswinds above 10 knots; however, a 15- to 20-minute waiting period after a downpour is usually sufficient to drain the water.

   • Be knowledgeable of the many variables associated with landing under wet runway conditions:
     
   • Landing weather forecast
     
   • Aircraft weight and approach speed
     
   • Hydroplaning speed
     
   • Conditions of tires — if the tread depth of the tires on an aircraft is greater than the depth of the water on the runway, then hydroplaning will not occur. Knowledge of the general condition of the tires (why we do pre-flights) should be helpful in a qualitative sense when potential hydroplaning conditions are expected.
     
   • Brake characteristics
     
   • Wind effects on the aircraft while landing on a wet runway
     
   • Runway length and slope
     
   • Glide path angle
     ... and finally remember, do not overlook or underestimate the effects of a crosswind because of its low magnitude.
     
     
2. Do not exceed 1.3 Vs plus wind additives at the runway threshold.
   
3. Establish and maintain a stabilized approach.
   
4. Use maximum flaps to provide minimum approach speeds.
   
5. Be prepared to go around from the threshold.
   
6. Do not perform a long flare.
   
7. Do not allow the aircraft to drift during the flare.
   
8. Touch down firmly and do not allow the aircraft to bounce.
   
9. If a crosswind exists, apply lateral wheel control into the wind.
   
10. Keep the aircraft centerline aligned with the runway centerline.
    
11. Anti-skid braking should be applied steadily to full pedal deflection when automatic ground spoilers deploy and main wheel spin-up occurs. Do not modulate brake pressure. The anti-skid system will not operate until the main wheels of the aircraft spin... don't lock your brakes before touchdown.
    
12. Be prepared to deploy ground spoilers manually if automatic deployment does not occur. Spoiler deployment greatly assists wheel spin-up during wet runway operations by materially reducing the wing lift and increasing the weight on the wheels, thus shortening your stopping distance.
    
13. Apply maximum reverse thrust as soon as possible after main gear touchdown; this is when it is most effective.
    
14. Get the nose of the aircraft down quickly. Do not attempt to hold the nose off for aerodynamic braking.
    
15. Apply forward column pressure as soon as the nosewheel is on the runway to increase weight on the nosewheel for improved steering effectiveness. Do not, however, apply excessive forward column pressure because the down elevator will, to some extent, unload the main wheels and decrease braking effectiveness.
    
16. If the aircraft is in a skid, align the aircraft centerline with the runway centerline if you can. Get off the brakes to maximize cornering capability and bring the aircraft back to runway center. If you are in a crab and cannot align aircraft centerline with runway centerline and attempted cornering is not effective, get out of reverse thrust to eliminate reverse thrust component side forces tending to push the aircraft off the side of the runway.

A good rule of thumb in normal operations is to do most of your reversing above 100 knots and braking below 100 knots.

The next article will start to deal with weather knowledge; the technical area I find the most candidates have the greatest weakness in during the Captain Board Interview. No way I can cover weather in one article, so I will occasionally write an article on weather... enjoy!

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