The wintertime hazards of operating in icing conditions are well known, and the majority of pilots are careful to avoid getting into icing trouble. However, when those skies clear we all want to get out there and fly, but many times the runway conditions are less than ideal for many days after a winter storm.
The problem of ice, snow, slush, and standing water plague many airports throughout the winter season. While these surface contaminants are generally of most concern to large aircraft due to critical stopping requirements, small aircraft are equally susceptible to many of the same problems. This month, I’ll discuss the problem of runway contamination and its effect on aircraft operation.
It goes without saying that a landing on a contaminated runway can be dangerous at very best. But what exactly is a contaminated runway?
The generally accepted definition of a contaminated runway is one that has standing water of at least 3mm, or snow, slush, and/or ice. Simply being wet does not cut-it in terms of contamination however.
Braking effectiveness predictions are possible with a wet runway, but once the water reaches a standing condition (large puddles, etc), or there is snow or ice, it becomes difficult to accurately predict the ability to properly decelerate to a stop. Even the current technologies for determining the friction coefficients of a wet runway have less than desirable correlations to reported conditions, and pilot reports of braking effectiveness have even less correlation to actual measured pavement friction.
Another problem is that not all surface contaminants have the same friction coefficients. For instance, while one would think that a cold icy runway might be the worst case scenario for braking, an icy runway with water or slush on top of it (like when it starts melting) is usually worse in terms of friction for stopping. Slush frequently exhibits properties of not only a wet runway, but also an icy one.
Dynamic hydroplaning will occur at higher speeds on slush due its density being lower than water. While it creates more drag than water, if it sits on top of ice, the overall low friction can easily negate the benefit from extra drag.
The main effect of runway surface contamination on takeoff is reduced acceleration. Beyond that problem, loose snow, ice, or slush can also be sprayed or thrown into gear, flaps, and control surface hinge areas potentially leading to control malfunction.
Landing on a contaminated runway is a far more complicated task to safely complete, so I’ll focus on that phase of flight.
The main problems associated with landing on a contaminated runway is loss of directional control or inability to slow down due to reduced friction between the tires and surface, or a loss of friction to the point of hydroplaning.
Loss of directional control is a real possibility especially with deep snow or slush. Anyone who has tried to taxi in even a couple inches of snow or slush knows that directional control is tricky at best in most light aircraft. Add to this a crosswind and the probability of loss of control increases during takeoff or landing. The problem here is that the tire-to-ground side friction is so low that a crosswind allows weathervaning.
Even worse is if the tires are locked up (not rotating) because in this case the side friction is reduced to zero. When that happens, only aerodynamic control will be effective, and only at relatively higher speeds.
One advantage for the twin-engine pilot is that if he or she is thinking quickly, the beginning of loss of control can often be neutralized by positive application of asymmetric thrust to yaw the aircraft back to the desired path. I wouldn’t recommend asymmetric thrust during takeoff, but it can certainly be used during landing.
I’ve had a few pilots argue with me against doing that, but if you’ve practiced simulated single-engine landings, then you already know how to handle the aircraft with asymmetric thrust!
Inability to slow down potentially leads to runway overrun, so it’s critical to consider runway length prior to making the landing attempt on a contaminated runway.
Although it’s true that water, slush, and snow add to the drag and would definitely increase takeoff distances, it also increases landing distances since braking cannot be performed to the level required to meet the performance manual landing distance.
Application of maximum braking on a contaminated runway will result in locked tires and little to no deceleration. Some pilots like to hold the nose off the ground to assist in aerodynamic braking during landing. This is not such a great idea for contaminated runway landings unless dealing with deep snow; and in that case you should be landing with skis, not tires!
While aerodynamic braking is useful, most of the braking in the later stages of landing comes from the tires, not the aerodynamic drag. The only way to assure the maximum weight is on the wheels, thus permitting maximum braking for the conditions, is to lower the nose to the ground early in the landing.
Also bear in mind that directional control becomes more problematic the longer the nose gear remains in the air, and this can complicate landing especially in a crosswind situation.
Braking action reports are generally associated with certain types of surface contaminants but this should be used only as a rough guide. For instance, while wet snow, slush, standing water, or ice by themselves generally equates poor braking action, a combination of any of those can worsen the situation.
Likewise, there can be distinct differences in the actual friction coefficient of a contaminated surface when considering the type of aircraft tire. A study of these differences has shown up to a 20 per cent difference in braking effectiveness depending on tire type.
According to research, the correlation between braking friction values as measured by some sort of friction testing device, and that of the actual aircraft in contact with the surface, varies from good to poor. This is primarily because the characteristics of the friction testers differ significantly from that of any particular aircraft, due to tire pressure, groundspeed, and other operational considerations.
However, the friction values are apparently not so bad when it comes to calculating a braking action report for snow or ice. But beware, the friction values as reported do not necessarily correlate to the braking action as reported since that is also frequently based on pilot reports.
Naturally, a pilot report of braking action depends on a multitude of factors, such as where on the runway touchdown occurs, the speed at which it occurs, and the firmness of the touchdown. You might wonder what firmness has to do with it, but suffice to say that sometimes a firm landing is required to get wheel spin-up early in the landing so that braking can commence.
Among the factors that determine landing in the distance suggested by the performance manual are: touchdown point, approach and landing speeds, approach to landing angle, pilot technique in the flare, and runway surface condition. Runway overruns in the wintertime are generally attributed to something gone wrong with one or more of the above factors.
Frequently, pilots calculate stopping distance only to find that the aircraft takes significantly farther to stop. More times than not, this happens because the assumptions behind those performance calculations are not met during the landing.
One frequently missed assumption of landing performance is that maximum braking effort is required to meet the landing performance for most aircraft. Unfortunately that’s something you just can’t do on a wet or contaminated runway!
Being on airspeed during the approach to landing is also critical since any excess airspeed approaching touchdown will likely extend the flare and delay touchdown. And remember that most of the braking effort needs to be accomplished early in the landing when considering runway length, not during the last 15 feet of runway, so “landing long” is simply not a good idea on a contaminated runway.
One side note: Tracks in the snow on a runway are hardly an indication that a safe landing may ensue. Other vehicles, as well as snow mobiles, may have made the tracks and have totally different traction characteristics than an airplane trying to maneuver on the same surface.
Sometimes tracks give the illusion that the snow is not as deep as it really is, leading to a landing attempt that ends in disaster. The key to avoiding situations like this is to get a reliable surface condition report prior to attempting the landing.
So what can a pilot do if sliding down the runway? The question of pumping the brakes comes up. Actually, the main cause of sliding is the lack of wheel spin up. If the wheels are not spinning, the brakes will do absolutely nothing when applied.
If the wheels are spinning prior to brake application, and then lock up, releasing the brakes is the only way to restore effective braking. Although we don’t really pump the brakes, multiple consecutive applications of the brakes is what an anti-skid system would do to maximize braking.
Believe it or not, antiskid systems on aircraft are far more effective than any human attempt at maintaining optimum wheel spin-up so that braking can occur. The reason for this is academic; the anti-skid system can detect abnormal changes in wheel velocity better than the pilot can. Too bad we don’t have anti-skid systems for light aircraft!
So the best way to approach a contaminated runway for landing is:
1) Be on speed, on centerline, and on glidepath during approach.
2) Make a positive touchdown with minimal delay touching the nose down, touching down on target, and
3) As soon as the nose is down, commence braking and continue with follow-through on control column to full aft as soon as speed allows. Failing any of the above, a go-around should be considered. Also, use braking action reports as a guide to what to expect and consider making a low pass to more closely inspect a runway reported as contaminated. Often, the pilot has little idea of what to expect until touchdown occurs, so gather all the information about the conditions as you can prior to the landing attempt!
This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.