When the Rubber Doesn’t Meet the Road

When the Rubber Doesn’t Meet the Road

When the Rubber Doesn’t Meet the Road

We’ve got a problem in the twin and turbine world, and I predict this problem is going to cause the demise of many nice and flyable airplanes this winter. What is the problem, you ask? Pilots accepting contaminated runways with a crosswind. I’ve preached about this issue to clients during training events for years, yet pilots continue to end up in a snowbank or a ditch in perfectly good airplanes. I usually get a call shortly after the event and the pilot feels terrible, knowing the limits were pushed and immediately understands his or her error within a short discussion.

I believe the solution to this problem comes from a better understanding of the aerodynamics associated with a takeoff or landing and then applying that knowledge to the real world. So, let’s dissect a landing with hopes of casting light on both the problem and the solution. Hopefully, the flight instructor community can collectively preach this same sermon and we can reduce the numbers of airplanes that leave icy runways this winter. 

There are effectively four ways that a pilot can interface with the medium in which the airplane operates (air): ailerons, elevator, rudder and power. I know I’m going to a get chorus of corrections from those who feel that I left out the lowly flaps (if installed), spoilers (if installed) and trim tabs. But, those flight controls are either secondary controls (flaps and spoilers), or only relieve control pressures (trim tabs). Reduced to the lowest common denominator, the ailerons, elevator, rudder, and power are the controls that must be managed in the heat of the moment to make a good (and safe) takeoff or landing.

The question is: What do those flight controls do during a takeoff or landing? Most pilots move the flight controls intuitively, meaning without much robotic action, and hopefully, that intuition is based on appropriate experience and honed by good instruction. We have a whole generation of pilots who learned to fly in directionally stable airplanes on the ground which means many pilots have a basis of experience that will fail them if the airplane suddenly becomes directionally unstable – this can happen in the blink of an eye with a contaminated runway and a crosswind.

The long debate over pitch and power is not going to be solved with this article. In fact, I’m not going to address pitch and power in this article for they are not applicable to my final point. Rest assured, I shall tackle that ginger subject in a future article. For this discussion, the proper use of the aileron and rudder is the focus.

When an airplane is on a long final approach (more than 200 feet above the touchdown zone), the ailerons and rudder have distinctly different responsibilities than when the airplane is on a short final. While on a long final, the ailerons are holding a heading and the rudder is simply ensuring coordination. If any crosswind exists, the airplane will be in a crab, with the heading of the airplane being determined by the pilot to defeat the crosswind. An airplane with a slow speed will have a bigger crab angle, and a faster one will have a smaller crab angle. A stronger crosswind will require a bigger crab angle and a lesser crosswind will require less crab angle. Easy, right? Yes, easy.

But, what do the controls do during a short final? At some point in the approach, usually about 100 feet (or so) above the runway, the pilot must make a mental shift and reposition the flight controls differently. The pilot must align the longitudinal axis of the airplane (line from nose to tail) with the alignment of the runway. So, the rudders have one (and only one) responsibility when landing: align the longitudinal axis with the runway. If a crosswind exists, the pilot must apply rudder pressure downwind to align the longitudinal axis with the centerline. When this rudder input is made in a crosswind, the airplane will drift from centerline unless something counters the effect of the crosswind. That something is the ailerons. 

Ailerons correct for drift, and they do so by banking the wings and creating a horizontal component of lift. How much aileron does a pilot apply into the wind? Answer: Whatever it takes to keep the airplane on the centerline. A big crosswind component will require more bank (forced by aileron control) and a small crosswind will require less bank. In other words, if the airplane lands off the centerline, the pilot flew the ailerons poorly. If the longitudinal axis is not aligned with the runway alignment at touchdown then the rudder was flown poorly. Still easy, right? Yep, still easy. It’s piloting 101.

Now, when the pilot touches down, the tire on the side where the wind is coming from should touch down first due to the banked airplane. Remember, the pilot is holding aileron into the wind to defeat that wind, so that bank is keeping the airplane from drifting. So, a one-wheel landing on the up-wind tire is mandatory if the landing is to be done correctly. 

What keeps the airplane from drifting downwind after the downwind tire (the second tire) touches down? The answer is NOT the ailerons, for the wings are now level and the horizontal component of lift is non-existent. The correct answer is rubber contact with the surface. That’s right, the friction of the tires to the ground is the only thing keeping the airplane on the centerline, and the rudder now has the job of controlling the direction of the airplane. Once the nosewheel touches down, there’s even more rubber helping to keep the airplane going straight. The nose wheel becomes more and more important for directional control as the rudder becomes less effective with decreasing speed.

Having said all this, here is my primary point with this article: If ice exists on the runway and a crosswind landing is attempted that has a side-force that exceeds the coefficient of friction of the tire to the surface, the airplane will drift with the force of the wind. Remember, the only thing keeping the airplane directionally stable once the aerodynamics stop is the friction of the rubber on the ground. If a layer of ice exists, all bets are off. You might stay on the runway, and you might not. 

If the ice between your rubber and the ground is “black ice” (which has virtually no coefficient of friction), you are just a passenger, and I hope you are a fortunate passenger riding in an airplane that stays on the runway. If it’s slushy ice with a moderate coefficient of friction, you’ll have some control, but there is no way to know for sure.

How much coefficient of friction do you have exactly? A pilot can get a clue at some of the big airports from braking-action reports, which give us an idea of what we’re going to experience. There’s also reports from other pilots who used the runway shortly before you which is super valuable information. But, wind and ice can create very dynamic
situations, and there are no assurances that the conditions have not changed. At smaller airports, it’s most often a guessing game. Don’t guess wrong.

I remember once landing a Saab 340 at the super long, super wide Runway 33L at KSPS (Sheppard AFB/Wichita Falls, TX). We had a full load of passengers and winter had arrived with much fanfare: ice, snow and plenty of wind. The nighttime touchdown was uneventful, but then came the rollout. As the Saab slowed on the rollout (certainly under 40 knots), the airplane started to slide. If you never been in a sliding airplane, it is one of the strangest and most disconcerting feelings ever. It’s a helpless feeling, where time slows to a turtle’s pace and gives you pause to think to yourself, “Why did I try this landing?! Please don’t go in the ditch!” The enormous width of that particular runway saved my bacon for the airplane did not leave the runway, but when it finally stopped, the Saab was pointing into the wind, off centerline and two wide-eyed pilots were staring at each other in disbelief. I proceeded to crawl the airplane to the terminal for fear that I’d lose control again. 

In the PA46 world, we usually lose two airframes (or more) each year to landing on contaminated runways with crosswinds. I suspect other airframes have commensurate losses. All of these accidents are avoidable. Simply put, if there’s a contaminated runway and a crosswind, do not land there. Find a runway aligned with the wind.

An off-runway event almost always results in a prop-strike, a wing ding or a nose gear collapse. Rarely does anyone get seriously injured, but the insurance claims are always high. It’ll cost you downtime, hull-value loss and betterment (a nasty little word in the insurance world that most owners are not prepared for in an insurance claim). 

Of course, on an icy runway, there’s also the threat of a FOD event, minimized braking effect and reduced visual cues. And all of those can cause serious problems that lead to an accident. But, the problem I see more often than not in the twin and turbine world is the attempted landing on a contaminated crosswind runway.

All of the above applies to a takeoff as well. If you understand the ground dynamics and aerodynamics of an approach and landing, you can apply that understanding to a takeoff and arrive at the same conclusion. That conclusion is that rubber-to-ground friction is also required for a safe takeoff. If ice is present and a crosswind exists, find another runway or put the airplane back in the hangar. It’s better to be conservative and fly another day than to attempt a crosswind takeoff on ice and end up in the ditch. I promise you’ll make headlines with your accident, and not the headlines you were hoping to make. 

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