Monday, September 1, 2008
Does your adrenaline level skyrocket on gusty days?
|We can all admit that, at some point, we’ve scared ourselves in a crosswind. Sure enough, most flying accidents occur during landing, and most of those are in crosswinds. Almost all crosswind-related accidents happen due to loss of control after touchdown; only a tiny portion involve a crash on approach or on a go-around. To stay safe, we should examine the true risks we face when landing in a crosswind, and the big risks come after touching down.|
We can all admit that, at some point, we’ve scared ourselves in a crosswind. Sure enough, most flying accidents occur during landing, and most of those are in crosswinds. Almost all crosswind-related accidents happen due to loss of control after touchdown; only a tiny portion involve a crash on approach or on a go-around. To stay safe, we should examine the true risks we face when landing in a crosswind, and the big risks come after touching down. Speed Matters
Fighter pilots say that speed is life. On a crosswind landing, however, extra speed on the approach isn’t your friend. Normal approach speed is 1.3 Vso
(stall speed in landing configuration). We were taught to fly that speed before we soloed, and we learned to make landings just fine. A Vso
of 1.3 is more than adequate for control authority on the approach. This speed allows aircraft power to be reduced to idle well before the flare, the nose to be raised to landing attitude, and the touchdown to happen at something slightly above stall speed.
|The highest risk during any crosswind landing occurs at the moment of initial touchdown, when the airplane has good aerodynamic control but poor roll control.|
Problem is, we tend to add extra speed on the approach because the airplane responds more crisply at higher speeds, making the approach feel better. That speed buys trouble later, when it really matters. The risk starts increasing when we balloon or float in the flare, due to excess speed, and start drifting downwind. The risk factor shoots up the moment the wheels touch the ground, and we have to deal with dissipating the energy of the mass in motion and keeping that mass on the runway. That’s when the fact that the energy of a moving plane increases as a squared function of the speed stares us in the face. So, if we touch down fast and lose control, even a little extra speed translates into a lot more damage when the plane hits something.
All of this means that pilots who know what they’re doing, and are thus at the least risk of wrecking their airplanes in a crosswind, touch down slowly. They’re at 1.3 Vso
by the time they’re 200 feet above the ground. So that the airplane decelerates at the maximum rate, they use all the flaps to maximize drag effect when the power goes to idle before the flare. They come into the flare—on top of speed, power off and flare to a nose-high attitude—without floating or ballooning, and not trying for a full-stall landing, but touching down slowly, under control.
Energy management means that in a steady wind—whether it’s right down the runway or a crosswind—there’s no reason to go faster than normal approach speed. The only reason to increase approach speed is if the wind is gusty, and then only by one half of the gust factor. So, if it’s blowing at 10 knots with gusts to 20, the gust factor is 10 knots, and the speed increase on final is no more than five knots. Propeller-driven airplanes accelerate well at approach speed, so even in significant gusts or wind shear, half the gust factor has been shown to be plenty adequate for the wind-speed changes.
If you think that you can’t keep the plane lined up with the runway unless you approach faster in a crosswind, that’s nonsense. You want to touch down as slowly as possible, right? Well, then at some point, you have to decelerate to the touchdown. If you come in fast, you have a lot of decelerating to do in the flare, and that means you float or touch down level, with the airplane still going fast. Neither one of those options is good. Extra speed also increases the risk of touching down flat, porpoising or both—an ugly event. Speed control means approaching at 1.3 Vso
, plus no more than half the gust factor. The airplane did its crosswind demonstration for the FAA at 1.2 Vso
, so if you’re at 1.3 Vso and can’t keep the airplane lined up with the runway—running out of aileron or rudder in the attempt—then it means you should go around and find a way to make a landing with less of a crosswind component.
Coming in fast and discovering at the last minute that you can’t keep the airplane lined up, as you float or balloon while drifting rapidly, isn’t conducive to keeping your airplane intact. Stay on top of your speed, with full flaps at 200 feet AGL, so you’ll know early in the game if you can stay lined up or are going to have to go around. Then, flare to a nose-high attitude, keep the wing down into the wind to control drift, and use opposite rudder to keep the nose parallel to the centerline (recognize that you probably will have to move one or more of the controls to the stop during the landing). And that’s okay. It’s okay to use all of the control deflection; that’s what it’s there for—to allow you to make the airplane go where you want it to go. Ailerons To The Stop
Great, you touched down on the upwind wheel. What comes next? Well, if you’re like way too many pilots, you’ll breathe a sigh of relief that you’re on the ground in a nosewheel airplane and you’ll promptly center the ailerons. Most of the time, the airplane hops a little bit, squeals the tires and slides somewhat downwind because there isn’t enough weight on the wheels to have good roll control. Fortunately, however, the crosswind component was so small that it doesn’t matter—most of the time. Remember, you’re in a high center of gravity tricycle.
The period of extremely high risk in any crosswind landing starts at the point of initial touchdown—where the airplane still has good aerodynamic control, but lousy roll control—and continues through the process of decelerating to a speed where it will have good roll control, about 10 knots or so. It’s during that time-oriented world of diminishing aerodynamic control and slowly improving roll control that almost all crosswind accidents happen.
There’s a simple reason why pilots lose control in crosswinds after touchdown: They don’t roll the ailerons all the way into the wind. That’s it. It’s one time in life when there’s a simple answer to a major problem. Once on the ground, the airplane has only the available tire friction to counter the massive effect of the crosswind on the side of the fuselage. By keeping the aileron all the way into the wind, the upwind wing is forced downward, putting a significant downforce on the upwind tire. With that action, there will be a lot of tire friction available to counter the crosswind. Should a gust attempt to get under the dihedral of the upwind wing, the deflected aileron will also keep the wing from being lifted up.
Once rolling, the plane needs to decelerate quickly through the region of risk. Step one is to use the aerodynamic drag from the flaps and pitched-up fuselage. That’s most effective for about the first third of the deceleration. After that, the brakes are more effective than aerodynamic drag, so hold the yoke just aft of neutral to put weight on the wheels, keep the aileron to the stop and get on the brakes. If you want to put more weight on the wheels, retract the flaps, recognizing that doing so increases your chance of raising the gear inadvertently. Do nothing else; just fly the airplane until it’s stopped. That means don’t change a radio frequency or shut off the strobes; address your full attention to directional control of the plane. Land Into The Wind
Finally, when faced with a very strong crosswind, do what you can to reduce the amount of crosswind component. We’re talking about a big-time crosswind, the kind of crosswind in which you’re not certain that you can safely land the airplane. The safe conduct of the flight is in question. Therefore, you, as pilot in command, are dealing with an emergency that could result in damage to the airplane and, possibly, injury to you and your passengers. Make a decision about how to increase the certainty of a safe landing. Is there another runway? I can’t tell you how many crosswind-landing accidents I’ve seen where the pilot accepted a horrendous crosswind on a paved runway instead of landing on a grass runway that was right into the wind. That’s about as ludicrous as running out of gas.
Can you reduce the effective angle of the crosswind by landing at a slight angle across the runway, more into the wind? It doesn’t help much on a narrow runway, but may make a difference on a wide one. Practice it sometime with an instructor to see if it may add a tool to your crosswind kit.
If there’s another airport within range that has a runway into the wind; divert and land on it. The wind will eventually die down or change direction. Then you can go to your intended destination.
There’s an alternative that’s legal and safe, but unconventional: If there’s a taxiway into the wind that’s clear of buildings, obstructions and people, it may be the best place to land. It’s your call as pilot in command. The FAR
s don’t prohibit landing on a taxiway; helicopters do it all the time, as do gliders. Taxiways are often as wide or wider than a fair number of small-airport runways, they aren’t soggy after a rain because they’re paved, and they’re well marked. As long as you look the area over for other aircraft, buildings and persons or vehicles, you’re not being careless or reckless. (At a controlled field, the tower can’t clear you to land on a taxiway and the landing becomes “at your own risk.”) A taxiway landing certainly isn’t a normal procedure, but in a strong crosswind, after mature reflection, it may be the best way to increase your level of safety by reducing the crosswind component to an acceptable velocity for a safe landing. And when all is said and done, what matters most is landing safely.