Technique: Don’t worry, be happy…feet

If you’ve ever seen an airplane off a runway, resting on a bent wing and flat tire, you may have seen the results of a ground loop. “Swapping ends,” as the taildragger pilots call it, is when the tail of an aircraft swings far enough to one side to spin the airplane around. The momentum, if great enough, will often tilt the airplane and cause a wing to strike the ground. The airplane can be seriously damaged, including bent wings, blown tires, and collapsed landing gear. With less speed, this uncontrolled turn may cause no aircraft damage, only giving a good jolt of adrenaline to the pilot.

Going loopy

An airplane’s motion about its vertical axis is called yaw, and it’s controlled by the rudder, which is controlled by the rudder pedals, which are controlled by your feet. That’s why happy landings and the prevention of ground loops start with happy feet actively working the rudder pedals.

Ground loops are caused by uncontrolled yaw, and although most occur on landing, they can happen anytime an airplane is moving on the ground, including during takeoffs and taxiing. During a perfect landing the airplane’s heading (the direction it’s pointed) corresponds precisely with its track (the direction it’s moving over the ground), and this track is aligned straight down the runway. Unfortunately, landings aren’t always perfect. Crosswinds can cause an airplane to land with a slight yaw, inept piloting may start a swerve, or a hole in a grassy field can send the airplane askew—and then the pilot better be quick to get it straight again. Mechanical problems, such as a broken tailwheel spring, a tire failure, or stuck brake, also can cause ground loops.

Ground loops occur more often on tailwheel aircraft because their center of gravity is behind the main wheels. When this rearward CG is yawed to one side, momentum wants that CG to continue in its original direction of travel. That’s part of Newton’s first law of motion: An object in motion stays in motion with the same speed and in the same direction unless acted upon.

A physicist I know speculated, “I’d say, when the plane touches down, the front wheels define the center of drag. The further out of alignment the center of mass [not precisely the same as CG, but close enough in this instance] is from the center of drag, the greater the tendency will be to spin the aircraft in a ground loop. I realize I’m mixing force and momentum, but think of the drag from the wheels as force pushing backward from the center point between the wheels and the momentum of the airplane as a force pushing forward from the center of mass. If you land perfectly lined up, both forces are along the centerline and there’s no torque. As the tail of the airplane gets further askew from the airplane’s centerline, rotating around the main wheels, this momentum could swing the tail completely around into a ground loop.”

Tricycle-gear airplanes are more forgiving of this tendency. With their CG forward of the main wheels, if an airplane lands with a little yaw, the CG wants to continue forward, but now it’s turning the nose in the direction of the desired travel, not away. One situation when a ground loop may be caused by the nosewheel is when a pilot has a particularly heavy hand on the yoke, or heavy feet on the brakes. Excessive pressure on the nosewheel can induce a side-to-side oscillation which a pilot may exacerbate with vigorous rudder inputs as he struggles to keep the airplane rolling straight. Too much pressure on the nose wheel could also cause a blowout at high speed, which might be disastrous.

Although most ground loops occur after landing, airplanes can ground loop anytime, including takeoff. Perhaps, the pilot applies the throttle too quickly and the left-turning tendency pulls the airplane off center. Or, the airplane wasn’t pointed precisely down the runway to start; that’s why you should never take off until you’ve completely straightened the airplane on the runway and centered the wheels. During acceleration a pilot may overcorrect a swerve that can quickly develop into a ground loop. Takeoff ground loops may be extremely severe if the pilot doesn’t cut the power quickly.

Happy Feet

All pilots, but especially taildragger pilots, must be constantly vigilant of any yaw during ground operations and correct it immediately; that’s why they’re taught to have happy feet, which more accurately might be called active feet.

Tailwheel airplanes also react better to quick jabs on the rudder pedals instead of constant pressure. If the nose is drifting left, a few quick taps on the right rudder will correct it. Pressing the right rudder will also correct a left drift, but then it may send the airplane off to the right. Pressing the left rudder pedal brings it back, but it may overcorrect again. A wild pilot-induced oscillation can start, which will usually end by the tail coming fully around in a ground loop.

During a perfect landing the airplane’s heading corresponds precisely with its track, and this track is aligned straight down the runway. Unfortunately, landings aren’t always perfect.An experienced tailwheel pilot told me that when he enters the pattern he says to himself, “Wake up, feet,” then he wiggles his feet and taps the rudder pedals, getting the feel of the rudder again. On final, he says out loud to himself and any bewildered passenger, “Happy feet, happy feet.” They may think he’s crazy, but he makes good landings.

Ground control exercises

Find a runway, or taxiway, wide enough for your aircraft to turn around. Perhaps a helpful air traffic controller will let you use an inactive runway—or fly to a little-used airport on a gray day when fair-weather pilots are at home.

Start by practicing using quick taps on the rudder pedals while performing the standard S-shaped taxi technique, used by tailwheel pilots who have limited visibility over the nose.

Next, taxi along one edge of the runway and then cut across it at a 45-degree angle and turn back to taxi along the opposite edge. Repeat until you’re out of runway. This pattern of turns and straightening out is good practice for preventing any unwanted swerves.

Also practice taxiing in figure-8 patterns, which will improve your ground control using the rudder pedals. If it’s breezy, you’ll also gain experience with the constantly changing wind direction. You’ll really feel the difference as the wind alternately strikes the nose, tail, and sides of the airplane. Try to position the ailerons and elevator in the correct positions for preventing a gust lifting a wing, too.

These exercises are good for nosewheel pilots, too, who sometimes forget they have rudder pedals.

All pilots, whether of nosewheelers or taildraggers, should be constantly vigilant for the dreaded ground loop during all ground operations. Keep the noisy end pointing forward by keeping those happy feet dancing on the rudder pedals.