Tailwheel Checkout Part 2: ‘What’s A Groundloop?’

Barry’s grin was so wide I swear I could see it from the back seat of the Bellanca Citabria. We flew about a thousand feet above the east Tennessee fields, gray-green patches of early spring earth peeking through wisps of morning fog. We were about an hour and a half into Barry’s first lesson toward his 10-hour tailwheel checkout, and we’d just completed our first takeoff. I’d planned it that way.

Federal Aviation Administration (FAA) regulations require a “tailwheel endorsement” to act as pilot-in-command of an airplane with the third wheel at the back end. The endorsement checkout needs to include normal and crosswind takeoffs and landings, go-arounds, and wheel landings. To carry passengers you have to log at least three full-stop takeoffs and landings in a tailwheel airplane within the 90-day period immediately preceding flight with passengers. But as we discussed last week, the insurance industry will likely require substantially more experience before putting their money behind a pilot new to tailwheel airplanes. Barry’s requirement of 10 hours flight instruction before solo, followed by 10 additional hours’ solo before carrying passengers, is very common.

Why the long checkout requirement, totaling half the FAA minimum requirement for the entire Private Pilot certificate? In a word: “groundloops.”

A “groundloop” is a picturesque description of a loss of aircraft directional control on the ground, such that the airplane pivots uncontrollably around its vertical axis and ends up with the tail leading the way — “swapping ends,” in taildragger parlance. Luckily, groundloops often are completely harmless (they usually happen at the very beginning of a takeoff roll or the very ending of a landing, when forward speed is lowest and controls are least effective), and they almost never hurt anyone. Done faster, though, a groundloop imposes a strong side-load on the main landing gear, often breaking it off. Plus, many tailwheel airplanes have a narrow gear track (the distance between main wheel tires), and may tip far enough to “drag a wingtip” in a high-speed groundloop. That goes for high-wing designs, too. The damage potential is even greater if the pilot resists a developed groundloop too much… sometimes you just have to go with the flow and let it spin around once you’ve “lost it.” In extreme winds it’s possible to groundloop a tricycle gear airplane, but the tailwheel plane is far more prone to “swap ends.” Here’s why:

All objects, in most cases, pivot naturally around their center of gravity. Tricycle gear airplanes are designed so the center of gravity (c.g.) falls forward of the main landing gear, so weight settles on the nosewheel. A tailwheel airplane’s c.g. is behind the main landing gear.

Because of c.g. placement, a tricycle gear airplane on the ground tends to go forward in a straight line. If a gust of wind or improper pilot technique causes the tail to wag, the location of the main wheels, behind the pivot point, inhibits movement around the vertical axis. Meanwhile the nosewheel, usually fixed within a narrow turning radius of straight ahead on the airplane, also resists pivoting movement. Try grabbing a tricycle-gear airplane with all three tires on the ground and pushing it sideways sometime, and see how far you get. “Nosewheel” airplanes are directionally stable around the vertical axis on the ground. Translation: They are quite unlikely to groundloop.

Inside Information: Airplanes with free-castoring nosewheels, like the Grumman singles, are almost as prone to ground-loops as are tailwheel designs, for the same reasons. That’s one reason for their typically higher insurance costs.

Tailwheel airplanes, on the contrary, must have their c.g. behind the main wheels, so the airplane’s weight comes down on the tailwheel on the ground. Hit a gust of wind or ham-fist (ham-foot?) the rudder pedals and the airplane will begin to pivot around its vertical axis… with nothing to stop it. Most tailwheels, are designed to allow the tail to swing. If pilot technique or control authority is insufficient to counteract the swing, the tailwheel airplane will continue this uncontrollable spin around the vertical axis. If enough energy is added to the equation, the maneuver may overload the main gear or rock the plane onto a wingtip. Because of their inherent ‘instability‘ on the ground, most larger tailwheel airplanes have a “tailwheel lock” that the pilot engages to keep the aircraft’s track aligned with its longitudinal axis and resist groundlooping during takeoff and landing. The wheel unlocks for taxi and parking.

Watch a tailwheel airplane’s rudder as the pilot takeoffs off and lands. You’ll see the rudder wagging back and forth the whole way, the movements are more pronounced at the beginning of the takeoff roll and the end of the landing roll, when airflow over the rudder is less, and the control is less effective. In a tailwheel airplane, the pilot’s feet are working all the time when the plane’s on the ground. It takes concentration, care and effort to maintain directional control even in calm air — and can be a real battle in wind.

Tailwheel instructors caution that you have to ‘fly the airplane all the way to the tiedowns,” because one lapse of concentration can cause the plane to careen out of control.

BOTTOM LINE: Checking out in and flying a tailwheel airplane is the kind of fun most of us dreamed about when we first discovered aviation. (For taxiing, there’s really nothing quite as cool as driving your airplane up to a parking spot head-on, locking up a brake and doing a half-pirouette to face the aircraft forward.) And it’s not difficult, it’s just demanding.

Next time we’ll look in more detail at training to fly a “taildragger”… including just what we did for an hour and a half before nosing skyward on that grin-inspiring morning.