Tailwheel Pilot: What’s a Ground Loop?

By Thomas P. Turner – Master CFI, CFII, MEI, Mastery Flight Training, Inc. | September 2015

Federal Aviation Administration 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, the FAA requires you log at least three full-stop takeoffs and landings in a tailwheel airplane within the 90-day period immediately preceding flight with passengers.1

Often the insurance industry will likely require a check out that includes a specific number of flight hours and a specific or minimum number of take offs and landings prior to you soloing your plane. As one underwriter told me, the insurance company’s checkout requirements are the result of the number of accidents and claims the insurance company pays on tailwheel aircraft.

Why the long checkout requirement? In a word: ground loops.

The word ground loop is a picturesque description of a loss of directional control (LODC) 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, if the ground loop happens at the very beginning of a takeoff roll or the very ending of a landing, when forward speed is lowest often the only damage done is to the pilot’s pride.

Sometimes, however, a ground loop imposes a strong side-load on the main landing gear, often breaking it off which not only damages the landing gear & the belly of the plane but also results in a prop strike & engine damage. Further, 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 ground loop. The damage potential is even greater if the pilot resists a developed ground loop too much...sometimes you just have to go with the flow and let it spin around once you have “lost it.”

The aerodynamics of ground loops
All objects 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. Whereas the c.g. of the tailwheel aircraft falls aft of the main landing gear.

Because of the 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 also resists pivoting movement. Try grabbing a tricycle-gear airplane with all three tires on the ground and pushing it sideways, and see how far you get. “Nosewheel” airplanes are directionally stable around the vertical axis on the ground. Translation: They are quite unlikely to ground loop.

Note: Airplanes with free-castering nosewheels and many homebuilts aren’t immune to ground-loops because the free-castoring nosewheel does not provide any resistance to the aircraft spinning around its vertical axis.

Tailwheel airplanes, as stated previously, 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. Tailwheel airplanes are directionally unstable around their vertical axis on the ground. They want to go backward—the tailwheel Cessna 120 I owned many years ago was hard to pull forward out of its hangar with both hands, but I could easily push it backward with one hand.

Most tailwheels, are free-castering and 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 its 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 directional instability on the ground, most larger tailwheel airplanes have a tailwheel lock. A tailwheel lock is a device the pilot engages that keeps the tailwheel centered during takeoff and landing, so the aircraft's track remains aligned with its longitudinal axis. This serves to resist groundlooping during takeoff and landing. The wheel unlocks for tighter turns during taxi and parking.

Dancing on the pedals Watch a tailwheel airplane’s rudder during takeoff and landing. 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 controls are therefore 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. Lest anyone get complacent this same caution is also appropriate to tricycle gear aircraft.

Checking out in and flying a tailwheel airplane is the kind of fun most of us dreamed about when we first discovered aviation. Because of the ground handling characteristics of a taildragger you have opportunities and runways available to you that would be unsuitable for tricycle gear aircraft. Controlling a tailwheel airplane on the ground is not difficult, it’s just demanding…and unforgiving. Once you’ve got the hang of it, though, the ability to maintain directional control on the ground means you’ll have a lot of fun with a taildragger in the air.

1 http://www.faa.gov/regulations_policies/advisory_circulars

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Holder of an ATP certificate with instructor, CFII and MEI ratings and a Masters Degree in Aviation Safety, 2010 National FAA Safety Team Representative of the Year, 2015 Inductee into the NAFI Hall of Fame and 2008 FAA Central Region CFI of the Year, three-time Master CFI Thomas P. Turner has been Lead Instructor for Bonanza pilot training program at the Beechcraft factory; production test pilot for engine modifications; aviation insurance underwriter; corporate pilot and safety expert; Captain in the United States Air Force; and contract course developer for Embry-Riddle Aeronautical University. He now directs the education and safety arm of a 9000-member pilots’ organization. With over 4000 hours logged, including more than 2500 as an instructor, Tom writes, lectures and instructs extensively from his home at THE AIR CAPITAL--Wichita, Kansas. Subscribe to Tom’s free FLYING LESSONS Weekly e-newsletter at http://mastery-flight-training.com/

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