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May 14, 2014
I bear on my forearman enduring reminder of the fallibilities of three-wheel vehicles. When I was five years old, I was attempting to secure my claim to the Outdoor Tricycle Land Speed Record when my leading wheel twisted on an uneven surface, dumping me across the handlebars and fender. I have an inch-long scar from those days when having a wound sewn up was viewed as a needless expense.
Tricycle landing gear, for all its reputation of docility, when mishandled can still dump us over onto our chastisement. Once introduced to the aircraft’s ground handling during initial training, we settle into a rudder-pedal-guided routine of wheeling around taxiways and runways, enjoying the stability provided by tri-gear’s forward-of-main-gear center of gravity. The nosewheel leads us along a straight path unless knowingly disturbed. And all is well, we think, expecting the airplane to behave like an automobile when it’s rolling along the ground.
Not so. As a three-wheeler, the modern airplane has a basic weakness that requires understanding and precaution. The forward quarter on each side of the nosewheel is not supported by outrigger protection, leaving the airplane—like my tricycle—susceptible to a tip-over. This weakness is probably most exploited by high-wing airplanes, which tend to have narrow main gear spans and high centers of gravity, but low-wing designs can fall prey to front-quarter tipping as well.
As the scenario presents itself, the airplane is moving in a different path than where the nose is pointed. This can occur during touchdown, from a crosswind’s drift, or uncorrected misalignment. It can arise from a taxiing turn begun with abrupt steering input at too great a taxi speed.
In any case, the landing gear resists the sideload imposed on it; in small misalignments, the natural self-correction of tricycle gear straightens out the aircraft’s longitudinal axis and a stable path is regained. Tailwheel aircraft, however, would tend to diverge into an even-more-curving path, as the aft-located CG tries to get around in front.
Thus, the tricycle-gear airplane doesn’t ground loop, but it sure can tip over toward the unprotected front quarter. The classic tricycle taxiing incident occurs with a strong tailwind and a fast turn entry; inertia and wind force the airplane to the outside of the turn, the inside tire leaves the ground, and the wind gains more purchase under the raised wing. The propeller blades may slash into the pavement and the outside wing tip can scrape, perhaps bending the main spar. These are expensive repairs, even though no one is hurt.
The more serious tipping mishap takes place during a landing, when the airplane’s longitudinal axis isn’t kept pointed in the direction the aircraft is traveling. A still-flying airplane is supported by air, and if the air is in motion across the runway centerline, it will carry the airplane with it. A pilot’s tendency is to focus on the receding strip of pavement that’s the object of his desire, and he steers back toward it with aileron and rudder inputs. Now the airplane is about to touch down with its friction-inducing tires misaligned with the direction of travel—which happens to be toward that unsupported front corner.
The prudent pilot sees that this isn’t right and will go around, or buy some time by adding power to forestall the touchdown until he has brought the nose of the airplane back into alignment with the runway, adding upwind aileron to stop the drift that began this chain of events. Power returned to idle, the landing takes place on the upwind main gear before the airplane settles onto the downwind tire and, eventually, the nosewheel.
Uncorrected, the airplane hits in a crabbed condition and tips toward the direction of travel, the propeller chopping into the runway as the tail and wing rise. If enough inertia is present, it’s possible to end up with the aircraft inverted on the runway, although a bent wing tip is the usual termination. Because of the higher speed and more rapid sequence of events, the landing tip-over can have more serious consequences than the taxiing faux pas.
If you know the airplane has a weak spot toward its front quarter, you will not ask it to make a taxiing turn at high speed, particularly with a following wind, and you’ll certainly avoid a crabbed touchdown. In the ground-handling instance, plan for the turn by slowing down well in advance, idling the engine, and braking to a slower pace than you were using on the straightaway. If planned correctly, you shouldn’t need to apply brake during the turn.
During the landing approach, note any tendency to drift off the runway centerline and initiate correction on final—not during the flare. Your goal is never to allow the aircraft to land with its nose out of alignment with the runway, or to be drifting laterally across the runway. Remove the threat with the throttle, which you should have under your hand at all times. Going around is always a wise choice, using what you’ve learned to improve your chances on the next attempt. If runway length is not a factor, use partial power to avoid landing until you regain proper alignment.
To prevent the crabbed touchdown and tipping tendency, regain a position over the centerline of the runway, then use coordinated controls to turn the aircraft into alignment. At this point, the wings are banked and must be leveled, or even rolled into a slight bank toward the wind to offset crosswind drift. As you do this, you’ll need to hold opposite rudder to prevent the airplane from turning back into a crabbed position. It’s important not to overcontrol; remember, the goal is a touchdown aligned with the runway, without drifting laterally.
Be aware of the tricycle-gear airplane’s weakness in its forward quarter. There’s plenty of control available to avoid such a mishap, so long as you use it properly.
Leroy Cook is the author of 101 Things to Do With Your Pilot’s License and a contributor to Flight Training magazine.
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