Taxiing is defined as the controlled movement of the aircraft under its own power over the ground. The key word here is controlled. At a minimum, you'll have to control the aircraft's speed and direction over the ground. But there is far more to taxiing than that. You should also be thinking about safety, minimizing wear and tear on the aircraft, and smoothness. Safety means knowing how to get from the tie-down spot or ramp area to the runway. Safety also includes minimizing propeller blast and maintaining "heads up and eyes out" while taxiing. It also demands a sterile cockpit while operating on the airport surface, meaning no nonessential cockpit communications (chit-chat) or duties (such as running checklists). These could cause distraction-wait until you're stopped in the run-up area.
Minimizing wear and tear entails using proper taxi technique so that brakes and tires don't wear out prematurely. Smoothness means that the aircraft's movements are free from abrupt starts, stops, and excessive lateral forces in turns.
Be a tortoise, not a hare
Controlling taxi speed is one area where understanding Newton's laws of motion helps. Newton's first law of motion states that: "Every body continues in its state of rest or of uniform speed in a straight line unless it is compelled to change that state by forces acting on it." The tendency of a body-or aircraft-to maintain its state of rest or of uniform motion in a straight line is called inertia.
Applied to an aircraft, this means that it is going to take more thrust to get it started moving forward than to keep it moving forward. So when starting to taxi from a standstill, gradually add power (usually no more than 200 or 300 rpm above idle) to get the aircraft moving. Is your airplane tied down on an asphalt ramp, and the asphalt has compressed beneath the tires so that the wheels rest in ruts? If this happens, it can take a lot of power to get the airplane moving-do its engine, nearby planes, and your fellow pilots a favor by pulling your airplane forward a foot or two during your preflight.
When the aircraft reaches the desired taxi speed, reduce power as needed to maintain that speed. A common problem here is adding excessive power to make the aircraft accelerate quickly. The result can be an abrupt start followed by excessive taxi speed or unnecessary braking. Remember, an aircraft does not, and should not, accelerate like a car.
Once under way, and as early as possible, perform a brake check. Here you are looking for typical warning signs of improper brake function such as weak braking action, squeaking or binding noises, spongy-feeling brake pedals, excessive brake-pedal travel, or outright failure while it is still manageable. Make sure the airplane continues in a straight line when you brake; if the brakes on one side are so weak that the airplane turns in a low-speed taxi, you could be in for a real surprise after landing if you have to brake more heavily during your rollout. When performing the brake check, be gentle. The check should be barely noticeable to passengers.
During long taxis, use the brakes intermittently, rather than continuously, to retard taxi speed. This will minimize brake wear and help to prevent overheating.
Another speed-control concept is momentum, which is simply mass times velocity; it aids greatly in turns and helps with directional control. To maintain momentum the aircraft needs to keep a constant taxi speed. Adjust the throttle as necessary to maintain this speed. A common mistake here is using large power adjustments to control taxi speed. One way to combat this tendency is to restrict the maximum power used for taxiing. Instead, make small corrections early, as soon as the taxi speed starts to change.
Be cognizant of surface slopes, headwinds, and tailwinds. These will all cause variations in taxi speed. If you get too fast, reduce the power to idle, and then use the brakes. Do not apply the brakes with power higher than idle. That causes excessive wear on the brakes.
Finally, what is a safe taxi speed? It is difficult to set any rule. What is safe under some conditions may be hazardous under others. A good practice is to avoid taxiing faster than a brisk walk. Remember that in congested areas, the safe taxi speed will be much slower. The primary requirement of safe taxiing is positive control-the ability to stop or turn where and when desired. You should taxi at a speed that is slow enough so that the aircraft can be stopped promptly and safely.
Directional control
Strive to taxi precisely, on the yellow taxiway line.
To taxi on the yellow line, you must first learn how to determine when the aircraft's nosewheel is centered on it. Ask your instructor to taxi on the yellow line for a while and then let you take over. While you're taxiing, have your instructor give callouts as to where the nosewheel actually is in relationship to the yellow line. For example, your instructor can tell you that the nose-wheel is one foot to the left, or six inches to the right. Then you can make corrections. If you're in the left seat of a side-by-side trainer, imagining that the taxi line is running up your right leg might do the trick.
Once you have developed the correct sight picture from the cockpit, the next step is learning how to maintain it. One of the most irritating things to many new student pilots is the rudder-to-nosewheel interconnect rigging. Students often think that if the rudder pedals are perfectly centered, the aircraft will taxi straight ahead. Unfortunately, this is rarely the case. Sometimes, getting an aircraft to taxi straight ahead requires the rudder pedals to be substantially displaced either left or right of center. This can take some getting used to, but knowing this up front can help to reduce your frustration.
Next, you must learn to anticipate steering corrections. A common problem here is weaving. Weaving (which is really overcontrolling) is generally caused by not looking out far enough ahead of the aircraft to anticipate the required rudder corrections. The aircraft's response is sluggish, and you may find that you're making your rudder (steering) inputs too late. Try looking out at least 50 to 100 feet ahead of the aircraft to determine rudder steering inputs. As soon as you detect that the aircraft is deviating from the desired heading, make a correction.
Lead your turns, both when entering and completing them, by applying rudder pressure just before the turn is to be negotiated, and releasing (or adding opposite) rudder pressure just before the aircraft reaches the desired heading. You can think of coming out of a turn as starting a turn in the opposite direction. At first, try to maintain directional control using only the rudders. If a tighter turning radius is required, apply the brake on the inside of the turn to effect a tighter turning radius.
Again, be mindful of your taxi speed. Excessive speed is risky while negotiating a corner, because the aircraft cannot roll like it does when turning in the air. With their small tires and closely spaced wheels, aircraft can swerve, tip over, or ground loop-make an uncommanded turn or course reversal-especially in strong winds. But don't slow down too much, or it will be harder to turn.
If you're taxiing a tricycle-gear airplane on grass, gravel, or any other rough surface-even deteriorated pavement-hold the yoke or stick all the way back. The resulting elevator deflection will help to lighten the load on the nosewheel, which could keep your propeller from striking the ground if the nosewheel hits a depression or pothole. Some airplanes, especially certain high-performance models, have very little clearance between the tips of the propeller blades and the ground, and a prop strike that stops the engine will necessitate an expensive inspection. For this reason some instructors recommend taxiing with the elevator up all the time, unless strong wind requires otherwise.
You must be cognizant of wind direction when taxiing on the ground. Your instructor will show you how to properly position ailerons and elevator to keep winds from getting under wing or tail surfaces. Your goal here is to prevent a wingtip or propeller from striking the ground. It's an important lesson to learn because, as many pilots will tell you, strong surface winds can make taxiing to and from the runway more difficult than any other portion of the flight.
Finally, let the nosewheel straighten out before coming to a complete stop. This reduces strain on the nosewheel and landing gear assembly and ensures that the aircraft will be going straight ahead when it next moves forward.
Safety
Safety is an attitude, a personal, conscious decision. It means following standard operating procedures and using best practices. Resist the temptation to adopt the unsafe practices that you undoubtedly will observe. For example, many pilots taxi too fast. Others spin their aircraft around into tiedown spots with high power. Some taxi dangerously close to other aircraft or structures. Remember, the little time saved is not worth the risk. Use a towbar if necessary to position for parking. Be careful of people and objects around your aircraft. And be conscious of your own prop wash, as well as the invisible hazard of others'-whether prop wash, rotor wash, or jet blast. Any time you have a doubt about wingtip or tail clearances, stop the aircraft and have someone outside check. If no help is available, shut down the engine and move the aircraft with the towbar.
Taxiing involves its own set of specific skills and knowledge. With a little help from Newton, the first and last steps of your flight from Point A to Point B just became a little bit easier.
Christopher L. Parker is a CFI and an aviation author, speaker, and FAA remedial training specialist. He is captain of a Canadair Challenger business jet based in Van Nuys, California.
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