Posted by: Korry | February 21, 2012

Techie Tuesday: How Does an Aircraft Move on the Ground?

In recent weeks, we’ve discussed how an airplane generates Lift and how a turbine engine works. With lift and power, an airplane can easily take flight. But that doesn’t clear up one of the most complicated parts of flying an airliner: moving on the ground. That’s the subject for today’s Techie Tuesday.

Now I know it may seem odd that I say taxiing an airplane is one of the more complicated parts of flying, but it is. You are (hopefully) never as close to other planes in the air as you are on the ground. There are fuel trucks, baggage trucks, catering trucks and other equipment that serve as moving obstacles. And that doesn’t begin to even bring up the subject of how complicated some airports can be to taxi around! Take a look at this airport diagram for Chicago’s O’Hare, one of the more complicated airport layouts I fly through on a regular basis:

Chicago O'Hare Airport Layout

The first thing you should know is that, unlike in your car, the wheels of the planes have no power linked to them at all. In fact, if the plane were up on jacks, you could easily spin the wheels in either direction with your hands. Yes, the main wheels have brakes to stop us, just like in your car, but to actually move the airplane forward we use our engines.

As we talked about last week, a turbine engine generates power by accelerating air rearward thus creating a force forward. The more power we apply to the engines, the faster we move forward. This applies when we are in the air and when we are on the ground. Unfortunately, this also causes a few problems for us to consider.

First and foremost, we cannot move backwards under our own power. This is one of the reasons why we use ground tugs to push us back from the gate instead of just firing up the engines, throwing the plane into reverse and moving backwards. (This can be a big issue if two planes mistakenly turn onto the same taxiway in opposite directions).

The second problem is that in order to start moving, we have to use a fair amount of thrust to “break away” from our stopped position. Often, this can be as much as 40% of full power. As you can probably imagine, using that much thrust means there is a TON of air being pushed behind us. In fact, it can be enough air to knock over ground trucks and sometimes even move airplanes around if they are directly behind us. That’s why you will sometimes get tugged INTO the gate if it’s a tight parking spot.

Once we’re away from the terminal, the difficulties of taxiing continue, although now the challenges are more situational. As the Chicago O’Hare airport diagram above easily shows, these airports can be very confusing. While the markings around an airport are all standard, the layout of the taxiways is generally anything but! That’s where the airport diagram comes in. It’s like our road map for each airport we use. New technology is greatly enhancing situational awareness for flight crews by showing them precisely where they are on an airport via a “moving map” display.

We also have to carefully watch for other airplanes, especially those on nearby taxiways, to ensure our wings and tail are clear, a task made particularly difficult since on most large airliners the pilots cannot even see the wingtips from the cockpit! You may recall this frightening video of an Air France A380 that clipped a Delta regional jet that was stopped:

So how do we actually steer on the ground? With our feet, believe it or not…and a small steering wheel called a tiller. The rudder pedals generally are used to maneuver the rudder, which is located on the vertical part of the tail. On the ground, the pedals also are linked to the nose wheel. This is done primarily so we can stay in the center of the runway when taking off or landing. Unfortunately, the pedals only let the nose wheel move about 7 degrees either side of center, so we can’t make big steering changes. To make sharper turns, we use the tiller. Most of the time, only the captain (left seat) has a tiller, although on some new airplanes such as the Boeing 777 there is a tiller on the right side, too. The tiller can turn the nose gear up to 60 or 70 degrees either side of center. Here’s a diagram that should help clarify this a bit:

Nose Wheel Taxi Diagram

The crazy part is that because modern airliners are so big, we often times have to swing really wide in making sharp turns (just like a limousine). When I was flying the 757, the nose wheel was so far behind the cockpit that sometimes I would feel like I was over the grass before we started making a turn onto another taxiway.

So as you can hopefully see, taxiing is quite complex. It takes a lot of concentration and situational awareness to do it safely and correctly. And hopefully this answers some of your questions about how an aircraft moves on the ground.

Do you have a Techie Tuesday question? Email me at lifesflightplan@gmail.com.

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