Eddie Sez:

Nobody teaches taxi technique, I suppose, because it is easy to get by with less than perfect taxi skills. Ninety-nine percent of us don't have any issues in this department, but passable technique could be costing you braking effectiveness, brake wear, or a few snide comments behind your back. Chances are you know this stuff, but you might want to consider some notes on taxi speed and smoothness.

The technical stuff comes from the references shown below, with my techniques shown in blue.


Taxi Speed

Flying the T-38 we used to say never taxi faster than the wing commander could walk. Nobody really did that. Years later, when block times were all important, we used to taxi at 18 knots because it made the math timing easier. I know that doesn't seem right, but a quick look at Flight Lessons / Block Times will explain that oddity.

These days the most important restrictions on our taxi speeds seem to be tire and brake temperature.

Tire Temperature Rise due to Taxi Speed

Figure: Temperature Rise vs. Taxi Speed, from Goodyear Aircraft Tire Manual, page 44.

[Goodyear Aircraft Tire Manual, page 44] The vertical dotted line at 35 mph (30 knots) indicates the recommended maximum taxi speed. On [this] chart, the curves constantly slope upward with higher taxi speeds. In other words, the faster an aircraft travels over a given distance, the hotter the tires will become. Many people would expect the shoulder area to generate the most heat. In reality, the bead and lower sidewall area are the hottest. There are two major reasons for this:

  1. All forces, in or acting on a tire, ultimately terminate at the bead. This is an area of high heat generation.
  2. Rubber is a good insulator; or said another way, it dissipates heat slowly. The bead area, being the thickest part of the tire, retains the heat longer than any other part of the tire

It would seem that once you've gone above 20 knots, further increases in taxi speed do not result in much larger increases in tire temperature.

Tire Temperature Rise due to Taxi Distance

Figure: Temperature Rise vs. Taxi Distance, from Goodyear Aircraft Tire Manual, page 44.

[Goodyear Aircraft Tire Manual, page 44] Even when an aircraft tire is properly inflated and operated at moderate taxi speeds, the heat generation will always exceed the heat dissipated. (This is indicated by the ever increasing slope of the lines.) The farther the taxi distance, the hotter the tires will be at the start of the take-off.

I haven't seen any distance limits in most the aircraft I've flown, though the B-707 did have a warning about going any further than five miles.

Brake Temperature

Figure: Brake wear per application, from Airbus, ¶3.2.1., Figure 5.

[Airbus, ¶3.2.1.] As shown in [the figure], energy is theoretically not the primary parameter for carbon wear, whereas it is the most important one for steel brakes. Nevertheless, applying more energy on the brake will have a direct effect on wear due to the induced in crease in brake temperature.

The consensus from most aircraft manufacturers is you should allow taxi speed to build to about 30 knots, then brake down to 10 knots, and then repeat. This is preferable to riding the brakes. More on this at: Procedures & Techniques / Carbon-Carbon Brakes.


Smoothness:

Think about applying pressures to the tiller, not moving the tiller. A lesson in story form: Flight Lessons / Smoothness

No airplane I've ever flown was more sensitive to tiller smoothness than the Boeing 747. Here's how the pros do it: German B-747 Taxi Technique.


References

Airbus Flight Operations Support: Proper Operation of Carbon Brakes, Guy Di Santo.

Goodyear Aircraft Tire Care & Maintenance Manual, Revises 1/11