When moving to and from the runway, does a large passenger aircraft turn by changing the thrust difference on its side engines, or by turning the front landing gear to a different angle? If it is by turning the front landing gear, through what device does the pilot control the angle?
During taxiing aircraft turn using their nose wheel.
The nose wheel is usually hydraulically controlled. The pilot operates it through a tiller. The nose gear can rotate usually quite far, sometimes to almost 90 degrees.
At high speeds, directional control is achieved using the rudder pedals. The nose gear is often linked to the rudder pedals, however the maximum rotation is limited. The nose gear steering does not exactly match to the the rudder steering at all speeds. At low speeds the rudder is ineffective so the nose wheel steering is dominant. For sharp turns the tiller is always used because of the limited rotation by the pedals.
A320 Nose wheel tiller
B747-400 Nose wheel tiller
While turning the nose gear is the primary means of turning while taxiing, differential thrust can be used when necessary. Offhand I can think of three instances where differential thrust should be used on an airplane such as the 747:
- If for whatever reason you are taxiing very slowly, and you're on narrow taxiways and need to make a 90 degree turn, you'll need differential thrust to keep the airplane from stopping in the turn.
- If your center of gravity is near the aft limit, there won't be much weight on the nose gear, and it might start skidding when you try to turn significantly without differential power.
- If the asphalt is hot and/or you're on reverted rubber, the nose gear won't have a lot of traction, and you really need to think about using differential power. I almost slid off a taxiway at Jeddah one day in such a situation even though my taxi speed was only around 8 knots. A general rule of thumb for the 747 in making 90 degree turns was to have your taxi speed down to 10 knots or less, otherwise the nose wheels might just skid.