Driving an aircraft around the airport can be done with several different techniques and tools. Most aircraft are equipped with some sort of method of steering the aircraft. Some general aviation aircraft utilize differential braking and a castering nosewheel. Other aircraft use a rudder pedal interconnect that links the nosewheel to rudder pedal inputs. Business jets often have a hydraulically powered nosewheel steering system. Some larger aircraft combine all of these methods into a multilayered system that allows for a tighter turning radius as well as responsive steering.
Using electrical power for control and hydraulic power to actuate the movement of the nosewheel, inputs can be made either by the rudder pedals or a tiller. Just as a tiller is attached to the rudder of a sailboat for steering, the tiller actuates the nosewheel for steering.
While the aircraft is on the ground, the electronic control module that powers the nosewheel steering is energized for usage. Steering commands may be made either by rudder pedal inputs or tiller deflection (or both) and are measured by a handwheel potentiometer which signals the control module to pressurize the steering actuator in the commanded direction. More simply put, the system converts the steering commands from electrical signals input via the tiller or rudder pedals to hydraulically actuated movements of the nosewheel.
In normal operations the tiller is used for almost all taxi operations. It provides the most control and the highest amount of nosewheel deflection, which allows for tight turns. In certain aircraft the rudder pedals may also be used either alone or in conjunction with the tiller for steering. Alone, the rudder pedals are capable of only 5 degrees of nosewheel deflection, but coupled with tiller inputs, the deflection is increased to more than 75 degrees. This allows the rudder pedals to be used for taxi operations that require small adjustments, or a combination of rudder and tiller inputs to gain maximum steering performance. The system is automatically disengaged when the aircraft becomes airborne; it can be manually disengaged, as well. Manual disengagement is usually used when the aircraft is being towed or pushed back from a gate, allowing the nosewheel to caster. A more dramatic scenario that would require manual disengagement would be un-commanded nosewheel steering during ground operations, especially during take-off and landing. Disconnecting the system in this scenario is crucial to maintaining aircraft control while on the ground.
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