The taxi regime is a very important part of the flight. It involves the ground movement of the aircraft on its power.
The way an aircraft taxi varies from aircraft to aircraft. In smaller aircraft, the nose wheel is steered using the rudder controls of the aircraft. In larger aircraft, the nose wheel is steered using a separate handle called a tiller, which is moved by the hand. Even in these aircraft, the nose wheel can be steered to a few degrees using the rudder pedals. This helps to steer the aircraft during the early parts of the takeoff when the rudder is not that effective.
Aircraft taxiing. Photo: Airbus
The nosewheel steering system
The nosewheel steering of most large aircraft is hydraulically actuated. The steering system consists of steering actuators and a control valve. When hydraulic pressure is fed to the actuators through the control valve it makes the nose wheel turn. In most systems, there are two actuators. In a turn to the right, the right actuator jack extends, and the left jack moves in making the nose wheel turn to the right. And in a left turn, the left actuator jack extends while the right jack moves in causing the wheel to turn to the left.
Once the desired turn is made and the tiller is released by the pilot, the control valve blocks off fluid to the actuators which automatically centers the nose wheel.
The steering system also has a safety bypass valve which is normally closed by the hydraulic pressure from the main hydraulic system of the aircraft. When the pressure is applied to the valve it is pushed down. If in the event of a hydraulic pressure loss, the valve is opened by a spring which allows the hydraulic fluid to flow freely into the actuators. This action puts the nose wheel in caster mode.
The safety bypass valve (when opened), allows nose wheel to caster. Photo: Oxford ATPL.
When in castering mode, the wheel acts like that on a shopping trolley and the pilots can taxi the aircraft using differential engine thrust/power or by the use of differential braking. As a matter of fact, many aircraft can be dispatched with an inoperative nose wheel steering system as long as the castering works. However, it can be quite a challenge to steer a large aircraft in caster mode.
Similarly, there is a nose wheel steering bypass valve that can be opened for towing purposes. In normal operations, this valve is closed. The valve can be operated by moving a towing bypass lever on the nose wheel. When the lever is positioned to towing position, the hydraulic fluid passes through the actuators and the tow truck can move the wheel as required. When the bypass lever is moved to the tow position, it is essential for the ground personnel or the engineer to put a bypass pin in the lever. This locks the lever in position.
The towing lever with the pin inserted. Photo: Anas Maaz.
This is an important step because, during the engine start, the nose wheel steering system is provided with hydraulic pressure. And if the towing bypass lever moves the bypass valve closes and allows full hydraulic pressure to be exerted on the steering system. With the tiller in the center position, if at that point the wheel is turned by the tow truck, the self-centering mechanism of the nose wheel steering will try to center the nose wheel. This could break the tow-bar and may even cause injury to people nearby.
Once the engine is started, and the aircraft is ready for taxi, the engineer should remove the bypass pin and show it to the pilots. This gives the pilots an indication that the pin is removed.
After engine start and completion of push back, the engineer must show the removed bypass pin to the pilots. Photo: Swissport
The taxi technique
The correct taxi technique is an essential piloting skill. In the following headings we will look at some of the taxiing techniques.
How much engine power is required to taxi
When taxiing in a jet aircraft, idle thrust is more than enough to keep the aircraft rolling. However, during the start of the taxi, a breakaway thrust is required to overcome the static friction between the wheels and the ground. Medium to heavy jets tends to start moving once about 20 to 25% of engine thrust is applied. As soon as the aircraft starts moving the thrust must be set to idle.
Above idle thrust is only required if the aircraft is made to come to a stop during the taxi. Excessive thrust application during taxiing must be avoided as it can cause exhaust blast damage and Foreign Object Damage (FOD).
In tight turns, differential thrust/power may be used. For example, if a right turn is desired, the right engine can be revved a little more than the left.
Most jetliners can be comfortably taxied with idle thrust. Photo: Virgin Atlantic.
The use of the tiller
The tiller should be used smoothly when initiating turns. Excessive tiller movement can cause oscillations which can be quite uncomfortable for the passengers. And, when in a turn, the turn must be continued in such a way that the tiller is never neutralized. In a turn, the aircraft naturally loses energy. So, additional thrust may be required. It is essential to add this thrust promptly to ensure that the aircraft does not come to a stop in the turn. If it does come to a stop, a lot of engine thrust will be required to complete the turn.
It can also be a little embarrassing and awkward if the aircraft were to come to a stop in the middle of the turn.
Following the centerline
During taxi and in any phase of the flight, a proper reference must be used to ensure that the aircraft is on the centerline. This varies from aircraft to aircraft. Usually (this works for most aircraft), keeping the centerline between the legs of the pilot ensures correct centerline tracking.
In the Airbus A320, the centerline can be tracked when it is placed between the two main display units placed in front of the pilot, which is approximately between the legs of the pilot. Photo: Airbus FCTM.
The oversteering method
This is a method utilized in large aircraft or those with long fuselages, such as the Airbus A321. In such aircraft, the nosewheel sits some distance away from the pilot’s seating position.
In particularly long aircraft, oversteer method must be used during taxiiing. Photo: Getty Images
This means, if the pilot were to turn perfectly on the taxiway line, the main wheels of the aircraft can cut the corner. On a narrow taxiway, this could make the main wheels inside the turn to get off the taxiway. To prevent this from happening the oversteer technique can be used.
During oversteer, the pilot should let the aircraft travel a little more ahead of the taxiway centerline. This way, the nose wheel tends to track near the outside edge of the turn or the taxiway allowing the main wheel to remain well within the taxiway. The pictures below shows the difference oversteering method makes.
The first picture shows an Airbus A330 that is being turned with the oversteer technique and the second picture shows the turn without the oversteer technique. Notice the blue dotted line of the main wheel inside the turn being a lot closer to the edge of the taxiway when it is not oversteered. Photo: Airbus.
Use of the brakes
Today’s aircraft are equipped with carbon brakes and these brakes are sensitive to the number of brake applications. Too many brake applications can cause the brakes to heat up. Thus, pilots should avoid stomping on the brakes frequently during the taxi. The most correct way is to let the aircraft speed up and once it reaches about 30 knots (maximum recommended taxi speed for most aircraft), brakes must be applied until about 10 knots is reached. Then the aircraft should be allowed to accelerate again. This way, the number of brake applications can be greatly reduced.
Heated brakes are less efficient at stopping the aircraft which is essential during a rejected takeoff.
Which pilot taxis the aircraft
In most airlines, the captain taxis the aircraft. This is an old tradition as in older aircraft, a tiller was only made available for the captain on the left side. As taxiing can be quite challenging in busy aerodromes, many airlines still prefer to give the taxiing job to the captain.
Modern aircraft are equipped with tillers on both the captain’s and the first officer’s side. And due to this reason, some airlines allow experienced first officers to taxi. A few airlines also have no issues with first officers taxiing, given that the operating captain is an instructor or an examiner in the airline.
Whoever taxis the aircraft, the non taxiing pilot must always monitor and guide the taxiing pilot using the relevant taxi charts.
In most airlines, the captain taxis the aircraft. Photo: Qantas.