Aircraft Nose Wheel Steering System | A320 Nose Wheel Steering System

Aircraft Nose Wheel Steering System general description and operation | A320 Nose Wheel Steering System description and operation

Aircraft Nose Wheel Steering Systems

Most aircraft include a nose wheel steering system that allows the pilot to control the nose wheel from the flight deck. As a result, the aircraft can be guided while operating from the ground. Some straightforward aircraft have castering nose wheel systems. Differential braking is used to guide such aircraft when they are taxiing.

Nose wheel Steering System General

Depending on the size and operation of the aircraft, nose-wheel steering systems are designed. Small aircraft incorporate simple mechanical systems, whereas large transport aircraft have complicated steering systems. We will talk about small and large aircraft in general, as well as the Airbus a320 nose wheel steering system in particular.

Small Aircraft Steering System

The majority of small aircraft may steer by means of a straightforward mechanical linkage system attached to the rudder pedals. On the bottom strut cylinder, pedal horns are attached to push-pull tubes. The strut piston axle and wheel assembly turn to the left or right as a result of the pedals being depressed.

Large Aircraft Nose Steering System

Large aircraft use a power source for nose wheel steering because of their mass and the need for positive control. Most power is hydraulic. Large aircraft nose steering systems come in a wide variety of configurations. The majority have comparable traits and elements. A small wheel, tiller, or joystick, normally positioned on the left side wall, is used on the flight deck to control the steering. On some aircraft, the system can be turned on and off. A steering control unit receives the controller input movement by mechanical, electrical, or hydraulic connections.

A hydraulic metering or control valve serves as the control unit. In order to spin the lower strut, it sends hydraulic fluid under pressure to one or two actuators built with different linkages. The fluid in the actuators and system is always under pressure thanks to an accumulator and relief valve, or equivalent pressurizing equipment. The steering actuating cylinders can now function as shimmy dampers thanks to this. Various gears, cables, rods, drums, and/or bell-cranks are components of a follow-up mechanism. Once the steering angle is attained, it resets the metering valve to its neutral position.

During takeoff and landing, several systems feature an input subsystem from the rudder pedals for minor degrees of turns done while controlling the aircraft at high speed. All systems often have safety valves to release pressure in the event of a hydraulic breakdown, allowing the nose wheel to swivel.

Through a shaft, the steering drum inside the flight deck control pedestal is connected to the nose wheel steering wheel. Through the use of cables and pulleys, the control drum of the differential assembly receives the steering signal as a result of the rotation of this drum. The metering valve assembly receives movement from the differential assembly through the differential link, which then moves the selector valve to the desired position. This gives the nose gear's hydraulic power source.

The open safety shutoff valve directs pressure from the aircraft hydraulic system into a pipe going to the metering valve. The metering valve then directs the pressurized fluid out of port A and into steering cylinder A through the right turn alternating line. Since there is only one port, pressure causes the piston to start extending. The extension of the piston gradually moves the steering spindle to the right since its rod attaches to the nose steering spindle on the nose gear shock strut, which pivots at point X. As the nose wheel rotates, fluid is pushed from steering cylinder B into port B of the metering valve via the left turn alternating line. The metering valve sends this return fluid into a compensator, which then sends it to the return manifold for the aircraft hydraulic system.

As said, the nose gear rotates as a result of hydraulic pressure. The gear shouldn't be turned too far, though. Devices that stop the gear at the chosen angle of turn and maintain it there are part of the nose gear steering system. With the help of follow-up linking, this is achieved. As previously mentioned, the steering spindle rotates the nose gear as cylinder A's piston extends. Gear teeth on the back of the spindle mesh with a gear at the base of the orifice rod. The orifice rod rotates in the opposite direction from the nose gear and spindle. The scissor follow-up links, which are situated at the top of the nose gear strut, receive this rotation from the two portions of the orifice rod. The attached follow-up drum rotates as the follow-up links return, transmitting movement to the differential assembly by cables and pulleys. The metering valve is moved back toward the neutral position by the differential arm and links when the differential assembly is operating.

Compensator unit

The compensator unit system maintains constant pressure in the steering cylinder fluid. A spring-loaded piston and poppet are housed inside a three-port housing that makes up this hydraulic device. The left port serves as an air vent to keep trapped air from obstructing the piston's movement at its rear. The metering valve return port is connected by a line to the second port, which is found at the top of the compensator. On the right side of the compensator is where you'll find the third port. The return manifold for the hydraulic system is connected to this port. When the poppet valve is open, it directs the steering system return fluid into the manifold.

When the piston is under high enough pressure to compress the spring, the compensator poppet opens. This system requires 100 psi. As a result, the pressure holding the fluid in the metering valve return line is sufficient. The metering valve and the cylinder return lines are both under the same 100 psi pressure. This keeps the steering cylinders under pressure at all times and enables them to serve as shimmy dampers.

Shimmy Dampers

Shimmy Damper

Most nose gear has a tendency to shimmy or oscillation quickly at certain speeds, and torque connections connecting from the stationary upper cylinder of a nose wheel strut to the moving bottom cylinder or piston of the strut are insufficient to prevent this. A shimmy damper must be utilised to control this disturbance. A shimmy damper uses hydraulic dampening to control nose wheel shimmy. Although the damper can be integrated within the nose gear itself, it is often an external element linked to the upper and lower shock struts. It keeps the nose gear steering system operating normally throughout all phases of ground operation.

Steering Damper

Large aircraft with hydraulic steering, as previously indicated, maintain pressure in the steering cylinders to give the necessary dampening. Steering damping is the term for this. Vane-type steering dampers are seen on several vintage transport aircraft. However, they serve to reduce vibration as well as steer the nose wheel.

Airbus A320 Nose Wheel Steering system

Description

A hydraulic actuating cylinder steers the nose wheel. The green hydraulic system supplies pressure to the cylinder, and electric signals from the BSCU (Brake and Steering Control Unit) to control the Nose Steering.

The BSCU receives orders from:

the Captain's and the First Officer's steering control wheels (command added algebraically),
the rudder pedals,
the autopilot.

The BSCU (Brake and Steering Control Unit) transforms these commands into nose wheel steering angle. That steering angle has the following limits, which depend on ground speed of the aircraft and the origin of the orders.

The nose wheel steering system receives actuating hydraulic pressure when:

The A/SKID & N/W STRG switch is on and,
The towing control lever (installed on nose wheel) is in normal position and, at least one engine is running and,
The aircraft is on ground.

The logic is designed in such a way that the nose landing gear doors must be closed in order for the green hydraulic system to apply pressure to the actuating cylinder.

The handwheel can turn the nose wheel up to 75° in either direction. A lever on the towing electrical box (installed on nose landing gear) allows ground crew to deactivate the steering system for towing the aircraft. This then allows the wheel to be turned 95° in either direction.

The pilots can use a pushbutton on either steering handwheel to prevent rudder pedal input or autopilot input from going to the BSCU. An internal cam mechanism installed returns the nose wheel to the centered position after takeoff.

Controls and Indicators

Indication on ECAM Page: The “STERING” LEGEND appears along with an ECAM caution if either the nose wheel steering system or the Anti-Skid system fails.

The steering handwheels, which are interconnected for operation from both crew, can steer the nose wheel up to 75° in either direction.

Note: The steering system automatically centers the nose wheel after liftoff.

Rudder PEDAL DISC Push Button: Pressing this button on either handwheel removes control of nose wheel steering from the rudder pedals until the button is released.

A/SKID & N/W STRG Switch: This ON/OFF switch activates or deactivates the nose wheel steering and anti-skid.

MEMO Display: When the nosewheel steering selector is in the towing position, “NW STRG DISC” is displayed in green. The legend is in amber color, if one engine is running.