Sistema de Direção da Roda do Nariz

Visão Geral

The nose wheel steering system provides directional control of the aircraft during ground operations — taxiing, takeoff roll, and landing roll — by rotating the nose gear assembly about its vertical axis. During flight, aerodynamic control surfaces provide all directional authority; on the ground, where aerodynamic forces are insufficient for directional control, nose wheel steering is the primary means of following taxiway centrelines, executing tight turns at gates and holding points, and maintaining directional control during crosswind takeoffs and landings.

Commercial transport aircraft nose wheel steering systems are hydraulic, with the nose gear turn angle commanded either through a cockpit tiller (a small steering wheel typically located on the outboard side of each pilot's seat) or through rudder pedal inputs. Tiller inputs allow large steering angles for taxi manoeuvres — typically ±70° to ±80° depending on aircraft type — while rudder pedal coupling provides smaller steering angles (±5° to ±10°) suitable for maintaining directional control during the takeoff and landing roll at speeds where large nose wheel deflections would impose excessive lateral loads.

Como Funciona

Tiller rotation is transmitted mechanically or electrically to a steering control valve (SCV) that directs hydraulic pressure to a dual-acting steering actuator (or a pair of actuators) mounted on the nose gear strut. The actuator rotates a torque link or steering collar assembly attached to the nose gear rotating sleeve, which in turn rotates the wheel axle. On aircraft with electrical tiller inputs (fly-by-wire steering), the Brake and Steering Control Unit (BSCU) translates tiller position signals into actuator commands, allowing simultaneous input blending from left and right seat tillers.

At high taxi speeds and during the takeoff and landing roll, steering authority is typically reduced automatically as a function of ground speed to prevent excessive nose wheel angles that could overload the nose gear or cause structural damage. Rudder pedal steering is generally active throughout the ground roll but provides only small angular deflections — the rudder is the primary directional control at the speeds where it becomes aerodynamically effective (above approximately 60–80 knots on most aircraft).

A centering cam mechanism returns the nose wheel to the straight-ahead position during gear retraction after takeoff, preventing the gear from retracting with a deflected nose wheel that would prevent it from fitting within the wheel well. This centering is typically hydraulic, using a dedicated centering actuator energised when the gear handle is moved to UP.

Componentes Principais

• Tiller: Cockpit steering input device, typically a small wheel or handle located outboard of each pilot. Provides up to ±75° of nose wheel steering authority depending on aircraft type.
• Steering Control Valve (SCV): Electro-hydraulic servo valve that converts tiller position commands into hydraulic pressure routing to the steering actuators. Typically incorporates position feedback for closed-loop control.
• Steering Actuators: Single or dual hydraulic cylinders attached tangentially to the nose gear upper torque link or steering collar. Convert linear hydraulic force into rotational steering moment.
• Bypass/Disconnect Valve: Allows the steering actuators to be hydraulically bypassed so the nose wheel can rotate freely during pushback operations (when the bypass pin is also inserted). Prevents hydraulic locking of the nose wheel against tug steering forces.
• Centering Mechanism: Hydraulic or mechanical centering device that aligns the nose wheel fore-and-aft during gear retraction to ensure wheel well clearance.
• BSCU (Brake and Steering Control Unit): On fly-by-wire aircraft, the central computer integrating tiller inputs, rudder pedal inputs, ground speed derating, and steering actuator position feedback.

Aplicações em Aeronaves

• Boeing 737-800 — dual tillers (captain and first officer); ±78° nose wheel authority via hydraulic System A; rudder pedal coupling ±7°
• Airbus A320-200 — dual tillers; ±75° nose wheel via Green hydraulic system; BSCU integration with anti-skid; steering disconnect for pushback
• Boeing 777-300ER — ±70° nose wheel steering; additionally has main gear steering (±8°) on the aft body gear for tight turn radius on large-radius taxiways
• Boeing 787-9 — standard hydraulic nose wheel steering; ±70° tiller authority; rudder pedal authority ±5°

Vantagens e Limitações

Nose wheel steering enables commercial jets to navigate the complex taxiway geometries of modern airports, executing turns at holding points and gates that would be impossible using differential thrust or aerodynamic controls alone. The system is hydraulically powered and therefore not dependent on engine thrust for operation, enabling precision manoeuvring with engines at idle or even with all engines off during towing.

The principal limitation is the mechanical loading imposed on the nose gear by tight-radius turns, particularly on wide-body aircraft. Tight turns at excessive speed can exceed the lateral load limit of the nose gear structure, and specific steering angle limits apply at different taxi speeds. Operators must also ensure the nose wheel steering bypass/disconnect is correctly engaged during pushback operations to prevent hydraulic system damage from tug-induced steering forces. Failure of the nose wheel steering system during taxi requires differential thrust management and, in extreme cases, tug assistance, significantly disrupting ground operations.