Landing Gear — Page 100, Lesson 137

Let’s start with the nose wheel, because that’s where the real precision work happens on the ground. Self-centring Before the landing gear retracts, the nose wheel must be perfectly straight — dead ahead. We call that self-centring, and it’s essential. Why? Because the wheel well, the stowage space, is tight. If the nose gear is even slightly off-centre when the hydraulic system starts forcing it up, there isn’t enough room. The gear will jam, and you can cause severe structural damage to the aircraft. So the system has to guarantee the wheel is centred before retraction. How is centring achieved? Two ways, depending on the design: a spring-loaded cam, or a hydraulic dashpot. The spring-loaded cam is exactly what it sounds like — a cam profile with a spring that pushes the wheel back to centre. The dashpot is a hydraulic damper — a cylinder with fluid that resists motion and gently brings the wheel back to the straight-ahead position. Nose Wheel Steering Now, to manoeuvre on the ground, the pilot needs a way to steer. Early aircraft used differential braking — you brake one main wheel harder than the other, and the aircraft turns. That works, but it’s crude. Modern large commercial aircraft use powered steering, almost always hydraulic. This is a big deal for fuel economy: with powered steering, you can set the engines at minimum thrust for taxiing, because you don’t need asymmetric power to turn. That saves fuel — a serious consideration with large jet engines. Powered steering is also more accurate, and it reduces tyre wear, brake wear, and noise pollution. But there’s a catch. Sometimes you want the nose wheel to be free to castor — for example, when towing the aircraft on the ground. So the steering system has a bypass in its hydraulics. When the bypass is open, hydraulic fluid can transfer freely from one side of the steering actuator to the other, and the nose wheel is free to swivel. When you select steering, hydraulic pressure closes that bypass, and the system becomes rigid — steering is engaged. How does the pilot control it? Two ways, depending on the aircraft type: a separate steering wheel — sometimes called a tiller — or by operating the rudder pedals. Built into the steering system are two key components: a self-centring jack, which helps return the nose wheel to centre, and a shimmy damper. A shimmy is a rapid oscillation of the nose wheel at speed — the damper absorbs that vibration so the wheel tracks straight instead of wobbling. Power Steering Systems Light aircraft keep it simple: the nose wheel is mechanically linked straight to the rudder pedals. But larger aircraft need powered steering arrangements. In a power steering system, the nose wheel is rotated by electric, pneumatic, or — most commonly — hydraulic power. Let me walk you through the hydraulic version, because that’s what you’ll see on the big jets. The system includes: - A cockpit steering wheel or tiller — that’s your input. - A control valve — this directs hydraulic fluid based on your input. - Steering cylinders — these are the actuators that physically turn the nose gear. - A mechanical feedback device — this holds the steering at the selected angle, so the wheel stays where you put it, not drifting back. - And a power source — normally the aircraft hydraulic supply, fed from the engine-driven pumps. So the chain is: you move the tiller, the control valve opens, hydraulic pressure goes to the steering cylinders, they rotate the nose gear, and the feedback device locks that angle in place. That’s the complete powered steering loop. One thing to keep straight: self-centring is about retraction — getting the wheel straight before it goes up. Steering is about ground manoeuvring — turning the aircraft while taxiing. They’re related, but they serve different moments in the flight cycle.