Landing Gear — Page 100, Lesson 139

We're now into the nose wheel steering system, and I want to walk you through how it actually operates hydraulically. This is the system that lets the pilot steer the aircraft on the ground by turning the nose wheel. First, where does the pressure come from? Normal nose wheel steering operating pressure is derived from the undercarriage 'down' line. So the same hydraulic line that pressurizes the gear to lower it also feeds the steering system. But there's a backup: a limited emergency supply is provided by a hydraulic accumulator. That's a storage device that holds pressurized fluid, so if the main pressure fails, you still have some steering capability for a limited time. Now, in the system shown in Figure 3.4, that pressure passes through a change-over valve. This valve is critical because it ensures the steering system is only in operation when the nose undercarriage is down. In other words, you can't steer the nose wheel while the gear is retracted or in transit — the change-over valve blocks the steering circuit unless the nose gear is locked down. Let me walk you through the steering operation itself. Pressure is directed through the control valve to the steering jacks. These are hydraulic actuators — they retract or extend to rotate the nose shock absorber strut within its housing. So the steering jacks physically push and pull the strut to turn the wheel. The pilot's input comes from the steering wheel, and that movement is transmitted through mechanical linkage to the control valve, in accordance with the amount and direction of turn required. So the further you turn the wheel, the more the control valve opens, and the more the jacks move. Here's a clever feedback feature: a follow-up linkage from the nose undercarriage gradually resets the control valve as the nose wheel turns. So as the wheel reaches the commanded position, that linkage progressively closes the valve down, stopping the jacks. When you release the steering wheel, the control valve returns to neutral under the action of its centring springs, and the nose wheel is free to castor. Castoring means the wheel can swivel freely — that's what allows the aircraft to be towed or to pivot on the ground without the pilot holding steering input. Now, the self-centring operation. This is what happens when you retract the gear. Each steering jack has an inner cylinder connected to the landing gear 'up' line. When the landing gear is selected up, that line is supplied with fluid under pressure. The steering jacks extend equally to centralize the nose wheel before pressure is applied to the nose retraction jack. So the sequence matters: the nose wheel is straightened first, so it aligns with the aircraft centreline before the gear starts to retract — otherwise the wheel would be cocked and might not fit into the bay. And the bypass valve allows fluid from the steering jacks to flow to the return line, so the jacks can move freely as they centralize the wheel. So the whole system has three distinct modes: normal steering from the down line, self-centring from the up line when you retract, and a limited emergency supply from the accumulator. The change-over valve keeps steering active only when the gear is down, and the follow-up linkage plus centring springs return everything to neutral when you let go.