Landing Gear — Page 106, Lesson 148

Let’s pick up with the landing gear retraction systems. We’ve already covered the pneumatic system, so now I want to walk you through the electrical retraction system, and then we’ll tie in the downlock and uplock operation that applies across these systems. First, the downlocks and uplocks. During retraction, the downlocks are released by the initial movement of the retraction rams. During extension, the uplocks are released by pneumatic pressure in the spring-rams. So the downlock holds the gear down, the uplock holds it up, and each is released by the opposite action starting. Now, the undercarriage doors. In these systems, the doors are operated mechanically, by a linkage on the shock absorber housing. That’s a key point — the doors aren’t driven by a separate hydraulic or electric actuator; they’re tied mechanically to the shock absorber. Now let’s look at the electrical gear retraction system. This is often fitted to light aircraft that don’t otherwise need a high pressure fluid system. So instead of hydraulics or pneumatics, we use an electric motor. The main and nose undercarriage units themselves are similar to those in fluid retraction systems, but the push and pull forces on the retraction mechanism come from an electric motor and suitable gearing. Figure 3.10 shows a typical system, where a single reversible electric motor provides the power to both retract and extend the gear. Reversible means it can run in either direction — one way to retract, the other to extend. Here’s the operation. The motor operates a screw jack. The screw jack provides angular movement to a torque tube. A push-pull rod from the torque tube acts on the drag strut of the nose undercarriage. And cables and rods from the torque tube act on the main undercarriage sidestays. Rubber cord is used to assist extension of the main undercarriage units — so that’s a mechanical assist to help the gear come down. Now, the downlocks in this system are imposed by over-centring of the drag strut and sidestays during the final movement of the operating mechanism, with the assistance of springs. Over-centring means the strut or sidestay is pushed just past the straight-line position, so it locks in place. Limit switches on the drag strut and sidestays cut off electrical power and brake the motor when the downlocks have engaged. And a limit switch on the torque tube stops and brakes the motor when the landing gear is fully retracted. So the sequence is: motor drives the screw jack, which turns the torque tube, which moves the push-pull rod and cables to the drag strut and sidestays. When the gear reaches the down position, the struts over-centre and the limit switches cut power and brake the motor. When it’s fully up, the torque tube limit switch does the same. Finally, the undercarriage doors on this system are operated by linkage to the shock absorber housings — same mechanical principle as before. Let me show you the pneumatic system diagram so you can compare the two. And here’s the oleo-pneumatic strut, which is the shock absorber housing we keep referring to. So to summarize the electrical system: single reversible motor, screw jack, torque tube, push-pull rod to the nose drag strut, cables and rods to the main sidestays, rubber cord assist for extension, over-centring for downlocks, and limit switches to cut power and brake the motor at both ends of travel.