Landing Gear — Page 106, Lesson 146

I want to walk you through the final movement of the landing gear locking down, and then we’ll move into a completely different way of powering that same gear — a pneumatic retraction system. Let’s start with the hydraulic sequence. When the landing gear door is fully open, it activates a valve called SV2. That’s a selector valve — think of it as the switch that routes hydraulic fluid. When SV2 is activated, it allows fluid to go to two places at once: to the MLG uplock, which releases, and to the MLG Jack, which retracts and pulls the MLG into the down position. MLG is the main landing gear. So the uplock is the latch that was holding the gear up — it releases — and the jack is the actuator that physically moves the gear down. Now here’s the important part about control. The return fluid — the fluid coming back from the jack — passes through a one-way restrictor, which gives a restricted flow. That restriction acts as a damper on the rate of undercarriage travel. In plain terms, it slows the gear down as it lowers, so it doesn’t slam into its mountings and damage the undercarriage mountings and structure. Finally, the MLG locks into place when it engages with the MLG downlock — that’s the latch that holds the gear down. There’s a note here that’s worth keeping. Restrictor valves are normally fitted to limit the speed, or rate, of lowering of the main undercarriage units. Why the mains specifically? Because the main units are influenced in this direction by gravity — they want to fall fast. The nose undercarriage often lowers against the slipstream, the airflow over the aircraft, so it doesn’t need the protection of a restrictor valve. That’s a nice contrast: gravity speeds up the mains, slipstream slows down the nose. Now let’s switch to the pneumatic retraction system, shown in Figure 3.9. The operation is similar to a hydraulic system, with one key difference: in a pneumatic system, pressure in the return lines is exhausted to atmosphere through the selector valve. So instead of fluid returning to a reservoir, the air just vents overboard. Here’s how the pressure is built and routed. Pressure is built up in a main storage cylinder by engine-driven air pumps. That air then passes through a pressure reducing valve to the landing gear selector valve. So the storage cylinder is the reservoir, the engine-driven air pumps charge it, and the pressure reducing valve steps the pressure down to a usable level before it reaches the selector. Now operate the selector valve to the ‘UP’ position. That directs pneumatic pressure through the ‘up’ lines to the retraction rams — the actuators that pull the gear up — and it opens the down line to atmosphere. So the down line vents, releasing pressure, and the up line pushes the gear up. Operate the selector to the ‘DOWN’ position, and it directs pneumatic pressure through a second pressure reducing valve and the down lines, to the uplock rams and retraction rams. Notice there’s a second pressure reducing valve on the down side. And there’s a note explaining why: a low pressure is used for landing gear extension, for the same reason restrictor valves are used in hydraulic systems — to prevent damage occurring through too rapid extension of the undercarriage units. So low pressure on extension is the pneumatic equivalent of the hydraulic restrictor. Now, the retraction rams themselves are usually damped to prevent violent movement. Here’s the mechanism: the hollow piston rod is filled with oil or grease. When the ram extends or retracts, that oil or grease is forced through the space between the inner surface of the piston rod and a stationary damper piston. That forcing slows the movement down. So the ram has its own built-in shock absorber. Finally, the locks. Uplocks and downlocks are similar to those used with hydraulic systems. The geometric downlocks are imposed by over-centring of the drag strut at the end of the retraction ram stroke. Over-centring means the strut is pushed just past its straight-line position, so it can’t fold back — it’s locked geometrically. The uplocks are spring-ram operated locks — a spring-loaded ram that snaps the lock into place to hold the gear up. So to tie it together: hydraulic uses fluid and restrictors to control lowering; pneumatic uses air, a low-pressure down line, and damped rams to achieve the same controlled, damage-free extension. Both end with the same result — the gear locked down by a downlock, and held up by an uplock.