Landing Gear — Page 106, Lesson 144

I want to walk you through the hydraulic gear retraction system now. This is the system that actually powers the landing gear up and down on a transport-category aircraft, and I want you to think of it as a sequence of events, not just a single action. First, where does the power come from? A hydraulic system for retracting and extending a landing gear normally takes its power from engine driven pumps. So the engines drive hydraulic pumps, and those pumps pressurize the fluid that does the work. Now, because a pump can fail, an alternative system is available in case of pump failure. On some light aircraft, instead of engine-driven pumps, there's a self-contained 'power pack'. That power pack houses a reservoir and selector valves for the landing gear and flap systems. It may include an electrically driven pump, or it may be powered by engine driven pumps. Here's the key characteristic of this type of system: it normally provides for powered retraction of the landing gear, but extension is by 'free-fall', with the assistance of spring struts. So the gear comes down under gravity, helped by springs, rather than being pushed down hydraulically. Now let's walk through the retraction sequence, because that's where the logic lives. When the landing gear selector is moved to the 'up' position, fluid is directed to the 'up' line, and a return path is created for 'down' line fluid. So you've got an up line and a down line, and the selector valve decides which one is pressurized. Here's the sequence. The 'up' line fluid flows to the Nose Landing Gear, the NLG, Down Lock, which is released. Simultaneously, fluid goes to the NLG Jack, which retracts. So the nose gear's down lock releases and the jack pulls the nose gear up. At the same time, fluid is also ported through the one way restrictor, which has a free flow path, to Sequence Valve 1, SV1. There it waits for the Main Landing Gear, the MLG, downlock, which releases, and for the MLG Jack, which extends and raises the Main Undercarriage. So you see, the nose gear goes first, and the main gear follows, and SV1 is the valve that sequences that timing. Let me show you this initial retraction sequence. Now, what happens as the gear locks up? When the nose undercarriage is fully retracted, it's retained in position by the NLG uplock. And note the type: it's Hydraulically Released, Spring Applied. That means the spring holds it locked, and hydraulic pressure is what releases it. That's a safety feature, so if you lose hydraulic pressure, the lock stays engaged. As the MLG reaches full retraction, it activates SV1, which allows the supply of fluid to the Door Jack. The Door Jack retracts, closing the Main Undercarriage Door. Finally, the MLG uplock, also Hydraulically Released, Spring Applied, engages, locking the gear up. One more detail here. On some aircraft, the selector valve is placed in the neutral position after the undercarriage is raised, leaving the gear un-pressurized for the period of the cruise. That's done to extend component life, because you're not holding pressure on the system for the whole flight. Now let's look at the down sequence, because it's not just the reverse. When the selector is moved to the 'down' position, fluid is directed to the NLG uplock, which is released, and to the NLG Jack, which extends and lowers the Nose Gear. At the same time, fluid is ported to Sequence Valve 2, SV2, where it waits, and to the Door Jack, which will extend to open the door. Now here's the interesting part: the door jack return fluid passes through SV1 and the One Way Restrictor, but this time on the Restricted Flow path. That restriction limits the rate of fluid return, and it acts as a door speed damper. So the same one way restrictor that let fluid flow freely on the way up now restricts it on the way down, controlling how fast the door opens. So the whole system is built around sequence valves and a one way restrictor that work differently depending on direction. SV1 sequences the main gear and the door closing on retraction, SV2 sequences the door opening on extension, and the restrictor damps the door speed on the way down. That's the complete hydraulic retraction and extension cycle.