Landing Gear — Page 94, Lesson 130

I want to walk you through the construction and operation of an oleo-pneumatic strut — this is the heart of how a landing gear absorbs the shock of touchdown and taxiing. Let's start with the construction, because you need the parts before you can understand the motion. Look at Figure 3.1 — it shows a simple oleo-pneumatic strut, and in this instance it's a nose undercarriage, which also includes a steering mechanism. So we have an outer cylinder, fixed rigidly to the airframe structure by two mounting brackets. Inside that outer cylinder sits an inner cylinder and a piston assembly. The interior space is partially filled with hydraulic fluid and inflated with compressed gas — either air or nitrogen. That's where the name comes from: "oleo" refers to the oil or hydraulic fluid, and "pneumatic" refers to the compressed gas. Now, the inner cylinder is free to rotate and move up and down within the outer cylinder, but those movements are limited by the torque links — also called scissor-links. These connect the inner cylinder to the steering collar. The steering collar arms are connected through spring struts to the rudder pedals, and a shimmy damper is attached to the steering collar. So you can see the nose gear is doing two jobs: absorbing shock and steering. Now let's look at how the strut operates, step by step. Under static conditions — that is, when the aircraft is parked or sitting still — the weight of the aircraft is balanced by the strut gas pressure, and the inner cylinder takes up a position approximately midway up its stroke. So the gas is holding the aircraft up. Under compression, such as when landing, the strut shortens. As it shortens, fluid is forced through the gap between the piston orifice and the metering rod. That restriction limits the speed of upward movement of the inner cylinder. So the fluid is being squeezed through a narrow gap, and that resistance is what slows the compression down — it's the damping effect. As the internal volume of the cylinders decreases, the gas pressure rises until it balances the upward force. So the gas is being compressed, and its pressure builds to match the landing load. Then, as the upward force decreases — say, after the landing impact is absorbed — the gas pressure acts as a spring and extends the inner cylinder. The speed of that extension is limited by the restricted flow of fluid through the orifice. So the same restriction that damps compression also damps extension. Normal taxiing bumps are cushioned by the gas pressure and dampened by the limited flow of fluid through the orifice. So every little bump on the taxiway is handled the same way: gas absorbs, fluid damps. Now for the steering side. Movement of the rudder pedals turns the nose wheel to facilitate ground manoeuvres. The spring struts are provided to allow for vertical movement of the nose wheel and to prevent shocks from being transmitted through the rudder control system. So when the nose wheel moves up and down over bumps, the spring struts flex so that shock doesn't travel back up into the rudder pedals and the pilot's feet. One important note on maintenance and inspection: evidence of strut gas pressure leakage will be given by the strut not extending as far as it should, and by uneven amounts of Fescalized metal showing on each main gear. Fescalized metal is the shiny material which forms the hard outer coating of the strut. So if one strut is sitting lower than it should, or you see uneven shiny wear on the main gear struts, that tells you gas pressure has leaked. That's the full picture — construction, the compression and extension cycle, the damping, and the steering integration.