With aircraft, there’s usually very little in the way of chrome ornamentation. Unlike the Cadillacs and Fords of the early 1960s, if an airplane has fins, they’re not for show; almost every part is there because it has to be. One of the most essential assemblies an airplane can have, namely its landing gear, is often regarded as having only secondary importance in terms of its overall engineering contribution to how the entire aircraft functions. However, being aware of how they can be attached (and what can go wrong) is indeed important.
THE THIGH BONE’S CONNECTED TO THE BACK BONE…
Be it a conventional gear, a tricycle gear, tandem gear, or even a retractable-gear aircraft, there are usually two main ways that wheels are connected to an airframe. Low wing aircraft, for example, often have the mains attached to the wings, inboard toward the fuselage and on the bottom surface (naturally). This allows a bit more spread between its legs, which confers greater stability on the ground and during crosswind landings. Some low-wing airplanes do use the fuselage as the attachment point, and the advantage here is a simpler wing structure, and the fact that bending loads from landing impacts don’t affect the wings, being directed to the fuselage instead. High wing airplanes often have the gear attached to the fuselage simply because it’s closer to the ground.
TUBULAR, OR SPRINGULAR?
Spring steel gear legs are durable, a bit more streamlined, and relatively easy to manufacture. They can be seen on many Cessna singles. However, at one point, they switched from the leaf spring design to tubular main gear. Why? One reason was that, tires aside, tubular gear gives a smoother ride on rough pavement (which is the opposite of what one might expect). In addition, it doesn’t bounce you back into the air quite as readily, should you arrive with a bit too much of a vertical descent rate (which makes landing a bit easier, from an energy management standpoint). They also tend to be a bit lower to the ground, making it somewhat easier to enter and exit the aircraft, and the shorter leg even lowers parasite drag slightly. Still another benefit is that, being a bit lower to the ground, an airplane is less sensitive to gusty winds during taxi (although this doesn’t excuse sloppy aileron technique). One of the most important things about tubular landing gear however, concerns the tires: once airborne, leaf springs automatically become a bit bowlegged, and upon landing, the aircraft’s weight causes them to bend outward again. The first parts of the tires to feel it are the outside edges. As the airplane’s weight transfers from the wings to the wheels, the tires get scrubbed sideways like erasers, and the same thing happens to them; they get worn down. Note: All landing gear that are not attached from immediately above the center of the tires do this, including tubular gear, but not as much. Still another way to attach wheels to wings is by means of a solid hunk of rubber; it’s low-tech, but it’s simple and durable. Here, the cushioning medium takes the form of rubber doughnuts or biscuits. The ride down the taxiway might not be as smooth as it is with a pneumatically cushioned suspension, but it works…speaking of which…
THAT’S OLEO, NOT OREO!
Probably because form follows function, there is less variation among nose wheels. The most common structure is a tube filled with air and oil. It’s just a shock absorber consisting of an oil-filled cylinder fitted with a hollow, perforated piston into which oil is slowly forced when a compressive force is applied to the landing gear (e.g., when you land). The air is confined inside the tube and behaves somewhat like a spring; the oil in turn provides a damping action. This tube is attached to the wheel, and fits inside a larger tube, which is attached to the airplane. (This is one place you should see chrome: the inner metal telescoping tube should always be shiny and clean.) Many Piper aircraft use this “oleo” strut on all three wheels. In my opinion, it provides a better ride during taxi, but this comes at the expense of possible leakage of air (and rather than a flat tire, a flat strut) if the seals fail.
Uncommon Knowledge: This type of mechanism was patented in about 1915 — to absorb the recoil shock of cannons — and was used as early as 1927 on the Model 10 Waco.
Drawbacks: The danger here, proper inflation. Under-inflation — whenever you see only a very little of that shiny cylinder showing, may lead to metal rubbing against metal (or metal slamming against metal, during a hard landing), not to mention dangerously reduced propeller clearance. Over-inflation makes for harder landings, a higher center of gravity, but worst of all, reduced tolerances to gear side loads.)
THE GENERIC CHECKLIST
Whether your aviation underpinnings are strong and stable, or soft and sprightly, folding or fixed: whatever you’re rolling on, there is always something to watch out for. Here are a few pointers:
- Proper strut extension. Your POH will tell you how many “inches” of the exposed shiny chrome shaft of the gear strut should be visible, and what pressure should be used to pump it up again.
- Cleanliness… Again, oleo extension tubes should be kept clean and free of grime, debris, or any particulate contamination whatsoever, which would only help abrade the seals, score the struts, and shorten their life span. If fluids are part of your landing gear, always check for leaks. It might not even come from the struts, but from the brake lines. If you see any red liquid, that might mean brake fluid — check along the underside of the fuselage near the top of the landing gear, and all along its length. Keep a watch out for anything that looks like it is seeping down along the strut. If there’s any oily liquid on the wheel, a seal is probably losing its grip on life, and needs immediate replacement.
- Fit and Finish. You might justifiably regard me as a master of the obvious here, but the integrity of any and all attachment points isn’t a ho-hum affair. Any loose hardware, and especially wrinkled skin, could be the result of a hard landing. Checking for proper alignment, articulation, and attachment of subassemblies for retractable gear aircraft is essential. Even seemingly secondary items like shimmy dampeners on the nose wheel of your 172 are important.
SPECIAL NOTES AND (un)COMMON KNOWLEDGE
- If there is anything “unconventional” about your landing gear (bungee cords on a Piper Cub, or the infamous free-castoring nosewheel of the Grumman Cheetah, be thoroughly familiar with its weaknesses and idiosyncrasies.)
- Just after pulling away from the tie-down or your hangar, gently tap the brakes to make sure there is no slippage or significant differences in braking action between either side. And of course during taxiing, the judicious use of reduced power rather than dragging the brakes is always wise.
- For those whose wheels aren’t welded, the only way to prolong your exclusion from among “those who have” is to be thoroughly familiar with the way they work, steadfastly shun distraction, and remember and verify the “U” in your “GUMP” pre-landing checklist.
- Last (but not least) are the wheels themselves. Check the tires for nicks and flat spots. (Don’t just give a look around the parts of the tire you can see, but push the airplane forward or backward a couple of feet also, to check the tires’ entire circumference.) Check for proper tire inflation and cotter pins holding the wheel hub onto the axle. Check for cracks in the wheel hubs, brake pad thickness, possible excessive rust and wear, the alignment of valve stem and tire markings (for those who follow this practice) and as with other boringly obvious things, check whatever else is on your checklist.
THE BOTTOM LINE: Landing gear assemblies aren’t all the same, but they all do the same thing: they make it much easier get around the airport (or to taxi without firewalling the power … and trimming the prop). Although they usually don’t engender much thought once we’re flying, they do deserve our attention to help us get around safely.