If you don't understand how something works, how can you be expected to know how to compensate if it doesn't work quite right? When it comes to airplanes, knowledge is everything, and experience gives that knowledge depth.
Some of you may think that what follows is unnecessarily simple. However, in aviation we sometimes wrongly assume someone understands something, and the amount of flight experience you may have accumulated does not always correlate to a specific level of knowledge, so we're keeping this coloring book basic.
Engine
With only a few exceptions, airplane engines are basically big Volkswagen Beetle powerplants: They have a flat cylinder arrangement, and are cooled by air that comes in the holes in the front and is forced down through the cylinder fins by baffles to exit out the bottom. It's not a wildly efficient way to cool an engine, but it has zero moving parts. If baffle seals are missing, broken, cracked, or worn, all of the incoming air is not being forced down between the fins.
There are two spark plugs per cylinder, which gives more efficient ignition. But the real reason there are two is for redundancy: Each plug in a cylinder is fired by a different magneto (there are two of those, too), so no single failure will stop the engine.
One big difference between your car and an airplane is that the engine on an airplane is a totally self-contained unit. Once started, it doesn't depend on anything connected to the airframe other than fuel lines to keep running. It doesn't need the airframe electrical system because the magnetos that fire the spark plugs generate their own electricity.
The magnetos depend on a ground wire that's connected to the ignition switch to kill them-and keep them dead when the engine isn't running. This fail-safe mode keeps them running if a wire fails in flight. To test this, just before shutting down, flip the ignition switch to Off, then quickly back to Both, to make sure the engine stops firing in the Off position.
The reason for the woven metallic cover on the spark plug wires and the way they screw onto the spark plugs is to cut down interference with radios.
Fuel
The fuel comes out of the tanks, through a selector valve, and flows through a gascolator that filters out foreign particles and gives any water-which always seeks the lowest point-a place to go. High-wing airplanes may not have auxiliary fuel pumps in the circuit because gravity feeds the fuel to their engines, while low-wing airplanes depend on an electrically powered boost pump to initially get fuel to the engine (where an engine-driven pump takes over). Check the pilot's operating handbook (POH) for more details-and any operational considerations, such as a restriction against transferring fuel into the tank being used-about the airplane you fly. On some airplanes fuel will drain into the low wing if parked on a slight slope and the fuel valve is on Both.
Flight controls
The rudder and ailerons of some aircraft are interconnected (usually by bungees) to simplify coordination. But in most aircraft, the rudder, ailerons, and elevator are independent control systems, with the ailerons and elevator coming together at the yoke.
The next time you're in an airplane, look under the panel. Use a flashlight, if necessary. In many aircraft you'll see a T-shaped steel-tube structure that pivots at the bottom in the middle of the cabin floor, and the control yokes are mounted on the arms of the T. When the yoke is pulled or pushed, the entire T assembly pivots fore and aft, pulling on cables or push rods that, in turn, move the elevator.
For aileron control, the shaft of the yoke is attached behind the panel to a sprocket on each end of the T. A chain that runs around both sprockets is attached to cables that run down both sides of the airplane, behind the upholstery, to the wing. Once in the wing, it may be redirected to the ailerons by pulleys. In other aircraft the aileron cable runs to bell cranks at the wing roots, which activate push rods inside the wings that translate your control inputs to the appropriate aileron deflection.
Landing gear
Most general aviation landing gear are separated into two camps: the oil-damped oleo strut and the steel spring (tapered rod or flat). However, they are often mixed and matched, with a solid spring being used for the mains and an oleo for the nose gear. Some airplanes use an oleo for all three, or a spring for all three.
The oleos use a combination of oil and gas (nitrogen or air) metered through various systems to supply a damping effect in both directions on touchdown. Because they are damped in rebound, as well as compression (touchdown), they don't bounce as readily as a spring gear does. If a strut is flat, it can be damaged on landing and will transfer more load to the aircraft structure.
The old-style flat spring steel main gear has been replaced with tapered-rod gear on many newer airplanes. When a tapered spring is used on a nosegear, steering can't easily be adapted to it. This is why Cirrus, American Yankee, and other aircraft have a full-castering nosewheel that relies on differential braking of the main gear for steering at slow speeds. Where the oleo has a bunch of moving parts, the spring gear is absolute simplicity, although energy that is put into it while being compressed on touchdown will be released on the rebound. It can't be damped as the oleo can.
Brakes
Disk brakes, as used in most airplanes, employ a system where a cylinder (or two or three) is pressurized within a caliper that is not rigidly mounted. Instead, it floats in and out on a pair of machined rods. The outer end of the caliper hangs over the brake disk and holds a brake pad on the outside of the disk. As hydraulic pressure forces the piston out on the inner side of the rotor, which is rigidly attached to the wheel, the two pads squeeze the rotor, which slows the wheel's rotation.
The entire brake assembly is quite simple, although it is dependent on O-ring seals to contain the pressurized fluid. If fluid is found on the ground or dripping from the bottom of the caliper, it's a good bet that an O-ring is leaking-and that the airplane should be grounded until it can be repaired. Brake fluid is contained in two different manners. Most general aviation airplanes have a reservoir on the firewall behind the engine. The level can sometimes be seen through the reservoir side, or the top must be removed. A line comes out of the bottom of the reservoir to the brakes on both sides, keeping the caliper pistons constantly full of fluid.
Another, less common design mounts a separate reservoir on the top of each of the master rudder pedals, usually the ones on the left if it's a side-by-side aircraft. To check and/or service these, you have to crawl under the panel.
What about the airframe?
There are three materials commonly used for airframe construction, with a few odd ones, such as wood, standing around the fringes of the rest. The materials used dictate the type of structure that must be designed to carry all flight loads and these can differ dramatically.
All-aluminum. Everyone is familiar with aluminum structure, but many don't realize that the outer skin is what gives the structure its rigidity and strength. It's like the shell of an egg, but it is supported by myriad carefully designed inner components, including ribs, spars, frames, and stringers. With the exception of the spars, which give most wings much of their strength, the rest of the inner structure serves two functions: They give the skin its shape and stabilize it, and also allow for smaller panels of skin as denoted by the lines of rivets. The smaller the panels, the lighter the skin can be-but more interior structure will be required.
Composite. Modern technology has given us various types of composites ranging from Kevlar to carbon fiber, all of which can be molded into almost any shape because they are nothing but cloth in a resin matrix arranged in a sandwich fashion with foam or honey-comb between the layers. They are incredibly strong and eerily light. They aren't affected by moisture, but they aren't crazy about heat or ultraviolet rays-modern paint has eliminated those fears. The materials can give very precise, clean surfaces and lines, which greatly improve the aerodynamics. The downside to composites is that they require extensive manufacturing tooling in the form of very precise molds. They also require specific expertise to repair.
Rag and tube. The classic steel tube fuselage is often mated with fabric-covered, wood-frame wings or all-aluminum wings. The steel tube fuselage has the advantage of being strong and offers excellent crash protection, but its fabric covering needs periodic replacement and the tubing will rust, if not properly protected. It is labor intensive to build. Today this structure is usually found on older aircraft, or utility or special-purpose aircraft (aerobatics, agriculture).
Electrical
Generally, aircraft electrical systems are simpler than those you find in modern automobiles. Because of that, each circuit generally has its own circuit breaker that is accessible in the cockpit. The most important thing to know about the electrical system is, as we mentioned earlier, that it's not needed to keep the engine running. If a short causes smoke in the cockpit, you can shut off the master switch to remove current from the system so the situation doesn't get worse-and the engine will keep running. However, if you have electric flaps or landing gear, you may find yourself landing without flaps or having to lower your landing gear using a manual procedure.
Instruments, pitot/static system
There are two basic variations of the static system and the airspeed system of which it is a part. Indicated airspeed is nothing more than the difference between the dynamic pressure created by air being rammed in the pitot tube and the static pressure of the surrounding air, calibrated in miles per hour or nautical miles per hour (knots). The static pressure can be taken several different ways, including ports on the fuselage (generally one on each side) far enough back that the airflow is neutral. Or, it can be measured by ports on a pitot mast sticking out of the wing (as shown above).
The static pressure ports are also connected to instruments other than airspeed. These include the altimeter and vertical speed indicator (VSI). Knowing that, if your static ports ice over or you lose your airspeed indication, one of the emergency fixes is to break the glass on one of the other instruments so it "breathes" cabin air. It's important to note that although close enough for an emergency situation, cabin air is usually less dense than actual, so the indicated altitude won't be precise. By the way, break the VSI glass; you'll need the altimeter.
Invest a little time learning your machine
It would take an encyclopedia to cover in detail every aspect of an aircraft's anatomy, but these are the high points. Now, head out to the airport and spend a morning with a mechanically oriented friend around an airplane asking questions. You never know when that kind of information may come in handy.
Budd Davisson is an aviation writer/photographer and magazine editor who has written approximately 2,200 articles and has flown more than 300 different types of aircraft. A CFI since 1967, he teaches about 30 hours a month in his Pitts S-2A Special. Visit his Web site (www.airbum.com).
Want to know more? (http://ft.aopa.org/links).
Related Articles
