A revealing look at your aircraft
As with a pilot's physical, the annual inspection requires the removal of some outerwear. Just as I have to take off my shirt and maybe shed a few other items of clothing so that the aviation medical examiner can poke, prod, and inspect where needed, so too does an aircraft technician remove panels, cowlings, fairings, and other components in order to get an up-close-and-personal look at pieces and parts that normally are inaccessible.
Now, I have no interest whatsoever in observing a physical examination being performed on a pilot or any other person, but an airplane physical is another story. Stopping by the maintenance shop while your airplane, or the one you fly regularly, is undergoing its annual or 100-hour inspection can be, if you'll pardon the pun, extremely revealing.
The typical inspection requires some deconstruction of the airplane. Seats and carpeting are removed from the interior so panels can be opened to inspect the main wing spar, flight control mechanism, fuel selector, master brake cylinder, and any other systems or important components accessible through the cockpit and cabin.
Likewise, the round inspection plates found all over the exterior skin must be taken off, or at least pivoted 180 degrees, to allow for a visual inspection of the inside of the wing and fuselage. Wheels and tires are removed, the engine is uncowled back to the firewall, and fairings on the wing tips and wing roots are unscrewed and set aside. Depending on the airplane, other sections of skin, fairings, or components may need to be removed.
After all of this disrobing, the airplane looks helplessly exposed. We humans depend on clothes to hide our developing humps and folds and flaws. An airplane is clothed in aluminum, carefully shaped to give it sweeping curves and sleek lines. Remove some of that skin and you're left peeking at an awkward-looking skeleton and some industrial-strength organs.
That's why I enjoy stopping by the shop when the airplane is being inspected. It's one of the few times that I'm able to look at the guts of the airplane and learn more about how it works. The only other opportunity is when it goes in for an unscheduled visit to the airplane doctor to treat an unexpected ailment, but on those occasions the mechanic is after a specific problem, and there might not be much to see.
Seeing an airplane exposed is an excellent way to better understand how the systems operate. You remember those fuel, oil, electrical, and hydraulic system diagrams and the accompanying detailed descriptions that made your eyelids droop when you read through the Systems Description section of the pilot's operating handbook or airplane flight manual. All of it begins to make better sense when you can see and feel the actual hardware.
What better way to understand the fuel system than grasping a fuel line and tracing it from the tank through the sump and selector and into the engine-driven pump? Wonder how the pitch trim wheel adjusts the tab on the elevator or stabilator, and what effect--if any--that has on the primary pitch control system? How does the autopilot interface with the pitch control system? Find out by tracing the cables from origin to destination.
What exactly happens--what pushes, pulls, clicks, turns, squeezes, or bends--when you push down the flap lever? Push the brake pedals? Pull the heater knob? Raise the gear? Drop it in for a hard landing?
Doing this kind of detective work also gives you a greater appreciation of the need for more careful preflight inspections and regular preventive maintenance. The first time my Cessna Skyhawk went through an annual inspection after I bought it, I assisted in the inspection. Assist is something of a euphemism--I did some of the grunt work, unscrewing inspection panels, removing seats, and the like. Even so, it was interesting and enlightening.
When the inspector examined the pulleys in the flight control cable system, he discovered that they were caked with a mixture of grease, dirt, and debris. It was so bad that the pulleys wouldn't turn, which meant that the steel-strand cables were moving across the frozen pulley tracks. One was beginning to fray. It was a forceful lesson in the need to keep pulleys clean and lubricated, to ensure that they were inspected at regular intervals, and to take a closer look in the cracks and crevices when doing the preflight walkaround.
One thing that always surprises me when I see a light airplane, say a Cessna 172, opened up for inspection is how little structure there is to the thing compared with most other vehicles with which we are familiar. The car you pull under the wing so you can transfer the contents of the trunk to the airplane's baggage compartment? It probably weighs more than twice as much as the 172 when it is empty of people and fuel.
For example, a 2005 Ford Mustang with a V-8 engine has a base curb weight of about 3,500 pounds, making it about 2.3 times as heavy as a 1,500-pound Cessna Skyhawk. You say you drive an H1 Hummer? How about a curb weight of 7,200 pounds, or nearly five times the weight of the airplane--which still manages to hold its own physically when next to that Hummer.
How do light airplanes manage to withstand the stresses of a full load of gas and people, turbulence, and hard landings when they are so, well, light? The answer is the engineering of the structure. Under the skin our airplane has a steel-tube frame that is welded together in a truss-like structure not unlike a bridge. Bridges are strong; so is our airplane.
Newer airplanes use aluminum formers and stringers to which the skin is attached, making the skin an integral part of the load-bearing structure. The latest composite (mostly fiberglass) airframes are mostly devoid of internal bracing; the fiberglass skin is the structure.
The best way to see exactly how the airframe structure is engineered on the airplane you fly is to look deep inside it when the mechanics have opened it up for inspection.
Next time the airplane you fly is down for a 100-hour or annual inspection, don't just fret about not being able to fly for awhile. Go to the airport and introduce yourself to the owner of the maintenance shop, if you haven't already made the acquaintance. Ask if it's OK to observe the inspection, or at least part of it, and poke around the airplane as long as it doesn't interfere with the shop's work.
The shop owner and inspector probably will appreciate your interest. More important, the experience will make you more knowledgeable and appreciative of the design, construction, and operation of your airplane. You'll learn something about the process of maintaining it, the shop that bears that responsibility, and the people who actually do the work. Not a bad way to spend a day.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.
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