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Can I Fly This?

10 preflight items that ground an airplane

A cracked exhaust stack can be hard to spot, but it can create evidence: gray dust left behind by exhaust gasses (below).
This fiberglass spinner developed a crack and was replaced.
A puddle on the ground beneath a wing could mean a leak from a fuel tank.
You'll lose a lot more fuel if you attempt to fly with a missing gas cap.
A hole in the tire is huge red flag not to fly, but even if the cord is showing, don't take a chance.

How often do you actually find something that grounds an airplane during a preflight inspection? Almost never, right? Because of that, how often have you thought about skipping--or have actually skipped--a preflight? After all, it flew fine an hour ago, so....

Mechanical parts do fail. How do we know something didn't fail just as the previous pilot was taxiing in from his flight? That's why we preflight the airplane before every single flight.

Of course, for a preflight to be effective, we must know what constitutes a problem big enough to render the airplane not flyable.

Actually, there aren't many things that immediately make an airplane impossible to fly. A tree falling across one wing comes to mind. A flat tire is another. A collapsed landing-gear strut should give us pause. Usually, however, discrepancies found during a preflight that ground an airplane are much less obvious and fall into the category of "Yeah, it'll fly, but for how long before this breaks?" Many of the maintenance items that ground an airplane have a certain amount of judgment attached to them: It doesn't look right, but is it bad enough to cancel the flight?

If you have to ask the question, you already know the answer. One way to analyze your decision is to ask a simple question: If this particular aircraft component were to fail on this flight, how serious would it be, and would I be able to deal with it? Better yet, would I want to have to deal with it?

What kinds of conditions are we talking about? How do we make the call? Here, in no particular order, is a list of no-go items you should be able to recognize.

1. Nicked prop

We have been told over and over to always inspect the prop for nicks, but how bad does a chip or gouge in the propeller's leading edge have to be before the airplane is no longer safe to fly?

We worry about nicks on props because an aluminum propeller is actually just a little wing that's spinning around. It is pushing against the wind but not in a smooth manner. In fact, every time the engine fires, it pulses the prop and pushes just a little harder, then relaxes for a nanosecond before pushing again. This means the prop is constantly being flexed, not unlike a wire being bent back and forth, and we all know what happens to a piece of wire--if you bend it enough, it breaks. We also know that a fast way to cut a piece of wire without cutters is to nick it with a file or something and start bending it back and forth. In a few bends it always breaks right at the notch.

That is because a nick is a stress riser--it concentrates the stresses right at the point of the nick. A sharp nick increases the forces many times over, as high as 15 times right at the apex. If it is rounded and smooth, the increase because of reduced cross-sectional area is still there, but the probability of fatigue failure goes down dramatically.

Any fresh, sharp nick should be filed out or dressed immediately, because it's impossible to know how long that propeller will fly without breaking. What are the consequences of losing a foot or so of your propeller? In all probability, the imbalance will immediately try to yank the engine completely out of the airplane, and it is usually successful. If you literally lose the engine, the airplane will be out of balance and will be uncontrollable.

So, if you find a fresh nick, get a mechanic to file it smooth. If you find a really deep nick, start looking for a ride home, because that bird may be grounded for a while.

2. Cracked spinner

Be very wary of a crack in the propeller spinner. A crack in any kind of aluminum is not going to stop by itself. The end of the crack represents a serious stress riser, and every time the part flexes, the crack is going to propagate. If a small hole is drilled right at the end of the crack, so that it terminates in a nice, round hole--assuming that's an approved technique for the specific situation in question--the crack is unlikely to go any farther.

Because of the stresses involved, the stop-drill technique seldom was enough for spinners, and most mechanics would patch them. About 10 years ago there was a shift from repairing to replacing cracked spinners, and you're not likely to see many patched or stop-drilled spinners today.

If you find a fresh crack in the spinner, don't fly the airplane. If the spinner comes apart in flight, it could come back through the windshield or damage a wing. Ask an aircraft mechanic to inspect it; he or she can reference the manufacturer's recommendations and see if there's an approved fix--or, if not, order a new spinner.

3. Fuel stains or leaks

The nice thing about aviation gasoline is that the dye that identifies its octane also leaves temporary stains on paint and aluminum, so we can easily see where it has been. Stains on the tops of wings are common and almost expected: a lineman overfills your tank and it runs down the wing or a cap leaks. Not the end of the world.

If you find that telltale blue stain on a rear baffle on an engine, however, you want to start looking around. Do the same if it's streaking down the belly behind a quick drain or dripping from the bottom of the wing under the tanks.

It doesn't take much to ignite avgas, or the right concentration of fumes--especially around an airplane, which has exhaust stacks up front and a cockpit loaded with electric switches and other gadgets that could generate a spark under the right circumstances.

When we see blue stains we want to know why they are there and where they came from. We want to do our wondering while looking at stains on the ground, not wondering about fumes while we're in the air.

4. Excess play in control surfaces

In theory, when you grab the trailing edge of the flaps, ailerons, elevator, or rudder and gently move them, you shouldn't feel any slop. That's in theory, because as airplanes age, everything in the system loosens up and it's not unusual to feel a little "give." But, it should only be a little. You shouldn't be able to move any control surface, including trim tabs, more than the tiniest fraction of an inch before meeting resistance.

The danger in feeling slop in the control system is that among other things, a failure in that system is serious no matter how you look at it. If the looseness can be traced to something like wear in the hinge bearings, then it's something to be watched, but the airplane probably won't be grounded unless the mechanic that looks at it say so.

If the looseness is excessive, don't bother looking for the cause. Go looking for a mechanic instead. Because of the possibility that the slack indicates an impending failure in the control linkage, if a lot of slack exists--even if everything seems tight--you could be inviting a control surface to flutter. Especially if it's an aileron or a trim tab.

This is a tough one to judge, so, when in doubt, find a mechanic and have it checked.

5. Worn tires with cord showing

This one's a no-brainer, as there's not much judgment involved. A badly worn tire is one thing, but if no cords are showing, chances are it'll get you home. If cords are showing, however, you have no way of knowing how much margin is left. So, to be on the safe side, ground the airplane. It might have one more landing left in it, but it might not.

6. Liquid under a brake caliper

Another no-brainer. Even if the bottom of the caliper is just showing a slow weep, don't trust it. The temptation is to top off the reservoir and get one more landing out of it, but there's no way of knowing when the leak/seep started or whether the brakes will work on the next landing. Don't push your luck; have a mechanic look at it instead.

7. Flat or collapsed strut

You can definitely hurt an airplane by flying with no fluid in the nosewheel strut. With nothing to absorb the shock, all of the taxiing and landing loads are rammed directly into the airframe. If you make a bad landing or you land on a really rough runway with a flat main-gear strut, damage could result. In a retractable-gear airplane that depends on full strut extension for the landing gear to fit properly in the wheel wells, trying to retract the gear with a flat strut could jam the system. If a strut is flat, have it serviced.

8. Missing gas cap

This may seem like a minor item, but is it? For one thing, there's no doubt in anyone's mind that if the tank is even close to being full, a lot of that fuel is going to be lost overboard by the siphoning action of the air flow across the tank opening. The decrease in pressure on the top of the wing can suck fuel out through the fuel-filler opening faster than you can punch the Nearest Airport button on your GPS.

However, depending on the way the fuel system is designed, a more insidious thing can happen. If it is a high-wing airplane that depends on gravity to keep the fuel lines filled, the slipstream across the tank opening can create a lower pressure in the tank--not a vacuum, just a lower pressure--that will lower the effective head of the tank. The head is the height of the fuel above the carburetor and the higher the head, the more fuel will flow through the line. Lower pressure in the tank will not stop the fuel flow, but it can slow it down enough that the engine doesn't get as much fuel as it needs, resulting in a very rough-running engine.

If you're missing a cap on an auxiliary tank, and you're running off the mains, you may not be immune. Many fuel systems vent their tanks together, so the lower pressure in the auxiliary tank could be communicated to the main tank via a vent line, causing that tank to quit feeding normally. This effect is especially noticeable when the tank in use is less than one-quarter full, where the head is reduced anyway.

9. Cracked exhaust flange or stack

A crack in an exhaust stack or flange won't immediately ground the airplane unless it is right at a juncture where a total failure will cause the pipe to drop off the engine. The most likely place to see a critical crack is one that's in the pipe at the base of any of the flanges or couplings. Even if the crack itself is hard to see, you're likely to find accumulated dust from the exhaust gas. Also, if bushings or tabs are welded to the stacks and hold straps that keep two pieces of stack together, cracks love to appear right at the edge of the welds. You don't want to fly with cracks like that because, if they fail, they will drop the stack off the engine, and hot exhaust gasses from that cylinder will shoot right into the cowling. That can lead to an in-flight fire, so we don't fly with cracked exhaust stacks.

10. Play in the prop

As you grab the end of the propeller blade and gently pull it forward and back (after making sure the mags are off, of course), and you feel a little click--as if something is moving slightly--that should wave a red flag. The propeller is the one place you don't want parts moving in relation to one another. Check with your flight instructor for the best technique for the airplane you fly; some props should be grasped closer to the hub, and pulling on the prop in some airplanes can result in normal sounds from other engine components.

You do not want to be in the air behind a propeller that is working against the crankshaft flange or the bolts that hold them together, if that's what's happening. Ask a mechanic to inspect anything that seems suspect; if it's loose, it needs to be tightened with a torque wrench and safety wired.

* * *

No aircraft is perfect, although we'd like to make sure they are as close to it as possible before flying. A good preflight is nothing like the aircraft's annual inspection, but it is still amazing how much critical equipment we can see, and check, during a good preflight inspection. It only takes about 10 minutes to do, but it pays off handsomely in the long run.

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.

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Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.

Budd Davisson
Budd Davisson is an aviation writer/photographer and magazine editor. A CFI since 1967, he teaches about 30 hours a month in his Pitts S–2A.

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