The Ultimate Preflight

Not everything that goes wrong with an airplane is catastrophic, but who wants anything to be wrong with their airplane? Here are some examples of glitches found in our personal airplanes during preflights over the years: a dozen or so broken exhaust stacks, a broken (not cracked, but broken) motor mount, one of the main through-bolts holding the engine case together was lying in the bottom of the cowl, a cracked oil cooler, leaking brake calipers, a cracked scissor link on the nosegear, etc.

Realistically, there are limits to what we can actually inspect during a preflight because we don't have access to the entire airplane without pulling out panels. In many cases, we can't even do a thorough inspection of the engine compartment because the cowling only has an oil door. Still, we should avail ourselves of what access there is and do our best to make sure there isn't a gremlin hiding in a corner somewhere that's just waiting to bite us in our personal empennage. So, if we can't pull out panels, just what can we check on a normal walkaround that's often overlooked?

Most POHs include a little walkaround map that usually starts with checking the oil on the right side of the engine cowl. We'll start in the same place and slowly work our way around the airplane back to where we started. Before we start, however, we'll assemble a paltry toolkit that includes a screwdriver/fuel tester, a clean rag and a flashlight. At least the first preflight each day should be done with a flashlight. Ideally, a flashlight should be used on all preflights.

Checking Oil
When loosening the dipstick, see if there's any "give" in the oil fill tube. If it's loose where it screws into the engine case, it'll start leaking there.

How does the oil look? Clean? Dirty? Dead black? Maybe it's way past the time to change it. And what's the level? Look in the POH for your engine and stay well above the minimum oil level indicated. If the oil is at a minimum or running low, that's because it's either leaking or burning. So, if you take off with it low, it's going to be even lower (or gone) later in the flight. Fill it up above minimum!

Engine Compartment
If the oil door is the only aperture in the cowling, chances are you're going to need a short stool because you can't see down into most of the smaller oil doors while standing on the ramp. Get up and shine your little light around in there. What you're searching for is anything that looks loose, along with indications of oil leaks.

Oil is supposed to be inside the engine and the fact that some of it isn't means that somewhere there's an open fault in the engine. The presence of oil probably just means a gasket or fitting is leaking, but tracking it down can also lead to the discovery of a crack.

What you're actually looking for is a change, and this concept applies to the entire preflight. If the engine is always dry and suddenly it's wet, something has changed and you need to find out what it is. Some engines are leakers, so a little oil isn't a change and you fly. A lot of oil is a change and you don't fly.

If you have more access to the engine compartment or even if you're limited to peeking through those ridiculous little hatches, you'll want to do at least the following steps:
• Check the cylinders around the bases, looking for oil leaks.
• Check around rocker covers, looking for leaks.
• Check the bottom of cylinder heads, looking for exhaust smudges that indicate exhaust cracks, bad gaskets or possible head cracks.
• Check the inside surface of the cowl itself for oil or exhaust smudges. Their location on the cowl gives an indication of where the problem is. If there are new smudges, find out where they came from.
• If you can, and the engine isn't hot, grab each spark plug to see if there is any looseness in either the ignition lead or the plug itself.
• Scan as much of the motor mount as possible, looking for cracks, dings or burned places.
• Check mufflers and heater muffs for smudges made by leaking cracks.
• If possible, trace the throttle cable from where it leaves the firewall to the carburetor or fuel control unit, and look for wear and tear, or loose connections.
• Trace each oil line and fuel line, looking for chafing or leaks.
• Grab the alternator belt and check for tension.
• Look for chafe marks, especially on hoses against metal or metal against hoses.
• Look in cowl air intakes for baffle cracks, leaks and foreign objects.

Propeller
Grab the tip and work it back and forth, looking for looseness. Run your fingernails up the leading and trailing edges. Your nails will catch in scratches, dings and cracks that your fingertips won't feel. Check the spinner mount screws to see if they're loose or if cracks are radiating from the holes.

Nosegear
Look for proper strut extension and signs of oleo leaking, and get down and really eyeball the scissors link. If it fails, the strut will rotate and fail, and a simple loose bolt or crack has now turned into an engine and prop overhaul. Rock the nose and see if the nosewheel shows any movement on the axle. Examine the steering linkage for any signs that it has been working against adjacent parts.

Main Gear Leg
Check for wrinkles in the fuselage skin where the main gear leg goes into the fuselage. Also, check the belly for wrinkles and, while you're down there, look for excessive oil (a little is okay---show me an airplane with a clean belly and I'll show you an airplane that doesn't fly much).

Wheel/Brake
Check the wheel pant for movement and run your hand along the bottom of the brake caliper, looking for hydraulic fluid. Even a little indicates a leak and it doesn't take much of a leak for a brake to go away on you. If there's fluid on the ground, don't go flying that day. Rock the airplane to see if the wheel is moving on the axle at all, indicating a worn or improperly adjusted axle nut.

Wing Strut
Look for signs of rust, corrosion or any indications that any of the bolts or rivets have been working. Especially look for nicks or dings in the struts where they've been hit by something.

Wing
Eyeball the leading edge for damage and continue down around the wingtip, looking for the same thing. Gently push the wingtip, looking for movement. One of the factors we're constantly looking for everywhere on the airplane is relative motion between two parts where there isn't supposed to be any. Examine nav-light lenses for security.

Aileron
Push the aileron full up so you can see the hinges. Examine where they attach to both the wing and aileron, looking for corrosion/rust, signs they've been working and/or cracks. Put the palm of your hand against the bottom of the leading edge of the aileron at each hinge and push up. You're looking for vertical movement, which indicates the hinges are loose on the wing or aileron spar. If it has external counterweights, e.g., a Cherokee, check the mount points inside the outboard end of the aileron for cracks. Repeat the same check for the flaps. When the flaps are up and locked, jiggle them and see if they're truly locked. Most modern airplanes are. Some older ones aren't supposed to be.

Fuel Level, Fuel Tanks
Everyone should have graduated fuel sticks for their tanks, which are readily available for most popular models of airplanes. If you can't feel the fuel with your fingertip, it's critical that you have an accurate way of measuring exactly how much fuel there actually is. It's a lousy place to be guessing and don't trust the fuel gauges, which are the most frequently inaccurate part of an airplane (that is, if you don't count the pilot claiming how fast his or her airplane is).

Also look for fuel stains under the wing around the tank. These are sure signs of leaks. At the same time, look at the tank vents to make sure insects aren't building hotels in them.

Horizontal Tail
If it's a stabilator, peek down inside the fuselage and/or tail cone and try to see the pivot point. Again, you're looking for cracks, rust or indications that it's been wearing against something. Move the surface up and down at the tip to see if there's any sign that it's moving independent of the fuselage.

On a normal, two-piece tail, rock the stabilizer to see that it has no movement and push the elevator down so you have a clear view of the hinges. Check them the same way you did the ailerons: Push up on the elevator spar and see if there's any vertical movement relative to the stabilizer.

If there are any wires or struts for the tail, check them carefully for nicks and chips, and examine the ends, both the top and the bottom, for signs of rust or movement.

Trim Tabs
Trim-tab hinges and actuating mechanisms seem to wear more than most parts of an airplane, and a loose or broken trim tab can cause flutter. Flutter is serious stuff and can tear the tail clean off an airplane, which isn't good. Just wiggle the trim tab. It's okay if it has a little movement, but it shouldn't have much.

Tail Cone
Check to see the tail cone is securely attached and that the position light is tight in its mount. Look down inside for birds' nests or anything else that looks as if it shouldn't be there. Also, use the flashlight to examine the elevator pushrod bolts. All bolts in an airplane should show no less than three and no more than five threads.

Tailwheel
Grab the top of the vertical stabilizer and rock the airplane back and forth while you watch the tailwheel. You're looking for relative movement between the fuselage and the spring, between the spring and the tailwheel, and between the tailwheel and the wheel itself. Listen for clicking, which will indicate something is moving.

Pitot System
Bugs are the pitot's biggest enemy, with dings being next. Some pitot tubes are mounted in such a way that fuelers with hoses have to work hard to avoid them, so watch for indications they've been hit or bent. If you change the angle of the pitot tube, you also change the airspeed indicator. And don't forget the static ports that may be located somewhere on the aft fuselage. Keep them clean.

Don’t Take Any Chances
From this point on, it becomes a pre-start, pre-takeoff checklist that requires a vigilance all its own. The bottom line on a preflight, however, is to get to know your airplane intimately, which includes the little dark places you don't normally look into.

The assumption many pilots hide behind is that the airplane has always worked in the past, so what are the odds of something happening on this particular flight? The exact opposite point of view should prevail---because everything wears out eventually, make the assumption that the last flight the airplane was on was when something important broke. Healthy pessimism has its place and one of those places is every time you prepare to go flying.