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.
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