Snakes in the Preflight

By Robert Rossier, EAA 472091

This piece originally ran in Robert’s Stick and Rudder column in the March 2021 issue of EAA Sport Aviation magazine.

Sometimes during the preflight, we’re really taken by surprise. Just imagine what went through my mind one dewy spring morning when I suddenly spotted the snake sitting atop the engine cowl. It wasn’t a big snake, just a small garter snake, seemingly content to rest on the cool, damp aluminum skin.

Of course, it wasn’t an actual snake, but rather a fake snake put there to help deter birds from entering the engine cowl. But still, it startled me, and it got me thinking. What other surprises might be in store as I followed my early morning ritual to ensure the airplane was safe for flight? I removed the cowl plugs and, after a thorough examination, was convinced no birds had taken up residence in the engine compartment. So where would the surprise come? What might be the real snakes in the preflight?

Performing the airplane preflight inspection is one of the first skills we learn when starting our flight training. Most pilot’s operating handbooks tell us what to check and what to look for, but our instructor or a good hangar flying session will often shed a bit more light on what can and does go wrong — either in general or with a particular make and model aircraft. In case we might have missed something along the way, here are just a few snakes in the grass to be wary of on our next preflight inspection.

Fuel Contamination

This is one issue we want to uncover before taking to the air. This can crop up especially following extended or heavy rains. A worn gasket or improperly seated gas cap can allow water to seep in and contaminate our fuel supply. When we draw that first sample, we want to look carefully for the proper color since the entire sample might be water. Particularly for aircraft with bladder tanks, we might want to rock the wings a bit in case condensation has accumulated in wrinkles in the bladder. We don’t want to stop checking until we get at least one good sample from a tank.

Fuel stains usually mean that fuel is or was leaking. We might wonder where that fuel ends up, but the answer is always “not in the engine.” Such stains could be an indicator of a developing and potentially serious problem. The solution could be as simple as replacing a worn gasket, but it might be more serious. It’s better to dig into this one than get the full surprise sometime in flight.

Missing Fasteners

A missing fastener is something we shouldn’t fool around with. Sure, aircraft are all designed with a factor of safety, so a single fastener on an engine cowl for example should not result in a failure. But how about two or three? What if they’re close together? Does it matter where they are? Missing fasteners can lead to an engine cowl separating from the aircraft in flight. That causes problems with engine cooling since the air isn’t channeled properly over all the cylinders. In one tragic case, an engine cowl separated in flight, wrapped around the horizontal stabilizer and elevator, and caused a loss of aircraft control. The result was an ugly, fatal crash.

Seat Rails

Problems with seat rails have been prevalent in some aircraft, allowing the seat to slide aft at the most inopportune time. Sometimes it’s a cracked or broken rail or latch mechanism. Sometimes it’s not ensuring the seat is properly latched. The way this has played out in fatal accidents is this: Imagine being on initial climb-out, when the nose is already pitched up, and the seat unexpectedly slides back. Maybe we have one hand on the yoke and the other on the throttle. Instinctively we hold on, and that causes us to bring the throttle to idle and pull the nose way up. The result is a low-altitude stall/spin scenario from which it is nearly impossible to recover.

Cracks and Wrinkles in the Skin

This can be a sign of the airframe being overstressed. Once this occurs, the airframe is compromised and may not provide the design factor of safety. It could be that something failed, or it could be that a structural failure is in the future. Have a certified airframe mechanic take a close look to find the root problem.

Thin Brake Pads

Here’s another sign of potential trouble. They can be hard to see for some aircraft with wheel fairings, and you might need a flashlight to peer in and see what’s what. But it’s worth a check. Typically, the brake pad on a caliper brake should be at minimum the thickness of a quarter. Any thinner, and the rivet head holding the pad wears on the rotor, which means gouging, more expensive maintenance, or disaster if the brakes don’t work properly when needed.

Rust or Corrosion

These come in a variety of forms and are a plague for aging aircraft. Look for rust particularly around fittings, fasteners, and antennas. It isn’t unusual for radio problems to stem from corrosion issues. Blistering paint can be a sign of a growing corrosion problem occurring beneath the paint. The sooner we find and treat rust problems, the less damage they cause, and the less likely we are to encounter expensive repairs and replacements.

Oil Leaks

These are not that uncommon, especially among some older aircraft. We might hear someone brush it off saying, “Oh, they’re all like that,” but don’t fall into the trap of thinking that such leakage is okay or normal. That’s why it’s a good idea to always keep track of oil consumption for an engine. Sometimes what appears to be a minor leak can be foreshadowing of a developing crack in the engine and upcoming catastrophic engine failure.

Not all leaks are from the engine. Hydraulic fluid, which is reddish in color, can come from the brake system or hydraulic landing gear. In either case, it should not be overlooked. Potential gear or brake failure is nothing to mess with.

Nicks in the Propeller

This is something we should always check for and should be taken seriously. Nicks can cause stress risers, which can ultimately lead to failure of the blade. Take a few inches off the end of a prop and the resulting imbalance and vibration can yank the engine right out of the airframe. Guess what that does to your center of gravity. Nicks should be dressed to prevent the stresses they cause.

Loose and Worn Belts

Problems like these can be a precursor to failure, which typically leaves us without an alternator. The resulting electrical system failure could mean no lights, no radio, no nav equipment, and so on. Here again, it’s better to check under the cowl to find and resolve such a problem on the ground rather than deal with an in-flight failure.

And that brings us back to birds’ nests. Particularly in the spring, when birds are nesting, it isn’t uncommon to find that some unsuspecting feathered friend has taken up residence in the presumed comfort and safety of the engine compartment.

On this day, it seemed the plastic snake had done its job as there were no birds’ nests in the engine compartment. However, I also checked in the tail cone, since some birds take up residence there as well. You just never know where trouble might be hiding.

Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.

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