By Lisa Turner, EAA Lifetime 509911
This piece originally ran in Lisa’s Airworthy column in the October 2020 issue of EAA Sport Aviation magazine.
How was it possible that I was still VFR but felt as if I was in a small dark room that was getting smaller? I alternated looking out into the mist for obstacles and examining the chart for towers and obstructions. As I was focused on the chart for one brief moment, I thought I heard the engine miss.
My mind is playing tricks on me.
I listened. There it was again — a faint blip in the beat of the engine. Was I imagining it?
Yes. You’re nervous. You’re imagining it.
I scanned the engine monitor and the gauges. Normal.
Then I heard it again. The engine missed a beat. It really was a new sound.
I took off my headset to listen. The wind was loud through the vents as I listened. I heard the frequency dip and then return. I put the headset back on.
I did hear something.
Without warning, the engine went full rough. It sounded like someone had removed half of the spark plugs. It was on the verge of completely stopping.
I started the emergency checklist that I’d committed to memory. I switched fuel tanks and flipped on the fuel pump. Before I could make an emergency radio call, the engine completely smoothed out, and even the little blip in frequencies that had started my panic was gone.
As student pilots our senses are overloaded. We’re trying to hear the instructor; we’re trying to see outside the airplane; we’re feeling the aircraft controls; we’re smelling combinations of combustion, sweat, and upholstery; our stomachs may be upset; our memories are trying to recall what to do next; and all of this is mixed with both exhilaration and some measure of fear. It’s amazing that we are able to learn and integrate experiences through all of this cacophony.
Our five senses form a composite of sensation. Most of the time we don’t even think about it. As our brain evaluates the nuances of each input for signs of danger, we are thinking of other things until the brain decides there is something to worry about and it elevates that signal to our conscious awareness.
(The problem in the story above — fuel starvation — was caused by a clogged fuel filter on one tank.)
Our sense of hearing and our ability to learn from sounds alone is amazing. Experienced pilots and mechanics almost have a sixth sense when it comes to sounds. I have seen these “airplane whisperers” identify engine make and model when the airplane flies overhead. To them, it’s no big deal.
There are dozens of sounds that can indicate problems. Over time, we can learn more about what to listen for that might indicate trouble. I’ve picked out my own top eight.
1. Engine sounds: clicking, missing, and rough running sounds when the engine is cold that may go away when it’s warm
What it is: The most serious is sticking valves, especially an exhaust valve stuck closed. The cause can be a buildup of deposits and/or corrosion. Normal combustion produces temperatures of more than 3,500 degrees Fahrenheit, a punishing heat even for components designed to take it. Heat is absorbed by the valve face, through the seat, and to the cylinder head. If the valve sticks shut, the gases that are supposed to exit the chamber get stuck, which prevents the intake valve from opening. The result is even more heat and engine damage — from pushrods to rocker arms to the valves themselves. A stuck valve may cause catastrophic failure of the rocker support, pushrod, cam follower, or cam lobe. An exhaust valve that sticks open is more common but isn’t much better in terms of results. If not fixed, any valve that is stuck open or shut will cause something to break.
What to do about it: Don’t allow yourself to be fooled if the noises go away upon warmup. Sticking valves may free up temporarily but don’t get better over time, so this is something to pay attention to and fix as soon as possible. This doesn’t mean after your flight — if you hear these sounds on startup, shut down and find out what’s going on.
The most wished-for outcome is to find a sticky valve stem and guide before damage is done. Then, review your maintenance schedule. If you’re up to it yourself, you can repair a stuck valve (under the watchful eye of your A&P mechanic) by following the procedure the engine manufacturer provides, such as Lycoming Service Instruction 1425.
2. Whistling or hissing sound; frequency changes; erratic idle
What it is: Most likely you are hearing the sound of an induction air leak. In normal operation, your aircraft engine needs a precise balance of fuel, air, compression, and a well-timed spark to produce efficient combustion. Any problem with this mix will affect performance. Induction air leaks are not uncommon. They will do the least amount of damage if caught and fixed promptly.
What to do about it: Think seals. When was the last time you tried to suck the air out of a zip-lock bag? If there’s a hole in the bag, you won’t be able to pull any suction at all. The most likely culprits in your airplane will be gaskets, hoses, flex elbows, clamps, head to intake tube joints, and even alternate air doors not closing all the way.
Just as you can use a soapy water solution to find air leaks in tires and tubes, you can methodically pressurize cylinder air on the intake stroke and check for leaks. The intake valve should be open with the piston at the bottom of its travel. Don’t try this unless you already know how to use a compression tester and you have an informed helper. The prop can escape your grasp and do serious harm.
3. Loud explosion noise, sometimes called a backfire
What it is: An “explosion” happens when unburned fuel ignites inside the intake or exhaust manifold instead of a cylinder where it is supposed to be exploding in a controlled burn. Or you can have a situation where deposits cause hot spots inside the cylinder and the combustion begins before the plug fires.
What to do about it: Uncontrolled burns and sparking outside of the cylinder can be difficult to diagnose. Even when we know that combustion is happening in the wrong place at the wrong time, there can be many contributing factors. Induction and exhaust leaks, incorrect timing, a faulty fuel injector, carbon deposits, sticky valves, and fuel pressure problems can all contribute to the condition.
See your A&P and get this solved quickly. Every misfire that you hear is doing damage to the engine and to associated engine components.
4. Thumping or that dreaded phalump-phalump noise
What it is: This is the noise that tires make when they are either flat, almost flat, or have a flat spot or big bulge.
What to do about it: A problem with a tire on our airplane is serious. A flat tire on landing can send us off into the weeds, into a ground loop, or into the expensive landing lights at the side of the runway. With loss of control, we are along for the ride, and most certainly we will end up paying for something expensive, whether it’s ours or someone else’s.
Get in the habit of taking a thorough look at your tires before leaving on a flight. If you hear this sound when you’re leaving on a flight, don’t leave until you’ve checked it out. If you hear it upon your return, maintain directional control if you can and get stopped to check it out. Depending on the sounds, you might be able to taxi to the ramp.
5. Squealing, screeching, or scraping when you brake
What it is: Brake pad/disc wear, warped rotors/drums, or a rock stuck between the rotor and the backing plate.
What to do about it: Since many of us fly airplanes with wheelpants, we might not be inspecting rotors and pads as often as we should. Like tire neglect, it’s easy to skip what we don’t see under a wheelpant. When you first start taxiing, brakes may squeak as you remove the rust that formed when the airplane was sitting, but then you should not be hearing any squealing or scraping. If you do, return to the ramp and take a detailed look.
6. Radio static or intermittent sound
What it is: Most often this is a faulty ground wire or wiring installation problem. It will cause radio noise and sometimes intermittent operation.
What to do about it: Trace the ground wire. Is there damage? Are connections tight? Do you have multiple grounds running to the radios? If so, change the signal path so you have just one path to ground. Are connectors properly installed? Check for loose terminals, broken or frayed wires, and damaged shielding. If you still haven’t found it, check the operation of the electrical systems (lights, strobes, fuel pump, etc.), one at a time, listening for noise, before going to the shop. Do this with each radio on and with the engine running, then again with the engine shut down. This might help you locate the fault.
What it is: This is the sound of something coming loose, something that came loose, or something about to come loose. Most often this is baggage shifting, parts coming off, or the dreaded, “Oops, I forgot to remove the tow bar.”
What to do about it: Check baggage tiedowns before leaving, use your hands to feel anything that acts or looks loose, and complete your entire preflight exterior checklist before getting into the airplane. Sliding cargo can be fatal if it drops to the tail and the airplane becomes uncontrollable. Balance your excitement to get in the air with a rational and thorough review of weight and balance and look for foreign objects outside the airplane that could produce a surprise.
8. Animal noises
What it is: No, these aren’t YOUR guttural utterances inside the airplane; these are unexpected barks, meows, and growls. Uh-oh! How did the cat get in there?
What to do about it: We routinely hear stories about the pet that jumped in when we weren’t looking and then surprised us en route. In that moment of surprise, try to control the airplane. If the squirrel, bird, cat, or dog is stuck in the wing cavity or baggage area and you’re by yourself, there isn’t much you can do except hope for the best and land somewhere. If it’s not causing a weight and balance problem and the animal is relatively calm, then you can decide whether to continue on or turn back.
Eight years ago, a small gray and white kitten adopted us. We named her Wrench since she was a constant companion in the shop. Two years ago, we restored a Piper Cub. Wrench loved sleeping in the back seat of the Cub. When the airplane was ready to go to its new owner, we flew it on a test flight and then placed it in the big FBO hangar at the other end of the airport.
The next day I asked, “Where’s Wrench?”
“I have no idea,” the mechanic, Adam, said. “I haven’t seen her since yesterday.”
My jaw dropped open, and I suddenly thought of all the bad things that could have befallen the cute little cat.
“Wait! Maybe she left in the Cub!” Jerry said.
We were both thinking the same thing as we jumped in the golf cart. The three-minute ride to the other end of the field felt like an hour. The Cub owner had just pulled up to the hangar.
“Wait a minute! I think we left something in the Cub,” we said as we ran over to the airplane parked in the corner. All was quiet.
“Wrench!” I shouted.
The FBO manager and the customer gave us odd looks.
We heard a muffled “meow” from the back of the fuselage.
“Wrench!” we exclaimed again with an immense sigh of relief.
Ignoring the continuing strange looks, we reached inside the airplane as Wrench tentatively came forward to be embraced and removed.
The two men watching shook their heads and then smiled.
The key to not hearing any “bad” noises consists of thorough inspection practices, thorough preflights, and attentive monitoring of engine data in-flight and post-flight. In particular, engine health can be directly observed with your engine monitor and keeping an attentive ear to how it’s running. Pilots and mechanics with lots of experience know all of these sounds by heart. What they don’t hear is the sound of my heart racing as I lift off on another wondrous flight.
Lisa Turner, EAA Lifetime 509911, is a manufacturing engineer, A&P, EAA technical counselor and flight advisor, and former DAR. She built and flew a Pulsar XP and Kolb Mark III, and is researching her next homebuilt project. Lisa’s third book, Dream Take Flight, details her Pulsar flying adventures and life lessons. Write Lisa at Lisa@DreamTakeFlight.com and learn more at www.DreamTakeFlight.com. For more from Lisa, read her Airworthy column each month in EAA Sport Aviation.