By Lisa Turner, EAA Lifetime 509911
This piece originally ran in Lisa’s Airworthy column in the March 2021 issue of EAA Sport Aviation magazine.
“Your 170 sure has been in the shop awhile, Bert. When are you getting it back?”
“Funny you should mention that. I’m picking it up tomorrow. It will be terrific to get back in the air.”
“An annual is the inspection that takes a year to do, right?” David said, laughing.
“I guess so. With the hours on it, they are replacing a lot of components, including the exhaust. There was too much corrosion to repair it.”
The next day Bert showed up at the aircraft repair shop. His beloved Cessna 170 sat outside, looking shiny in bright white with a red stripe. He felt a pang of excitement as he approached the aircraft. It had been in the shop for a month, and he was anxious to get in the air. The day was clear and calm.
“Everything should be good to go,” Ted, the shop manager, said.
“Did you take it for a flight?”
“No, we thought we’d let you do that. The ground run was okay.”
“Nothing fell off, huh?”
“No,” Ted said with a laugh.
Bert took his time on the preflight. He knew that an annual could introduce problems, between new components and installation mistakes on reassembly and rigging. Everything checked out.
After starting the engine, he listened carefully. The sound was a little different than what he was used to.
Aha. The new exhaust.
On the run-up, Bert felt like something wasn’t quite right, as if the engine was not making full power.
It should be even better after the annual. It must be me. It’s been a while since I’ve flown it.
Bert took off on Runway 25. The tail came up, and the airplane traveled slowly down the tarmac.
It feels slow. Should I abort or take off?
In the midst of Bert agonizing over what to do, the airplane lifted off and labored into the air.
“The engine isn’t making full power,” Bert said out loud to himself.
I don’t like this.
Bert checked controls and gauges. Tanks full. Everything looked fine. He put the nose down a little to get more airspeed. The O-300 Continental sounded steady. But something still wasn’t right. Bert’s euphoria from getting into the air after more than a month evaporated and was quickly replaced with concern.
After a long, slow climb to pattern altitude, Bert nursed the aircraft around for a return to the airport. He was perplexed and worried.
He landed and taxied over to Ted’s shop.
“That was fast,” Ted said, coming out of the open hangar.
“Something is wrong, Ted. I can’t get full power.”
Ted frowned. “Okay, let me flight test it.”
Bert returned to the shop later in the day to see the mechanics poring over the aircraft. He thought they would find a simple problem.
“Well, gee, when I brought it to you, it was running fine! What did you do to it?”
In embarrassment, the mechanics acted like they hadn’t heard him.
“Aha!” Sarah said. “A blockage in the exhaust! Look.”
She pointed to the borescope screen. They could see the interior of a pipe at a welded intersection. It wasn’t fully cut out; there was a partial, small hole.
“Let me check the others,” she said.
“Well, that sure would do it,” Ted said.
After an inspection of the rest of the interior of the exhaust piping, they found that four of the six had never been fully cut out at the weld intersections.
“That’s the oddest thing I’ve ever seen,” Sarah said. “It’s a brand-new exhaust set.”
“Indeed, really strange,” Ted said. “No wonder the airplane couldn’t make full power. We’ll replace this as soon as we can. I’m sorry, Bert.”
Bert gave a sigh of relief.
The exhaust system represents the lungs of an airplane, breathing in and breathing out. Our airplanes have to maintain a balance of air in and air out to maximize performance. If one or the other is degraded, we’ll see that it affects both. For this discussion, let’s assume that the intake side is okay. Gaskets and filters are fine, and there are no leaks.
Overlooking problems in the exhaust system can result in performance degradation (to the point of having the engine quit or not start), fire (catastrophic in an aircraft), and carbon monoxide poisoning, which can be fatal. One short piece of reading that I consider informative for all aircraft owners is FAA Advisory Circular 91-59A, Inspection and Care of General Aviation Aircraft Exhaust Systems. I’ve provided the link at the end of this article.
Exhaust gas temperatures reach 1,500 degrees Fahrenheit and higher. Fifty percent or more of the energy unleashed by a traditional internal combustion engine is wasted as heat. These really high temperatures are not constant but cyclic, one reason that exhaust gas temperature (EGT) readings on a cockpit gauge can be misleading all by themselves (see sidebar).
Here are the top four issues I’ve found involving exhaust systems on small aircraft, how to spot them, and what to do if you find a problem.
Installation – Amateur-Built Aircraft
Because of the variation in aircraft from builder to builder, I find a large assortment of problems having to do with the assembly and installation of the exhaust system on homebuilts. They all revolve around two things: not following instructions and a lack of understanding of how the system works. The good news for homebuilts is that the insidious corrosion of older GA aircraft is rarely present, simply because of the relative newness of most homebuilt aircraft. With that said, if your homebuilt has considerable years and hours on it, take a look at the section on leaks and corrosion later in the article.
What we find.
Loose links and flanges, misalignments, over and under torqued nuts at the cylinders, missing lock washers, missing springs, wrong nut types, and aircraft structure and components (wires, fuel lines) too close to the exhaust. We also see problems with the heater system that open the occupants up to carbon monoxide poisoning in flight if the exhaust gases leak into the cockpit (see Heat Systems below).
What to do.
Pull out your instruction manuals, including the engine manual. Don’t have them? Bought the airplane secondhand? See if you can get a copy from the manufacturer or from another builder to copy. Put together an inspection checklist if you don’t already have one.
On your inspection, pay extra attention to areas around the exhaust structure. Inspect joints to see if they are excessively loose. Inspect clamps and make sure the hardware you see matches what the instructions say should be there. Look carefully at the gasket area where the exhaust meets the cylinders. Nuts should be properly torqued and have the correct hardware. Look at the plans, diagrams, and instructions to make sure everything is what and where it should be. Have a local A&P mechanic take a look, too, and round out your inspection checklist with what was found.
Route wires and oil and fuel lines away from exhaust components with a 1.5-inch or more clearance. Check the engine cowl interior to exhaust distance with a couple of layers of aluminum tape stuck straight up from various components to see how much the installed cowling scrunches them down (technical term) to give you an idea of clearance. You can also use aluminum tape on the interior of the cowling in areas where you think the exhaust might be a little close. Be careful. Once high heat damages fiberglass, it can be a tough repair. Repeated similar inspections once you get flying are critical to catching problems early.
Note: It’s okay to put a fire sleeve on the fuel line where it travels near heated components in the engine compartment, but don’t place heat tape (wrap) on the exhaust unless the manufacturer specifically calls for it. It robs the exhaust of its designed heat sink ability and can shorten its life.
In light aircraft, two of the most confounding problems are intake leaks and exhaust leaks. Symptoms may present in similar ways because the in and out of air, fuel, and spent gases have to be in balance. Since we’re talking about exhausts here, let’s assume that you have your intake side tight and leak-free.
What we find.
Most small aircraft exhaust systems are made from mild steel (low carbon steel), stainless steel, or Inconel (a nickel-chromium alloy). The more expensive and highly durable Inconel is used more in turbocharged applications because it can better handle stress and heat. However, the downside is that Inconel deteriorates from the inside out (think pinholes), is harder to inspect, and is expensive. Our smaller, simpler airplanes will typically have mild steel or stainless-steel exhaust systems.
We find leaks most often at the cylinder exhaust flange gaskets, at the outside radius of bends in the pipe, at misaligned joints, at loose connections, and at clamps and welds.
What to do.
Do a thorough visual examination using an inspection checklist and a high-powered flashlight or shop light. Note any areas that are discolored. Any discoloration from the adjacent “normal” areas is suspect and can include green, blue, orange, brown, sooty/black blotches, or streaking. General wear will show a bluish-black appearance and surface bumps and scars. Corrosion will look characteristically rust-colored and may be flaky.
Use your hands to grip (no, not right after flying!) the sections and lightly apply pressure to identify anything that seems loose or rattles, and to check the relative tightness of links and joints. Slip joints are supposed to allow some movement to protect the rigid sections from cracking under vibration.
Take an awl and tap on suspect areas and any bulges or deformations with the blunt end. On areas that look damaged, use the sharp end to lightly poke the areas. You may discover that it breaks through in areas that are discolored or rusty. Better to find it now than in flight. Listen for changes in sounds as you tap different areas.
Leaks aren’t obvious? Make sure with a leak test. You’ll need a shop vacuum, some clean rags, and a sprayer with soapy water. Clean the inside of the vacuum canister (for the first time ever, right?) because you don’t want to inject debris into your exhaust system. Attach the vacuum hose to the outlet port of the vacuum. On dual aircraft exhausts, plug one outlet. Use the rags to plug other areas you have open. Turn on the vacuum and spray the soapy water liberally where you think you have a leak. Bubbles will be the result. Use a camera or your phone to get close-ups so you can get attention to the right spot later.
Although this is an easy task that can be done in a short time, you need to be careful not to over pressurize the exhaust system — 3-5 psi is best to prevent damage to systems. In other words, don’t use a high-powered air source.
These are the things that we least expect. The story I began with is true, and very rare. We’re usually dealing with leaks rather than restrictions. However, when we do have a restriction, it can be serious.
What we find.
Nests, broken baffles that have blocked the outlet, and other odd things. In many cases, the aircraft cannot be started or the engine dies quickly after starting, leaving us scratching our heads.
What to do.
If you suspect you have a restriction, use a rubber mallet or the end of a screwdriver to tap the sections, particularly the areas near the outlets. The sound will be distinctly different once you’ve found something — more like a dull thud than the brighter, metallic sounds from tapping the other open areas.
Obvious plugs can be straightforward, such as nests and broken internal parts, but others may not be easily visible. In this case, a borescope is a wonderful tool to have. A plumber’s drain snake can also be helpful. I’ve seen nests that were not visible and other odd items that made me think a gremlin had been playing in the hangar, like toy trucks and stuffed animals.
The most serious leak you can have in your airplane is one where exhaust air in any amount finds its way into the cockpit. Carbon monoxide (CO) is produced in abundance in our small, piston-powered airplanes. Odorless and colorless, CO gas will silently incapacitate you with potentially fatal results.
Long ago, inventors cleverly designed systems to scavenge heat from the engine as an efficient and inexpensive way to provide carburetor warming and pilot and passenger comfort. Air is drawn into areas away from exhaust components and run through a shroud surrounding the hot exhaust piping. In some cases, with more simple designs, the air is simply drawn from an area of high heat in the engine compartment. This works well until a leak develops in the exhaust.
What we find.
Heat exchangers and shrouds are tough to inspect on your preflight because other components are in the way. However, we’ve found that shroud and exchanger conditions often follow the rest of the system. If you’re seeing deterioration in the visible sections, then it’s also likely in the hidden sections.
What to do.
Even if your preflight is thorough, it may be difficult to identify a leak in the heat exchanger. Let the condition of the rest of the system be a guide. At your condition or annual inspection, you should disassemble the hidden areas to identify damage and leaks. If you identify any problems in the visible sections before that, a detailed inspection of the exchanger is in order.
Because CO poisoning is often fatal, I’d skip the stick-on chemical CO detectors and invest in a good aviation electronic model with a warning system. It will set you back a few hundred dollars, but it’s well worth the price for saving lives and peace of mind.
Just as the other systems on your aircraft are specialized in function and fit, so is the exhaust system. I don’t recommend nonaviation people doing welding repairs on the system if you identify a crack or leak. The reason is that we’ve seen more rapid failures in repair areas and damage to adjacent materials. It’s a better idea — albeit more expensive — to remove the system and send it out to a shop specializing in exhausts if you find multiple breaches.
For some reason, the entire exhaust system often gets overlooked on most preflights. Psychologically, we are looking for things that have changed since the last time we flew. We look for things like water in the fuel, nicks in the propeller, and a lack of air in the tires, but we don’t think that the steel on the exhaust is going to change flight to flight. And it oftentimes doesn’t look like it’s deteriorating because we can’t see the changes from flight to flight. Just like the surprise of our child suddenly becoming taller than we are, the system can surprise us with problems. When we look carefully, it’s right out in the open.
Make sure your exhaust system is on your checklist in a detailed way. As I have mentioned before, not as “check exhaust,” but as “check for discoloration around cylinder gaskets,” “look for heat damage on components near exhaust,” and “look for cracks, bulges, deformation, and color changes on all visible exhaust components.”
Yes, the list gets longer and longer. But the chances for misfortune become fewer and fewer.
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 DreamTakeFlight.com.