Bill Ross
Magneto Maintenance 101

Magneto Maintenance 101

Understanding magnetos and manufacturer recommendations for maintenance can help ensure your safety. 

The following is an excerpt from Bill Ross’ book “Engine Management 101.” Published by Superior Air Parts Inc., this book is a compilation of what Bill has learned during his 35-plus years of experience as a pilot, aircraft owner, piston aircraft engine industry leader and FAA A&P/IA.

Although the basic magneto has provided very reliable service to aircraft operators for over 100 years, it’s still a very misunderstood part of your engine. For example, while many owner-pilots think it needs the aircraft’s battery to operate, the fact is your magneto is a self-contained unit capable of full functionality that is independent of the aircraft’s electrical system. 

There is no battery power required to start or sustain an aircraft magneto operation. Oh, there are a few that would say, “Without the battery (to turn the starter), the magneto is not able to function.” 

Obviously, they did not grow up on our airport in South Alabama where hand propping was the normal procedure. Many of the aircraft that I cut my proverbial pilot’s teeth on had, and still have, no electrical system. Therefore, the only way to start the engine was the “Armstrong Method.”

The basic aircraft magneto uses a strong rotating magnet inside a coil. There is a voltage produced in this primary coil, which is then stepped up even higher in a secondary coil. This is achieved by having more windings in the secondary coil than in the primary. The increase in voltage is enough to excite the spark plugs to then spark-ignite the fuel/air mixture for combustion. 

While the magneto is very reliable and robust, it is intolerant of neglect or abuse. This article is not intended to make you an expert on magneto theory, function and maintenance but should prove helpful in understanding the periodic maintenance recommendations by the respective manufacturers. As pilots and owners, if we understand these recommendations and why they are important to safety, we can ensure our maintenance provider is correctly following them.

Understanding the mag check

The magneto check is one of the first things we learn as student pilots. Run the engine up to a specific rpm and switch to the right mag, then the left. If you see a “suitable” drop in the engine’s rpm, you’re good to go. Or are you? 

One of my first jobs as a pilot (besides flight instruction) was flying charter in various piston-powered aircraft. Oftentimes, we were paired with another pilot because we had a contract with the Army Corps of Engineers, and the rules required two pilots even if we were in a Beech Baron with a single yoke.

We had another pilot on our staff named Charlie. Charlie and I flew a lot together, and he taught me a lot about flying and engine operations. He was in the first class of flying sergeants to graduate from Columbus, Mississippi, during the onset of World War II. 

During his career, he flew B-25 bombers and then had a 34-year career as a captain with Pan Am. Anyway, early on in our flying partnership, he told me the funniest story about magneto checks. When he was in primary training and just about ready to solo the Stearman, he would watch the instructor reach up and move the ignition switch back and forth and notice a little rpm drop on the engine. 

For each and every flight, he would watch in confusion as to why the instructor went through this ritual. Was it superstition, an ode to the flying spirits, or nervousness on the part of the instructor? Whatever the reason, our friend Charlie had no idea why the Army Air Corps instructor repeated this odd ritual. 

So when it came time for him to solo, Charlie reached up and did the same pre-takeoff ignition ritual as the instructor. 

It wasn’t until after he soloed that his instructor covered magneto checks in ground school. Remember, this was a civilian/government contractor after all, and well, sometimes they do things a bit odd.

The point here is even today, with all the information available about runup mag checks, do we still get caught up in the ritual and not really look at the health of the ignition system as a whole? No doubt we all do.


Magneto internals at 500-plus hours. These magnetos checked OK in runup.
Does the mag drop really mean anything?

After many years and thousands of hours in the left seat, I’ve learned that simply having an “acceptable magneto drop” during engine runup is not necessarily a good indicator of the overall health of your magneto system. Even if the drop is only 50 rpm, there are many issues that could well be lurking inside the magneto that are just waiting to come out and cause problems. 

During my career as an A&P, I have personally performed analytical inspections on many engines that were destroyed due to improper, or an overall lack of, magneto maintenance. 

Yes, I know there are many “experts” in the General Aviation industry professing that you can learn everything about the health of your magneto by simply performing a lean of peak test while in flight. 

I also hear a lot of talk about how today’s sophisticated engine monitors are capable of detecting imminent ignition system issues. While to some degree this can be true—such as determining spark plug performance, spark plug lead and minor magneto issues—even the best engine monitor is not capable of predicting the future. 

Contrary to the opinions of many, a lean of peak magneto test or engine monitor cannot predict some of the serious issues I have witnessed with ignition systems. These include worn or fractured gear teeth, lubrication distress, cracked housings—the list goes on and on. While a magneto problem may not seem too serious, believe me, it can be disastrous. 

Magnetos need maintenance, too

There are very specific maintenance checks recommended by the manufacturer that are extremely important to both the safety and reliability of the engine and aircraft.

The problem is most aircraft owners, and some mechanics, are not aware that these recommendations even exist. For example, manufacturers of both the Bendix-style and Slick magnetos “recommend” a 500-hour inspection for their units. 

Personally, I know very few owners that actually know of, and fewer who actually comply with, this recommendation. 

“It’s only a recommendation,” they’ll say. Well, that’s true. But these are often the owners who are plagued with difficult starts, a rough-running engine and an ongoing list of other unscheduled maintenance practices. Remember, the purpose of scheduled maintenance is to prevent unscheduled maintenance.

In addition, Bendix-style magnetos have a recommended four-year-in-service or five-year-from-date-of-manufacture replacement/overhaul, whichever occurs first. 

Therefore, if a Bendix-style magneto has been on the shelf for three years and is then installed on an engine, it only has two-year useful life remaining before recommended overhaul. Again, this is a recommendation, but following it can be beneficial to ensure system reliability and flight safety. 

Why do these particular units have this type of recommendation? Magnetos that sit on the shelf can be severely damaged by corrosive attack due to inactivity. Conversely, magnetos that have been in service for a period of time can become worn due to contamination and operation. Magnetos that house excessive dirt and electrolytic debris are prime candidates to ruin your day.

Oh, they may check fine on the ground during runup, but they may well have internal issues that will show up during your flight. Some scenarios could be cross-tracking of the spark within the distributor block. If the spark reaches the incorrect pole, a preignition event can occur leading to detonation and engine failure. Cracks in the distributor drive gear can lead to fracture of the gear and stop the distribution of spark to different cylinders. 

Over the years I have performed several analytical inspections pertaining to preignition/detonation events that exhibited fractured away teeth on the distributor gear. Once the gear could no longer be driven, the distributor finger would fire only on one cylinder. This leads to a preignition/detonation event and destroying the engine. 

Even though I wish they could, engine monitors cannot predict events like these. Like with many parts of your aircraft and engine, the only way to help keep issues like these from ruining your day is to follow manufacturer’s recommended maintenance, inspection and replacement intervals.

Keeping your magneto healthy

Think about it: We routinely change oil in the engine, but we don’t change oil in our magnetos even though the manufacturer says we should. Why is that? 

During the typical 500-hour engine inspection, the maintenance provider should disassemble, clean and inspect the magneto internal parts. At the same time, the breaker points and condenser are normally replaced and the magneto is lubricated. Inspection of the gears for cracks or abnormal wear is very important to the function of the magneto and health of the engine.

In addition, the impulse coupling (if equipped) is inspected for excessive wear and correct functionality. The impulse couplings are a spring-loaded component that aids in engine starting by accelerating the rotating magnet and retarding the timing. 

Having an impulse coupling that is not functioning properly can lead to the engine firing in full advance timing and thus lead to a kickback during starting. Kickbacks can wreck your starter and severely damage engine components. 

After inspection, the magneto is reassembled, tested and installed back on the engine. Next, the magneto-to-engine timing is set in accordance with the engine manufacturer’s instructions and specifications. 

The best advice to help you get the most out of your engines and components is to simply follow the manufacturer’s recommendations for continued airworthiness.


Bill Ross is a graduate from the University of South Alabama and was employed by Continental Motors for 15 years holding positions in engineering, analytical, air safety and technical product support. Ross is now Vice President of Product Support for Superior Air Parts and committed to the company goal of making flying affordable. When not working at Superior, Ross can be often found flying his family’s 1941 Boeing Stearman, working on antique aircraft or exposing young people to the joys of flight and potential careers in aviation. Send questions or comments to .

My Engine is 50 Hours From TBO…

My Engine is 50 Hours From TBO…

When it comes to what to do when your engine reaches TBO, your choices range from doing “everything” to doing “nothing.”


The following is an excerpt from Bill Ross’ 144-page book “Engine Management 101.” Published by Superior Air Parts, Inc., “Engine Management 101” is a compilation of what Ross has learned during his 36-plus years of experience as a pilot, aircraft owner, piston aircraft engine industry leader and FAA A&P/IA.   

TBO —what does it really mean? 

Today, there are many in the industry making a case for flying the aircraft until it breaks and not necessarily adhering to manufacturer’s recommendations for continued airworthiness. To me, that’s just asking for trouble. 

How do you know when the aircraft will “break?” Will it be on short final at your home airport, or at night, in the clouds, with your family on board? Is the risk worth it to you? I certainly hope not. 

My opinion is you can safely fly past TBO without consequences if you and your maintenance provider follow the engine manufacturer’s recommendations.

The ABCs of TBOs

Let’s look at the term “time between overhauls” (TBOs). The FAA requires manufacturers to publish a TBO for each of their engines. These aren’t numbers that are pulled out of a hat. The engine manufacturers establish these recommendations based on typical maintenance and typical engine operation. 

Engines are required to have both an accumulation of actual operating time and calendar time recommendations. Mistakenly, many pilots try to “extend” their engine’s overhaul—and the cost thereof—by not flying as often as they should. The fact is, lack of consistent use is probably one of the worst things you can do to an aircraft engine. 

Aircraft engines that are sedentary for many months, and sometimes years at a time, are more likely to have internal damage than those that are maintained and flown regularly. Most engine manufacturers recommend that if the engine is going to be inactive for six months or longer, it should be preserved in accordance with their respective instructions. 


TBO: Your time has come

One of the questions I get asked most frequently about TBO is whether an owner should overhaul or replace their engine. It’s a good question, and it has more than one answer. Fact is, when your engine reaches overhaul time, you basically have six choices:  

1) Purchase a new engine.

2) Purchase a rebuilt engine.

3) Have your engine overhauled.

4) Patch the engine of leaks, address any low compressions and accessory issues. 

5) Wait too long.

6) Do nothing. (Really. That is an option.)

OPTION 1: Purchase a new engine.

When you purchase a new engine, every part is new and meets factory-new specifications with zero time, a new serial number and a factory-new warranty. 

Aside from those points, there is really no technical benefit to the new engine. It will not necessarily provide you with any more power, smoothness, better performance or longer service life than any of the other options we will discuss.  

OPTION 2: Purchase a rebuilt engine. 

Rebuilt engines are different from overhauled engines, even though people often use the terms interchangeably. The rebuilt engine is assembled at the engine’s original manufacturer using various parts from the manufacturer’s used/reclaimed stock. 

When rebuilding an engine, the manufacturer is not required to disclose the total hours (i.e., total times) on those “stock” items. Therefore, you could have a crankshaft or crankcase that has a lot of hours or several previous TBO intervals on it. 

Nevertheless, rebuilt engine components must meet factory-new fit tolerances, but specific parts can be machined undersized to meet specifications. There is nothing inherently wrong with this practice, but it could result in an unusable crankshaft at the next overhaul. 

Rebuilt engines are issued new serial numbers and granted zero-time status by the manufacturer. 

OPTION 3: Have your engine overhauled.

Historically, an engine overhaul has been the most economical option for aircraft owners. During an overhaul, your original engine is sent to a third-party overhauler, where it typically receives new cylinder assemblies, hardware, gaskets, bearings and other piece parts. 

The overhauled engine is not granted zero-time status. The engine’s total time is continued from the point of overhaul. For example, the engine may have 2,000 hours and zero time since major overhaul. The engine still has 2,000 hours total time. 

An overhauled engine, if worked up by a reputable facility, can greatly enhance the aircraft’s value. Why? Simply because it keeps the engine/airframe serial numbers as matching pairs. That means a lot to some owners.

OPTION 4: Patch engine leaks; address low compressions and accessory issues.

Patching the engine should be only considered if the internals of the engine are determined to be in good condition. That means no metal in the filter, and oil consumption is within the limits set forth by the manufacturer. 

Most aircraft engine failures that I have investigated or have performed analytical inspections after the fact, failed due to either a malfunction in the fuel delivery or ignition system. Therefore, always follow the manufacturer’s recommendations for continued airworthiness for critical items including the magnetos, alternator and the fuel delivery system. Like the engine as a whole, each of these components have specific service intervals that should be followed. 

For example, ignition systems typically have 500-hour inspections, and some have requirements for overhaul after four years in service or five years from date of manufacture. 

Remember that doing a preflight runup and magneto check at the end of the runway is not always an indicator of good ignition system health. Preventive maintenance can go a long way in providing enhanced reliability and safety.

OPTION 5: Wait too long.

One of the airplanes my father and I own had the original engine it was delivered with back in 1966. When we purchased the aircraft, it only had about 1,200 hours total time. Many of you might think, “Wow, it had lots of time left on the engine.” Which it did… 

…but we were continually repairing oil leaks, reworking accessories and worrying about becoming stranded away from home base. 

This continued for a number of years until one day I was performing an oil change. I invited my daughter to ride with me around the patch in order to warm the engine oil. We came back, landed and removed the cowling for draining the engine oil. 

When the oil began to flow, I noticed what looked like pieces of metal flowing from the drain hose. That’s not good. I pulled the oil screen and we could almost read the part numbers for the metal coming from the engine. 

I remember the look on my father’s face. It wasn’t a look of financial distress, but rather, relief. Now we had a valid excuse to overhaul the engine. The trouble here is we could have waited too long. 

Not only was it a risk to our safety to push the envelope on this engine, but also by continuing to fly it, we could have done irreparable damage to the engine that would have been more costly to repair than a standard overhaul. 

OPTION 6: Do nothing.

You read it right: the last option is to do nothing. By definition, TBO is just a “recommendation.” It’s not a law. Many engines go beyond TBO and perform very well. 

But before going that route, you need to have a thorough evaluation of the engine’s current state. Questions that I ask of owners that question me about going beyond an engine’s TBO are:

1) What is the calendar time since the engine was new or last overhauled?

2) What is the oil consumption?

3) Do you have any persistent oil leaks?

4) Have you discovered any wear material in the oil during analysis? 

5) What is the reliability of the accessories including the magnetos, alternator, carburetor or fuel injection system?

6) What price do you put on your peace of mind?

7) What price do you put on the safety of you and your passengers? 

While there are plenty more questions I could ask, these seven hit the high points when it comes to an engine’s overall health—or lack thereof.

My engine has plenty of life left!

It may well, but you should also ask yourself this question: what is the calendar time since the engine was new or last overhauled? 

Why does that matter? Remember, inactivity of an engine can be very damaging due to internal corrosion. That fact than an engine has low operational hours does not mean a thing to me.

For example, look at the ads that list aircraft for sale. I have never once seen an ad that actually stated the engine’s calendar time. You see many with 500 or 600 hours since major overhaul or since new all the time. 

What buyers should be asking is, “What is the engine’s actual calendar time?” and  “How long has the engine been sedentary?” Generally, you would like to see the aircraft flown at least a couple of times per month. 


“Oil, that gold. Texas tea...”

Another critical thing that helps determine the overhaul health of an engine is its oil consumption. Does it have any persistent oil leaks? Or have you discovered any wear material in the oil during analysis? Regular oil analysis can be a very helpful diagnostic tool.

With regard to oil consumption: are you constantly filling the oil every time you fill up with fuel? If so, your engine is likely a candidate for replacement or overhaul. Increased engine oil consumption could be the result of actual engine usage, leaks or a combination or both. 

An old engine that is beginning to show these symptoms is probably getting close to needing overhaul, replacement or significant repair. The question to ask yourself is: how long can I continue to bale the engine together?

To TBO and beyond

I am not advocating that you rigidly adhere to your engine’s recommended TBO numbers, but the manufacturer is required to provide one as a point of reference. 

Clearly, you can safely fly past TBO, provided you follow the manufacturer’s recommendations for determining continued airworthiness of the engine. Those owners and maintenance providers that follow the proper maintenance guidelines will beat others to the engine’s TBO numbers and beyond through overall lower operating costs and improved safety. 

Basically, if your engine is not producing abnormal wear metal, has good compressions, does not consume oil at an alarming rate and is maintained in accordance with manufacturer’s instructions, more times than not, you can safely fly past TBO without any issues. And there are folks who do it day in and day out. 

My point here is that you can expect long life out of your aircraft’s engine if you take care of it and do the maintenance and inspections when and how the manufacturer recommends. 


Bill Ross is a graduate from the University of South Alabama and was employed by Continental Motors for 15 years holding positions in engineering, analytical, air safety and technical product support. Bill is now Vice President of Product Support for Superior Air Parts and committed to the company goal of making flying affordable. When not working at Superior, Ross can be often found flying his family’s 1941 Boeing Stearman, working on antique aircraft or exposing young people to the joys of flight and potential careers in aviation. Send questions or comments to .

Engine Management 101: Understanding Cylinder Baffling & Keeping Your Engine Cool

Engine Management 101: Understanding Cylinder Baffling & Keeping Your Engine Cool

The following is an excerpt from Bill Ross’ new book “Engine Management 101.” Published by Superior Air Parts, Inc.“  Engine Management 101” is a compilation of what Ross has learned during his 35-plus years of experience as a pilot, aircraft owner, piston aircraft engine industry leader and A&P/IA. (To get a free copy of Ross’ book, see the sidebar on page 35. —Ed.)

 A major topic of interest for any aircraft owner should be the proper maintenance of their engine’s baffling. I’ve witnessed too many instances where an aircraft owner or their mechanic has installed a $30,000 to $90,000 engine only to use the old deteriorated baffling. Why not spend a little bit more and do all you can to protect that big investment? 

In reality, I don’t think it is so much a question of economics but rather a lack of understanding about the functions and performance of an engine’s baffling.

With fuel system calibration being number-one (on fuel-injected Continental engines), neglecting baffling condition is probably the number-two most common maintenance issue I see under the cowling. The fact is, engine baffling that is left in a state of disrepair will likely result in reduced cylinder life and other operational performance issues. 

Demystifying baffling

Engine baffling serves a crucial role in cooling the cylinders evenly and keeping the entire engine within specified temperature parameters. Small imperfections or problems in the baffling can result in ill effects to the cylinder and oil temperature. 

Some of the detrimental effects can include accelerated wear of cylinder and valve train components, glazing of cylinders, and in some cases, reducing detonation margins to dangerous levels.

Over my many years as an A&P and technical representative, I have assisted in solving cylinder head temperature (CHT) issues many times. In the majority of cases, abnormal CHT is usually not the result of some manufacturing or material defect in the cylinder itself. 

In fact, as I cover in many parts of “Engine Management 101,” there are many things that can influence CHT, including fuel system calibration, engine timing, flight profile, climate and most of all, damaged or improperly installed baffling. 

I have witnessed many mechanics chase CHT issues by adding fuel on the injection adjustment well above the manufacturer’s recommendations. This is merely placing a bandage on the root of the problem!

Owners and mechanics can be quick to blame the cylinder’s manufacturer, but in my 30-plus years, I have found only a couple of instances where the cylinder was indeed the cause of the problem. 

When troubleshooting engine temperature problems, I always ask the owner or mechanic about the condition of the baffling. Without question, 100 percent of the time it is reported as “OK,” “good” or even “perfect.” As I probe further into the problem, more times than not, the condition of the baffling is, in fact, nowhere close to being OK. 

Bird: 1; baffling: 0

I remember one instance where the owner reported high CHT problems. We assisted in troubleshooting over the phone for several days and were told the baffling was perfect. Finally I traveled to his home airport and when I examined the airplane, I actually found the remnants of a bird’s nest in the baffling. The bird had long ago moved out. 

I guess birds don’t like CHTs above 400 degrees either! I just had to ask the mechanic if this was his idea of “good” or “perfect” baffling. He and the owner were ready to remove cylinders, change the fuel pump, etc.—anything but address the actual problem.

Anyway, we cleaned and resealed the baffling and that immediately brought CHT down to normal levels.

The owner was embarrassed and offered to pay all my expenses for the trip. I declined and advised him we would use this time to work with his mechanic. Today—not because of me—this mechanic is one of the best in the industry. I encouraged him to attend many of the recurrent training classes on engines and airframes. While experience is good, training is sometimes better.

Don’t be bumfuzzled by baffling

In order to appreciate the importance of engine baffling, I think we must understand its function and how it works. The accompanying illustration (above) shows how the airflow moves through most General Aviation aircraft cowlings. In the majority of installations, the cooling air flows down through the cylinders and out through the bottom of the cowling. 

This airflow is accomplished by creating a confined high-pressure area through the proper placement and installation of the baffling. The higher pressure on top of the engine allows for the air to flow directly to the low-pressure area on the bottom. 

The baffling constrains the air so it flows directly around the cylinders. The intercylinder baffling directs cooling air to the vital portions of the cylinder head, and in some cases helps to balance the cooling airflow to eliminate hot spots.

When properly installed, the baffling is designed to create a seal between the upper and lower portions of the engine. It is very important that the rubber pieces of the baffling conform to the engine cowling, allowing no air to escape over or around. 

This is why it is vital that all air gaps be sealed properly. The general rule of thumb for most installations is to direct as much cooling air as possible down through the cylinders. The residual heat is carried away through the bottom of the cowling. 

For those of you who fly aircraft equipped with cowl flaps, you are able to regulate the amount of air flowing through the cylinders simply by regulating pressure within the cowling.

Check your engine baffling carefully during preflight inspections and make sure your maintenance provider checks it closely during the next 100-hour or annual inspection. You do not have to remove the cowl and do a detailed inspection during preflight; just a quick glance in the cowling will let you know if the baffling is problematic or perhaps folded the wrong way. 

You can review with your mechanic how the baffling should look during your preflight inspections. Here are just a few items to look for:

• Cracks

• Incorrect fit

• Incorrect positioning

• Gaps not sealed properly

• Torn or cut rubber seals

During engine installation, make sure your engine baffling is either in pristine condition or is replaced. This small investment in new or repaired baffling will protect the larger investment you made under the cowling! 

For example, I recently overhauled the engine in my father’s Alon Aircoupe and that overhaul included new baffling. Now, thanks to the new powder-coated baffles and rubber seals, the trusty Continental C90 gets lots of cooling-air love. 

The attention given by you and your mechanic to ensure your aircraft’s engine baffling is installed and maintained in accordance with manufacturer’s instructions will go a long way in providing cylinder and overall engine longevity.

Bill Ross is a graduate from the University of South Alabama and was employed by Continental Motors for 15 years. As Vice President of Product Support for Superior Air Parts, Ross is committed to the company goal of making flying affordable. When not working at Superior, Ross can be often found flying his family’s 1941 Boeing Stearman, working on antique aircraft or exposing young people to the joys of flight and potential careers in aviation. Send questions or comments to .