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Do I Need to Worry?

Last month we got an email from John, who had some questions about his report. His F250 was showing traces of coolant in the oil, and lead, from bearings, was elevated. He had the engine out of the truck pending repairs and wanted to know: how much lead is too much? Did he need to replace the bearings?

“Do I need to worry?” is a common question, and one there’s not one easy answer for. We’ve had people pull the bearings out of a Corvette when lead was only a few ppm above average and we said in the report, “You don’t need to do anything about this yet.” (For the record, that guy called us and said his bearings looked fine and was kind of honked off about it.)

We’ve had people with metals that are high all along, but not changing, and it never turns into a problem. And we’ve had people not pursue what appeared to be a problem, and regret it in the end (this is especially problematic when the engine is in an airplane).

So how do we decide what’s a problem and what’s not? It would be great if there was a magic number, but there’s not. We assess each engine individually, mainly focusing on these things:

How your sample compares to your trends
How your sample compares to average
The balance of metals to each other
Whether you’re using additives

Trends

If you have them, trends are the most helpful thing we look at in determining your engine’s health. It takes three samples to get a good trend going (though we can often tell if something is amiss earlier than that).

All engines are different, as are their drivers, how they’re used, and where they are in the country. As such, it’s very helpful to sample a few oil changes in a row, at least at first, and have a baseline established for your specific engine. Consistency counts. If your engine is wearing a lot but it’s doing so steadily, it’s possible that the metal isn’t a problem. Problems tend to get worse over time – not remain stagnant.

Figure 1 is a good example where lead (a bearing metal) doesn’t appear to be a problem. That engine has more lead than average, but it’s consistent. Since the owner wasn’t having any problems, our recommendation was to just watch lead as time goes on.

But on the other hand, look at Figure 2. Lead read at just 12 ppm in this sample—that’s well within the average range, but we marked it because lead had always been much lower than this. If this had been his first report, we might have thought lead was okay. But since we know that lead is usually low, we told him the bearings are wearing more than they were and to watch for abnormalities like low oil pressure.

Universal averages

Of course, when you start sampling, you don’t have trends to rely on. So our second line of defense, when we’re looking at your numbers, is universal averages.

We have averages established for most of the engines out there, though we’re always adding to our database as new types of engines (and transmissions and generators and other machinery) are being made all the time. When you do your first sample, we’ll compare your metals to averages for your specific engine.

It’s helpful for us to know what kind of engine you have. Look at Figure 3, for example. This is a comparison between the Toyota 1.8L 1ZZ-FE (used in Corollas and Vibes), and the Oldsmobile 455 (used in older motorhomes and the Cutlass and Trans Am). Toyotas don’t wear much, whereas the Olds 455 makes a lot of metal.

If we don’t know what kind of engine you have, we might end up comparing your numbers to the wrong set of averages, or just a generic engine file. We can still tell if something is way out of line, but the more subtle differences between your engine and averages are harder to see.

Along those same lines, some vehicles come with many different engine options, so just telling us the year, make, and model of your vehicle isn’t always enough. The 2006 Silverado, for example, could have one of five different gas engines or the 6.6L diesel engine in it. We have different averages for each of those engine types. Take a look at Figure 4. The metals are similar in those engines, but they’re different enough to matter when we’re determining if something is too high or not.

Generally speaking, we’ll mark a metal in bold when it’s twice average or more. But not always—there are also times when we don’t mark elevated metals, if we know something else is going on.

We test a fleet of armored Sprinter vans that operate in New York City, for example. The vehicles are loaded up with armor and spend their entire lives idling and driving in unforgiving traffic conditions. It’s no surprise that the engines wear more than average. (See Figure 5.)

Balance of metals

We also look at the balance of metals relative to each other. In Figure 6, lead is not reading twice average but we marked it anyway. According to averages, lead and iron should be at about a 1:1 ratio. In this sample, the lead: iron ratio is more like 4:1. This balance tells us the bearings are wearing more than the rest of the engine, and that can be a sign of trouble too.

Additives

Another factor to consider is the use of additives and/or leaded fuel. Lots of people use Restore, which has copper and lead in it, and although in that form those elements aren’t harmful, they do make your numbers read high.

Likewise, if you’re using leaded fuel, racing fuel or certain octane boosters, fuel blow-by will cause high lead readings. The highest lead reading we’ve seen in any BMW S65 engine was 1055 ppm. The rest of the metals looked great, though, and the customer had mentioned using an additive, so we were pretty sure the lead in his sample wasn’t a sign of an impending bearing failure.

How much metal is too much?

So how much metal is too much? In truth that number is different for every engine. You already know that we take a lot of things into account in trying to answer that question. Usually we’ll call you to get more information if we’re not sure, and we’ll suggest giving it an oil change or two to see how trends shake out. If something is seriously out of line we can usually tell, even if we don’t know your engine type or how you use it.

We will say this, though: it’s pretty rare for a major mechanical problem to happen unexpectedly overnight. Most engines will give at least some warning before things go south, and that’s why you do analysis. Follow the trends to see what’s normal for your engine, and when deviations occur, you’re informed enough to make a good decision.

By Kristin Huff|2024-09-19T10:07:41-04:002023|Articles, Gas/Diesel Engine, Marine|Comments Off

Antifreeze: The Silent Killer

After analyzing engine oil for 30 years, I can safely say the thing that kills more engines than any other is antifreeze seeping into the oil. We call it the “silent killer” since there is normally no indication this dreadful contaminant is about to strike until after the damage is done. Neither you nor your mechanic can see it in the oil. The dealer won’t know it’s there.

We like to say engines speak before they fail, but in this case, you aren’t likely to hear much of anything until you hear just about the worst sound an engine can make. Oil analysis is the only way of knowing this sneaky killer is closing in on you. We can see it in the oil at a trace level, long before any harm is done.

We call people daily to let them know anti-freeze contamination is about to ruin their day. A typical reaction is, “What? That engine is running fine!” And they are right—the engine will, in most instances, run perfectly well until a bearing spins, oil pressure drops, and the engine destructs to the point of no salvation.

Some engine configurations are more susceptible to the problem than others: V-6s and V-8s for instance, are perhaps more prone than other engines. But no engine is immune (except air-cooled engines!). One would think that after 100 years of building engines, the automakers would get it right. To my knowledge no one has.

The problem with design

The engineers who design engines do a marvelous job of building lighter, more efficient and faster engines. But for every step forward in the process, there are compromises.

Building lighter engines necessitates working with new alloys for the various parts that are bolted together. Gaskets are used to seal between the parts. To get an engine perfectly right, they have to use parts that expand and contract with heat at the same rate, and gaskets that are hardy enough to seal well even after they age and suffer millions of heat cycles. You can imagine the engineers tossing in their sleep while wrestling with this dilemma.

A classic example of the problem was a Jaguar in-line 6-cylinder engine I once owned. I loved that engine with its long, high-end torque curve and mellow growl. It was probably the first duel overhead cam design that managed quiet chains in the days before belts were used. But for all its wonderful assets, there was this one drawback: they used an aluminum alloy head on a cast iron block. If you managed 50,000 miles on a head gasket, you were a very fortunate person.

With an in-line design, the anti-freeze contamination usually develops at the head gasket. With the V-designs, a more common source of the problem is intake manifold gaskets.

The manifold gasket supports the air/fuel system mechanism and straddles (and is bolted to) the heads. You can imagine the complexity of the problem of heat cycles. Block expansion forces the heads up and away from the crankshaft. The lowly intake manifold is not in a position to move in concert with the expansion. It would be like trying to ride two horses standing on the two saddles.

The result of this set-up is that intake manifold gaskets fail. Antifreeze starts seeping into the oil. It often takes quite a long while before the problem manifests itself in a failure, but it can also happen quickly. Since there are usually no obvious symptoms of the problem, the unwary engine owner usually drives the engine to oblivion.

Another common question we hear is, “How long until it fails?” Unfortunately, it’s impossible to predict how long an engine with an antifreeze problem will last. Many variables factor into the equation: the type of engine, how it’s driven, the environment it’s operated in, and—the most unpredictable of all—Lady Luck. Some people can limp along for ages with a slight trace of coolant that never turns into anything serious. Others turn up a trace and then WHAM! Faster than you can say “spun bearing” the engine fails.

Don’t quote me on this, but if I had to estimate the severity of the problem in car and truck engines today—judging from our oil samples—I would suggest 1–2 % of the cars and trucks in the road today are in the process of failing from antifreeze contamination of the oil. Fortunately, most antifreeze problems can be detected early with oil analysis, and in most cases we can save the engine before a failure. We would like to save all of them. But we can’t save anything until see the oil from it.

By Jim Stark|2024-09-19T10:09:24-04:002023|Articles, Gas/Diesel Engine, Marine|Comments Off

Pre-Ignition and Detonation

The information (and harrowing pictures!) that follows is reprinted courtesy of the FAA.

Pre-ignition. Detonation. Both can be deadly. But what’s the difference? And how can you avoid them?

This engine is from a Beech S35 Bonanza’s fatal accident. The #6 piston was eroded and began to melt. The rings and piston skirt were compromised by thermal expansion and metal transfer. Note the deep pitting and erosion of the piston face. This caused combustion gases to bleed into and over-pressurized the crankcase, forcing engine oil out the breather. The connecting rods then failed due to the lack of lubrication and smashed holes in the crank case, causing loss of power and engine failure.

Normal combustion vs. pre-ignition

Normal combustion is an orderly, progressive burning of the fuel-air mixture in the cylinders. The gasses within the cylinders are ignited from the top. The flame then travels down in an organized way. This combustive force, equally applied to the piston in a stable manner, pushes the piston down. The downward motion of the piston is then mechanically transferred to the propeller. This makes pilots very happy.

In a pre-ignition event, combustion is abnormal. It happens when the air-fuel mix ignites before the spark plug fires, while the piston is still moving up in the compression cycle. The ignition can be caused by a cracked spark plug tip, carbon or lead deposits in the combustion chamber, a burned exhaust valve, an ignition system fault, or anything that can act as a glow plug to ignite the charge prematurely.

When this happens the engine works against itself. The piston compresses and at the same time the hot gas expands. This puts tremendous mechanical stress on the engine and transfers a great deal of heat into the aluminum piston face damaging the piston. Engine failure can happen in minutes.

Detonation

As the name suggests, detonation is an explosion of the fuel-air mixture inside the cylinder. It occurs after the compression stroke near or after top dead center. During detonation, the fuel/air charge (or pockets within the charge) explodes rather than burning smoothly. Because of this explosion, the charge exerts a much higher force on the piston and cylinder, leading to increased noise, vibration, and excessive cylinder head temperatures.

The violence of detonation also causes a reduction in power. Mild detonation may increase engine wear, though some engines can operate with mild detonation. However, severe detonation can cause engine failure in minutes. Because of the noise that it makes, detonation is called “engine knock” or “pinging” in cars.

High heat is detrimental to piston engine operation. Its cumulative effects can lead to piston, ring, and cylinder head failure and place thermal stress on other operating components. Excessive cylinder head temperatures can lead to detonation, which in turn can cause catastrophic engine failure. Turbocharged engines are especially heat sensitive.

Some causes of detonation include:

improper ignition timing
high inlet air temperature
engine overheating
oil in the combustion chamber
carbon build-up in the combustion chamber

A combination of high manifold pressure and low rpm creates a very high engine load, which can also cause detonation. In order to avoid these situations:

When increasing power, increase the rpm first and then the manifold pressure
When decreasing power, decrease the manifold pressure first and then decrease the rpm

Pre-ignition and detonation results

The explosion of pre-ignition and detonation is like hitting the piston with a sledge hammer. The automotive term for the sound it makes is “ping” (something pilots cannot hear in aircraft). The ping sound is the entire engine resonating at 6400 hertz. It sounds like a ping, but it is an explosion with enough power to make the engine resound like a gong.

Both pre-ignition and detonation put tremendous mechanical stress on the engine and transfer a great deal of heat into the piston deck. This can cause the piston to melt (EGT is 1600 degrees; aluminum pistons melt at 1200 degrees). The force of these explosions can knock holes in pistons, bend connecting rods, overcome the lubrication film in the rod bearings, and hammer the babbitt out of rod bearings. Engine failure can happen in minutes.

The bent connecting rod at the start of the article is a good example of the damage pre-ignition and detonation can do.

These cylinder #2 spark plugs are packed with melted piston material.

Here’s what happens

This is a cylinder head showing signs of pre-ignition or detonation.

The carbon coating that normally lines the head dome is knocked off. There is melted piston material in the head and the cylinder sleeve is scored by the overheated piston.

This is the same piston , but note that the piston deck is eroded.

The rings are broken. The piston skirt is scuffed from rubbing on the cylinder wall. A piston in this condition allows combustion gases into the crank case. This over-pressurizes the crankcase and blows engine oil out of the crank case breather — all of the engine oil, in minutes.

Soon after the engine oil departs the connecting rods try to make a break for it, resulting in giant holes in the crank case.

How do I detect pre-ignition?

A rough-running engine can be the first sign of pre-ignition or detonation. High EGTs or CHTs can be a sign of a problem so be sure to keep an eye on that if you can.

Below are common indications of detonation. You should have increasing oil temperature, not pressure. The top left gauge is RPM. The top right is manifold pressure.

What do I do when it happens?

Since excessive heat can be so damaging, your main priority is to cool the engine:

Reduce power
Increase airspeed
Enrich the fuel mix
Open the cowl flaps.
Land immediately!

Preventing Pre-ignition

Do not take off unless the run-up is perfect
Maintain the ignition system
Pay attention to cylinder compression tests
Use the proper heat range spark plugs
Make sure cooling baffles are in good repair

Preventing Detonation

Lean the engine per the flight manual
Keep engine load to a minimum
Do not over boost
Use only the recommended fuel grade
Make sure engine timing is properly set
Make sure cooling baffles are in good repair
Be wary on hot, dry days
If in doubt, run rich

By Kristin Huff|2024-09-18T13:48:58-04:002023|Aircraft, Articles|Comments Off

Sudden Landing? Remember TWIT

Aviation is fraught with acronyms. There are so many acronyms that it is humorous to think the government still thinks they are a memory solution instead of the problem. I hate to add one more, but TWIT is one that may cause an unfortunate event in your life to turn out to be a good story instead of a tragedy. It is the word to remember should your (single) aircraft engine quit. You know, when the heat pump stops heating, when the cooling fan stops cooling, and/or when the roar of the engine that has always been your security blanket leaves nothing but silence ringing in your ears, uncovering your naked fear.

More than twenty years ago I was viewing the remains of an upside-down C-150 in a bean field with a couple guys from the Sheriff’s Department, when a civilian turned up. He was a lanky guy in a black suit with a camera slung across his shoulder. He asked me if I was the party that had left the airplane in this condition, and I admitted that I was the culprit. He said he was not only the undertaker in Paris, Illinois, but the newspaper photographer too, and when he gets a call like the one he got this morning, he didn’t know if he’d be taking pictures or measuring the body. With a grin, he pulled out a tape measure. “Looks like its only pictures this morning,” he said.

The saving grace of TWIT

I’ve had a long time to think about that and decided your early “engine out” training, as good and thorough as it may have been, could be improved upon with TWIT. Any engine out practice is good because if and when it really happens, you don’t need the tension of suffering the sensation for the first time. Few of us do first-time experiences well. So it is nice when you find yourself gliding around up there, sans engine, to have done it a few times before.

Everyone knows the first thing to do, which is to trim best glide speed. Don’t know it? Well, anything in the vicinity is okay. The idea, of course, is to give you the most time aloft for the altitude you are carrying. The reason that is important is that some prayers take far longer than others to complete. And if you happen to be one of those people who feels the need to pray with your eyes closed, well, you don’t want to waste all the glide time left with your eyes closed. It is a good idea to take a look at the terrain.

Trim

So the first letter to TWIT is for Trim, which hopefully gets you somewhere in the neighborhood of best glide. Your airspeed will be about 65–75, depending on the airplane. Our father…

Your flight instructor thought the next important thing to do was find a field to land in. With all due respect, that may not be the best idea. If you can hit a particular field from 5,000’ you are a better pilot than I am. You may pick the best field in your range of vision, but actually being able to hit it from altitude takes a lot of practice that we don’t generally work on. (Well…maybe the astronauts.)

My experience is, you will pick a field, and then see a better one as you get lower, and then a better one when you get lower than that. What makes it good is your ability to land on it, not its humongous size. It only takes a few hundred feet to land an airplane and you will find most fields at least that long. It is far more important to be landing into the wind so that length of field works. It is also nice to land without hitting anything. It is cheaper to buy a quarter acre of corn from a farmer than have to pay for his custom and recently rebuilt outhouse, which his insurance company will value in the tens of thousands of dollars.

Wind

So the second letter of TWIT is Wind. The first of the two turns you are going to make is the turn to base. You need to be adjacent to the wind. You probably know the wind direction, at least generally. If it is, for example, south, then turn east or west. If it is west, turn north or south. Which direction you turn will be decided on which direction has the best fields and the least structures. You don’t need much of a field, but when it comes to choosing one, the more options you have, the merrier.

If you are headed for a town or city, use some of that glide time to make a 180. A long walk out is better than hitting an unmovable object. Hopefully this is going to be the longest base leg you have ever flown. It will give you time to check your drift and verify the wind direction and speed. You might even stumble across a landing strip, if your GPS hasn’t already found one for you. If the wind is left, you are on a left base. If the wind is right, you are on a right base.

Initiate

You don’t pick your field until you see one that fits your sight picture for turning final. Few of us could hit a field from 5,000 feet, but all of us can hit the runway when turning final (well, almost all of us). If you are on left base, you are looking for the field on your left. If you are on right base, you are looking right. When the sight picture is what you are used to seeing, make your turn to Initiate the landing, which is the third letter to TWIT. It is time to get your flaps and wheels down if the wheels are retracted. (Shame on you if you did either of these things on that long base leg glide you were on.)

Talk

No second guessing at this point. It is time to congratulate yourself on getting from way up there to way down here without having done anything foolish, and you can already hear yourself telling a story about your successful off-field landing. Now that you are kicked back and gliding to final, it is a good time to call ATC, FSS, or perhaps your Maker, and let them know where they may find you, while you still have a little altitude left. You may have already done this earlier when you tuned your transponder to the emergency code, but it wouldn’t hurt to call in again, just to prove you can still talk without stuttering. I can speak here from experience — the last thing on your mind when your engine quits will be the radio. That’s why TWIT ends in Talk. You may be comfortable enough, or have enough presence of mind, to actually talk with someone other than yourself, once you have the field made.

Next time your engine gives up the ghost and you have already run through all the nasty words you can think of, try yelling TWIT to yourself. It may help you get down to terra firma safely.

By Jim Stark|2024-09-18T13:59:14-04:002023|Aircraft, Articles|Comments Off

Critical Component Failures

This article, by Mike Busch, originally appeared in the March 2010 issue of Sport Aviation as part three in the series “Reliability-Centered Maintenance.” We are reprinting it with the permission of the author.

To properly apply reliability-centered maintenance (RCM) principles to the maintenance of our piston aircraft engines, we need to analyze the failure modes and failure consequences of each major component part of those engines.

In this article we’ll examine the critical components of these engines, how they fail, what the consequences are of those failures on engine operation and safety of flight, and what sort of maintenance actions we can take to deal with those failures effectively and cost-efficiently.

Crankshaft

It’s hard to think of a more serious piston engine failure mode than a crankshaft failure. If it fails, the engine quits. Yet crankshafts are rarely replaced at overhaul. Lycoming says their crankshafts often remain in service for more than 14,000 hours and 50 years! TCM hasn’t published this sort of data, but TCM crankshafts probably have similar longevity.

Crankshafts fail in three ways: 1) infant-mortality failures due to improper material or manufacture, 2) failures following unreported prop strikes, and 3) failures secondary to oil starvation and/or bearing failure.

We’ve seen a rash of infant mortality crankshaft failures in recent years. Both TCM and Lycoming have had major recalls of crankshafts that were either forged from bad steel or were physically damaged during manufacture. Those failures invariably occurred within the first 200 hours after a newly manufactured crankshaft entered service. If a crankshaft survives the first 200 hours, we can be pretty confident that it was manufactured correctly and should perform reliably for many engine TBOs.

Unreported prop strikes seem to be getting rare because owners and mechanics are becoming smarter about the high risk of operating an engine after a prop strike. Both TCM and Lycoming state that any incident that damages the propeller enough that it has to be removed for repair warrants an engine teardown inspection. This applies even to prop damage that occurs when the engine isn’t running. Insurance will pay for the teardown and any necessary repairs, no questions asked, so it’s a no-brainer.

That leaves us with failures due to oil starvation and/or bearing failure. We’ll talk about these when we look at oil pumps and bearings.

Crankcase

Crankcases are also rarely replaced at major overhaul, and they often provide reliable service for many TBOs. If the case stays in service long enough, it will eventually crack. The good news is that case cracks propagate slowly, so a detailed annual visual inspection is sufficient to detect such cracks before they pose a threat to safety. Engine failures caused by case cracks are extremely rare.

Camshaft and lifters

The cam/lifter interface endures more pressure and friction than any other moving parts in the engine. The cam lobes and lifter faces must be hard and smooth in order to function and survive. Even tiny corrosion pits (caused by disuse or acid build-up in the oil) can lead to rapid destruction (spalling) of the cam and lifters and the need for a premature teardown. This is the number one reason that engines fail to make TBO. This problem mainly affects owner-flown airplanes because they tend to fly irregularly and sit unflown for weeks at a time.

Camshaft and lifter problems seldom cause catastrophic engine failures. The engine will continue to make power even with severely spalled cam lobes that have lost a lot of metal, although there is some small loss of power. Typically, the problem is discovered when the oil filter is cut open and found to be full of metal.

If the oil filter isn’t cut open and inspected on a regular basis, the cam and lifter failure may progress undetected to the point that ferrous metal circulates through the oil system and contaminates the engine’s bearings. In rare cases, this can cause catastrophic engine failure. A program of regular oil filter inspection and oil analysis will prevent such failures.

If the engine is flown regularly, the cam and lifters can remain in pristine condition for thousands of hours. Some overhaul shops routinely replace the cam and lifters with new ones at major overhaul, but other shops use reground cams and lifters and most knowledgeable engine experts agree that properly reground cams and lifters are just as reliable as new ones.

Gears

The engine has lots of gears: crankshaft and camshaft gears, oil pump and fuel pump drive gears, magneto and accessory drive gears, prop governor drive gears, and sometimes alternator drive gears. These gears typically have a very long useful life and are not usually replaced at major overhaul unless obvious damage is found. Gears rarely cause catastrophic engine failures.

Oil pump

Failure of the oil pump is occasionally responsible for catastrophic engine failures. If oil pressure is lost, the engine will seize quite quickly. The oil pump is very simple, consisting of two gears in a close-tolerance housing, and is usually trouble-free. When trouble does occur it usually starts making metal long before complete failure occurs. Regular oil filter inspection and oil analysis will normally detect oil pump problems long before they reach the failure point.

Bearings

Bearing failure is responsible for a significant number of catastrophic engine failures. Under normal circumstances, bearings have a very long useful life. They are always replaced at major overhaul, but it’s quite typical for bearings that are removed at overhaul to be in excellent (sometimes even pristine) condition with very little measurable wear. Bearings fail prematurely for three reasons: (1) they become contaminated with metal from some other failure; (2) they become oil-starved when oil pressure is lost; or (3) they become oil-starved because the bearing shells shift position in the crankcase saddles to the point where the bearing’s oil supply holes become misaligned (“spun bearing”).

Contamination failures can be prevented by using a full-flow oil filter and inspecting the filter for metal on a regular basis. So long as the filter is changed before its filtering capacity is exceeded, particles of wear metals will be caught by the filter and won’t contaminate the bearings. If significant metal is found in the filter, the aircraft should be grounded until the source of the metal is found and corrected.

Oil starvation failures are fairly rare. Pilots tend to be well-trained to respond to loss of oil pressure by reducing power and landing at the first opportunity. Bearings will continue to function properly even with fairly low oil pressure (e.g., 10 psi).

Spun bearings are usually infant mortality failures that occur either shortly after an engine is overhauled (assembly error), or shortly after cylinder replacement. Failures can also occur after a long period of crankcase fretting (which is detectable through oil filter inspection and oil analysis), or after extreme cold-starts without proper pre-heating. These are usually random failures, unrelated to hours or years since overhaul.

Connecting rods

Connecting rod failure is responsible for a significant number of catastrophic engine failures. When rod fails in flight, it often punches a hole in the crankcase and causes loss of engine oil and subsequent oil starvation. Rod failures have also been known to result in camshaft breakage. The result is invariably a rapid loss of engine power.

Connecting rods usually have a very long useful life and are not normally replaced at major overhaul. (The rod bearings, like all bearings, are always replaced at overhaul.) Some rod failures are infant mortality failures caused by improper torque of the rod cap bolts. Rod failures can also be caused by failure of the rod bearings, and these are usually random failures unrelated to time since overhaul.

Pistons and rings

Piston and ring failures can cause catastrophic engine failures, usually involving only partial power loss but occasionally total power loss. Piston and ring failures are of two types: (1) infant mortality failures due to improper manufacture or installation; and (2) heat-distress failures caused by pre-ignition or destructive detonation events. Heat-distress failures can be caused by contaminated fuel or improper engine operation, but they are generally unrelated to hours or years since overhaul. Use of a digital engine monitor can usually detect pre-ignition or destructive detonation episodes and allow the pilot to take corrective action before heat-distress damage occurs.

Cylinders

Cylinder failures can cause catastrophic engine failures, usually involving only partial power loss but occasionally total power loss. A cylinder has a forged steel barrel mated to an aluminum alloy head. Cylinder barrels normally wear slowly, and excessive wear is detected at annual inspection by means of compression tests and borescope inspections. However, cylinder heads can suffer fatigue failures, and occasionally the head can separate from the barrel, causing a catastrophic engine failure. Cylinder head failures can be infant mortality problems (due to improper manufacture) or can be age-related. Age-related failures seldom occur unless the cylinder is operated for more than two or three TBOs. Nowadays, most major overhauls include new cylinders, so age-related cylinder failures have become quite rare.

Valves and valve guides

It is quite common for valves and guides (particularly exhaust) to develop problems well short of TBO. Valve problems can usually be detected prior to failure by means of compression tests, borescope inspections, and surveillance with a digital engine monitor (provided the pilot knows how to interpret the engine monitor data). If a valve fails completely, a significant power loss can occur.

Rocker arms and pushrods

Rocker arms and pushrods (which operate the valves) typically have a very long useful life and are not routinely replaced at major overhaul. (Rocker arm bushings are always replaced at overhaul). Rocker arm failure is quite rare. Pushrod failures are caused by stuck valves and can almost always be avoided through repetitive valve inspections and digital engine monitor usage, as discussed earlier.

Magnetos

Magneto failure is uncomfortably commonplace. Fortunately, aircraft engines are equipped with dual magnetos for redundancy, and the probability of both magnetos failing simultaneously is extremely remote. Mag checks during pre-flight run-up can detect gross magneto failures, but in-flight mag checks are far better at detecting subtle or incipient failures. Digital engine monitors can reliably detect magneto failures in real time if the pilot knows how to interpret the data. Magnetos should be disassembled, inspected, and serviced every 500 hours—doing so drastically reduces the likelihood of an in-flight magneto failure.

The bottom line

The “bottom-end” components of these engines—crankcase, crankshaft, camshaft, bearings, gears, oil pump, etc.—are very robust. They normally exhibit very long useful lives that are many times as long as recommended TBOs. Most of these bottom-end components (with the notable exception of bearings) are reused at major overhaul and not replaced on a routine basis.

When these items do fail prematurely, the failures mostly occur shortly after engine manufacture, rebuild, or overhaul, or they are random failures that are unrelated to hours or years since overhaul. The vast majority of random failures can be detected long before they get bad enough to cause catastrophic engine failure simply by means of routine oil filter inspection and laboratory oil analysis. There seems to be no evidence that these bottom-end components exhibit any sort of well-defined steep-slope wear-out zone that would justify fixed-interval overhaul or replacement at TBO.

The “top-end” components—pistons, cylinders, valves, etc.—are considerably less robust. (The “top end” of a piston engine is analogous to the “hot section” of a turbine engine.) It is not unusual for top-end components to fail prior to TBO. However, most of these failures can be prevented by regular inspections (compression tests, borescope, etc.) and by use of digital engine monitors (by pilots who have been taught how to interpret the data). Furthermore, when potential failures are detected, the top-end components can be repaired or replaced quite easily without the need for engine teardown. Once again, the failures are mostly infant-mortality failures or random failures that do not correlate with time since overhaul.

The bottom line is that a detailed failure analysis of piston aircraft engines using RCM principles strongly suggests that what the airlines and military found to be true about turbine aircraft engines is also true of piston aircraft engines: The traditional practice of fixed-interval overhaul or replacement is counterproductive. A conscientiously applied program of on-condition maintenance that includes regular oil filter inspections, oil analysis, compression tests, borescope inspections, and in-flight digital engine monitor usage can be expected to yield improved reliability and much reduced maintenance expense and downtime.

Magnetos are an exception. They really need to go through a fixed-interval major maintenance cycle every 500 hours, because we have no effective means of detecting potential magneto failures without disassembly inspection.

By Mike Busch|2024-09-18T14:03:20-04:002023|Aircraft, Articles|Comments Off

Emergency!

Unless you rent just one plane a lot, you never really know about a rental plane. You would like to think that it sees careful maintenance all the time, and I’m sure most of them do, but as long as some other person is flying it, it could have problems lurking that don’t show up every flight. Like most pilots, I would like to own a plane someday — something I could fly a lot and get familiar with. Unfortunately, a Republic SeaBee isn’t in the cards right now, so I’ll be renting for the time being. That leaves the possibility for unknown problems lurking that have to be dealt with on the fly (so to speak).

Half power

For me, my first experience with a problem in a rental was actually during flight training. I was flying a Cessna 152 out of Fort Wayne International. I was just going up solo for some touch and go’s and on my first climb-out the engine went to about half power. Thankfully it stayed at half power and I was able to fly the pattern and land without incident.

When I got back in to the flight training building and told my instructor what happened, I got a sense that she didn’t believe me. And sure enough, when we both took it up, everything was fine. She mentioned something like, “I’ll bet there was water in the tanks” (I thought, No, I sumped the tanks and they were clean — I do work in a lab, dammit!) and that’s the last I heard about it. Of course, I was renting the plane, so I don’t know if it had happened before or after my incident, or if something was fixed afterwards that may have been the cause.

In any case, I didn’t panic and made a nice landing (the plane was reusable) so really didn’t think much about it until several years later.

When suddenly…

I have had my private pilot’s license for a few years now, and I have been renting a 172N with the Continental O-300. I got checked out in it just fine and had taken a few flights previously by myself. On this particular day, I went up with my Dad (Jim Stark, Blackstone’s founder) and his wife on a sightseeing trip.

We flew north for about 30 minutes looking at the lakes of northeast Indiana, and we had just finished a turn south to head back when the engine started shaking. No little shudder either, but the kind of shaking a dog would make trying to pass razorblades — at least what I would imagine a dog would look like. My dog was never so dumb as to eat razorblades to start with.

Anyway, the engine started shaking really bad. My father (also a pilot) initially said “Get the carb heat on!” and I thought, of course! Carb heat! Continentals are prone to carb ice and I have had nightmares of having to force land an aircraft for just that reason. I’m not sure if I would have thought of that myself, so I was sure glad to have him sitting in the right seat.

I pulled on the carb heat so hard I thought the knob might come off. We sat expectantly for a minute, both waiting for the engine to smooth out. Unfortunately, that didn’t happen. We still had power, but the shaking was bad enough that the thought of a 30-minute flight back to Fort Wayne wasn’t appealing, so I looked at Dad and said, “We’re going back to Angola to land!”

Angola is a town about as close to the northeast corner of Indiana that you can get. It has a beautiful airport with a paved 5,000-foot east-west runway. We were only about five minutes away, but as you can imagine, it seemed to take about an hour to get there.

The carb heat was on the whole time yet the engine never smoothed out, so I figured the engine had some serious issues. The landing was uneventful and as we pulled up to the ramp the engine was still shaking, so we decided to do a mag check.

The right mag check produced no change, but the engine almost died on the left mag. We tested this several times to make sure it was correct and then shut the engine down. One of the best things about the Angola airport is they have an airport car that’s available for situations just like this. It was a late ’90s Ford Explorer that shook almost as bad as the airplane, but hey, beggars can’t be choosers.

I called the FBO where I rented the plane, told them the situation, and then drove home. After an hour’s drive, I pulled into the FBO, gave them the keys and paid my bill. Yes, full price for the time I had the airplane. No discounts for having to make an emergency landing and no allowance for my stepmother needing new underwear. It was okay though, I was just happy to be alive and back in Fort Wayne.

The bright side

Now, the good thing about renting an airplane is, when it breaks, all you have to do is say “Your plane is messed up” and leave. I don’t worry about having to schedule/pay the mechanic, call the people who were renting it afterwards and tell them to make other plans, no hangar fees, no insurance, no fixing knobs that got pulled off.

The bad part about it is that you really never get to know the aircraft and engine ¾ what’s normal operation and what’s not. After a few days, I get a call from the FBO manager who said the engine had a stuck valve. I was fairly amazed because I suspected the horrible mag check denoted something electrical as the problem.

We happened to be doing the oil analysis on this engine, so I checked that to see what it looked like. Aside from a little excess copper, it looked pretty good. However, the O-300 does have bronze exhaust valve guides, so this should have been a warning, at least to be on the lookout for valve problems.

Signs of the problem

Since this incident, I have learned that a really bad mag check is a common symptom of a stuck valve. Some other common symptoms are below.

Morning sickness — When an engine starts rough first time consistently, not just in the morning, without plug fouling
Temporary roughness on climb out or in cruise — this happens when a valve momentarily sticks, then shakes loose
Intermittent rough idle that’s not caused by carb ice (this needs to be ruled out)

If I had been the sole operator of this aircraft, I might have identified some of the other symptoms and put two and two together. Instead, I was not aware of any issues at all. In all fairness, maybe there weren’t any, I don’t know. But I know these symptoms now and I’ll be sure to look for them in the future.

By Ryan Stark|2024-09-18T14:17:40-04:002023|Aircraft, Articles|Comments Off