Aircraft | Blackstone Laboratories

Landslide!

Imagine the awesome event of a landslide. There’s no doubt it’s a brutal force of nature. If you’re unfortunate enough to be caught in one, you might not survive. A landslide is gravity pulling terra firma down a slope with such force that it takes all things, natural and manmade, with it. The very earth that supports us unmoors from its surroundings, changes shape, and becomes destructive. While it may not be obvious at first glance, this landslide can help us understand oil analysis.

Take a picture

Back to your mental image of the landslide: it starts off with a few pebbles rolling down a hill. Those pebbles strike others, and the dirt slide gains momentum. The process escalates and the mass of the movement increases. Larger rocks and patches of earth are dislodged, and the process continues until the whole hillside is involved, taking trees, boulders, and anything else in the way. Now stop: Take a mental picture of the landslide in full force. Step back and look at the frozen picture. Everything on the hillside that started off peacefully and at rest is in the process of roaring toward the bottom of the slope.

If you looked at your picture of the landslide from afar, you’d see a cloud of dust and dirt at the front edge of the sliding mass, and lingering far behind it. The dust cloud itself would actually hide much of the larger detail of rocks and trees crashing along the slope. Without looking at the larger debris contained in the mess, could you determine the makeup and extent of the landslide from the dust cloud alone? For the most part you could, and that’s how oil analysis works.

Normal vs. abnormal

One of the limitations to oil analysis is that we can only tell you about the wear metals that we can see with the spectrometer, which are between about 1 and 15 microns in size. (How big is a micron? One-millionth of a meter. One inch contains 25,400 microns. The period in this sentence is about 615 microns.)

If you have a mechanical problem with your engine, the oil filter should collect the larger metallic particles (usually those larger than 40 microns). These are the boulders in the landslide. There is also a wide range of rocks and stones present in the landslide that don’t become airborne. They still ride the slide to the bottom of the hill, but they don’t hang suspended in the dirt cloud. These are the particles that fall out of suspension and don’t make it to the lab with your oil sample.

Then there’s the dust cloud. We compare the “dust” we see in an oil sample to what is average for a particular type of engine, transmission, differential, etc. We expect all mechanical machines to produce wear in the course of normal operation. But there is normal wear, and there’s abnormal wear. When we find abnormal wear in your “dust cloud,” we may be looking at a potential landslide in your engine.

Avoiding the trees

Fortunately, we don’t have to wait for a landslide to occur before we can determine what’s going on your engine. While the dust cloud accompanying the slide is a one-time occurrence, we can repeatedly analyze the oil from your engine and see trends developing. One snapshot gives you a look at whether the dust we find appears normal or abnormal. But a series of snapshots gives us a clearer picture of the condition of the engine. By trending the results from one oil change to the next, we can see whether the dust cloud is growing or subsiding. If it’s growing, eventually there will be boulders and you’ll need to take action to save the engine.

Occasionally we are asked about the dust: How much is too much? In other words, when someone has a particular metal that’s reading high, they often want to know how high it needs to be before they really start to worry. The answer is, there’s no single answer.

Lots of things affect the amount of wear we find — the type of engine it is, how it’s driven, and what conditions it’s operated in. What’s more important than the level of wear is the wear trend that’s developing. Someone who routinely races a Subaru 2.5L engine in Las Vegas is probably going to find higher wear on a routine basis than a person who uses the same engine to mainly go to work and the grocery store in Minnesota. If the racer’s wear is always high, and it always reads at about the same high levels from sample to sample, we’d probably consider it normal for that particular engine. If the grocery-store engine was producing low, steady wear, and then wear suddenly jumped up to the racer’s levels, we’d worry. That’s why it’s important to establish wear trends for your particular vehicle, and why we can’t always say, “Okay, when iron gets to X level, it’s a definite problem.”

Avoiding the full-on, catastrophic landslide is not hard to do if you practice routine oil analysis. To keep the boulders and trees out of your engine, pay attention to what you find when you change the oil or have it changed. Some metals are normal in new engines, but once past 10,000 or 15,000 miles, you should not be normally finding any metals that you can see. If you do find metal, it’s probably not too late to stop the slide — but it’s better to avoid it in the first place, if you can, through the trend analysis of your engine’s wear.

By Jim Stark|2024-09-19T09:01:39-04:00June 12, 2024|Aircraft, Articles, Gas/Diesel Engine|Comments Off

Blackstone and the Post Office

(TL;DR: You can use the labels that are on your kits now, but if you’d like new ones, you can print one here.)

“I am FED UP,” said the customer on the phone. “Do you even have my sample? I mailed it a month ago.” I looked up his tracking number and he wasn’t exaggerating – he mailed it September 15, and we had just received it that day, October 15. Sound familiar?

Why is it taking so long for samples to arrive? And what are we doing about it? Read on, Blackstone fans. Have we got a story for you.

The Post Office makes some changes

“I think the post office isn’t charging us enough.” Ryan Stark, Blackstone’s president, and my brother and business partner, said to me one day last November after reconciling the checkbook. He’d noticed that for the last few months, the amount we were paying in postage had dropped significantly.

Stick with me, this is all going to tie together.

Last summer, just as we were all realizing the pandemic was not simply going to disappear, I learned the post office was ending their Merchandise Returns program. Because our samples came back to us on MR labels, we needed to create a new one, so I had my printer start working on it.

A major part of that process is getting approval from the USPS at various points along the way. And that’s where the process slowed…then slowed down some more…and then, like molasses on a winter sidewalk, came to a creeping halt.

We called USPS. How’s the label going? No reply. We emailed. How’s the label coming along? Nothing. Time passes. Months go by. Sometimes we’d get a reply – “We should have an answer for you soon!” But then…nada.

Back to the money

Meanwhile, the issue of not paying enough postage was still a problem. What do you do when you think the USPS isn’t charging your business enough? You call them – so I did.

I first contacted my local post office – the ones who deliver us samples every day, the ones who know who we are and what we do. “I think we’re not being charged enough,” I explained. “Nope, that’s not me,” she said. “They take care of that in Indianapolis now.” She gave me a number, so I called down to Indy. “Huh,” the Indy person said. “Let me look into it.”

Reader, you can see where this is going.

I got nowhere in November, so I called again in January. “Hey!” I said. “I still don’t think we’re getting charged enough!” “Hmmm” said the voice on the line. “Let me ask my supervisor about that.”

Time marches on. After calling and emailing various USPS representatives throughout February and March, I got fed up in April and sent an email blast to every single USPS contact I had, including the ones in Washington, D.C.

That one got some attention.

They started looking into what was going on, and to make a long story short, the issue culminated in a conference call with several USPS bigwigs. “Well,” said Bigwig #1, “you owe us (insert a huge amount of postage here. Nope, it was more than that).

It turns out that when the USPS stopped their Merchandise Returns program, our local post office stopped charging us for our incoming samples. We were still being charged for outgoing mail, but we hadn’t paid postage on incoming samples since the MR program ended in August.

After much gnashing of teeth and some heated words on my end (would they ever have caught the problem if I hadn’t kept after them? We’ll never know), we settled on a plan to pay the outstanding postage.

As part of this reconciliation, one of the USPS Bigwigs suggested we have samples returned to us in a Tyvek envelope, to help catch spills. Well, oil spills aren’t really the problem with getting samples delivered, but I tucked the idea away for the future.

Back to the labels

Meanwhile, the new label still had not been approved. And people still needed kits. While all this was going on, we continued to print and send out hundreds of thousands of old labels on kits. What choice did we have? Now those old Merchandise Return labels are now on kits that are sitting in garages, hangars, and marinas all over the country.

So when did we get it resolved? We officially started printing our new, USPS-approved labels more than a year after the old label was officially discontinued. The thing is, the post office reassured me that it would be fine to continue to use our old label – we would just have to pay more when people returned them.

Which is fine. Fine, fine, fine. Except, for some post offices, it’s not so fine. Most of those old, Merchandise Return-labeled kits get here no problem. But occasionally, a post office will hold on to it and not deliver it because it’s the old label, even though they said we could keep using them.

At this point, there’s nothing we can do about the thousands of old labels that are in circulation except try and get the word out. So that’s why you’re reading this. If you have old labels on your kits (they say Merchandise Return right on them), click here to ask for new ones. We really do want to receive your samples. And we don’t want you to have to wait for a month to get your results.

But wait, there’s more!

So while all of that was going on, Travis – a long-time Senior Analyst-turned-coder – had an idea. “What if,” he said to me one day, “we do a test to see if putting samples in a Tyvek envelope helps with the return postage time?” Because although oil spills aren’t a significant problem, it does seem to be a problem that the mailer is 1) small, and 2) clearly headed for a laboratory. Putting the oil into a Tyvek envelope might solve both issues. So we started a test – for one month, we sent all outgoing kits with a labeled Tyvek envelope for returning the sample to Blackstone.

The results were immediate and striking: this was a winner. We didn’t even run the test for the full month. The data Travis put together showed that return times were cut in HALF (from an average of 8.74 to 3.48 days) when samples came back to us in the Tyvek envelope. (See the sidebar.) We stopped the test and immediately started including Tyvek envelopes with each kit order, for return samples.

USPS supporters

Despite the problems, we are proud supporters of the United States Postal Service. No other carrier offers service to every single part of the US, no matter how remote. Lots of people don’t have access to UPS or FedEx, though if you want to use them to send in your samples, that’s absolutely fine.

The changes we’ve made to our label and the return package are already paying off in getting samples to us in a timely fashion. If you need new return envelopes and labels for your kits, let us know – we’re happy to send them out!

______________________________________

Update! The Post office has discontinued their First Class Return labels (my new mantra: change is good…change is good). We are now using Ground Advantage labels. All the same things in this article still apply. You can use the First Class return labels, but your sample will arrive faster with a Ground Advantage label. You can print one off right here.

By Kristin Huff|2024-09-19T10:15:59-04:00July 19, 2023|Aircraft, Articles, Gas/Diesel Engine, Industrial, Marine|Comments Off

Protecting from Corrosion

Considering the relative inactivity of much of the general aviation fleet, it’s not surprising that corrosion is a hot topic. It’s also fodder for aviation oil makers to claim their oil is better than others at protecting engine parts from corrosion.

The frustrating thing when you can’t find the time to go flying is, your beautiful bird is languishing alone in a dark hangar accumulating rust on its parts and dust and bird doo on its wings. What to do?

If you can’t fly it, you don’t want to just ground-run the engine since it’s pretty well accepted that doing so may cause more harm than good. In the end, the path most often chosen is to “leave her sit.” But that’s the maddening part. You just know that corrosion has begun at the cylinders, cam, and lifters, as well as all the other parts that are parked above the engine’s oil level. “Dammit! Maybe I should go out and shoot some landings.” But you don’t. So the question still stands…what to do?

We get a lot of questions about which oil protects aircraft engines best from corrosion. If there were a sure answer as to which oil is best, someone would surely have come up with it. Since they haven’t, perhaps we should reconsider the question. Maybe what we should be asking is, “What can I do to prevent corrosion in my (not flown frequently enough) aircraft engine, regardless of the oil I use?”

Turned around that way, there may be an answer.

Water orbs

Oil and water don’t mix at the atomic level. Since there is no such thing as dry oil — both hot and cold oil suck moisture from the air like a sponge — the only way these dissimilar types of matter can coexist is for the moisture to ball itself up into minute spheres, so tiny that they can exist in suspension. If the water orbs get large enough they will precipitate, that is, fall out of suspension. But there is almost no limit to how tiny they can be. The longer the oil sits undisturbed, the more water it will accumulate.

Oil routinely has some moisture in it, usually at levels between 40–400 ppm. In amounts greater than that, it can start to make your oil look like chicken gravy. Once moisture sets in, heat and/or pressure are the only way to get it out. If you go out and fly for an hour, the oil temperature and agitation will dismiss the moisture droplets like unruly elementary students. The moisture accumulation process will start all over again once you pull idle cut-off, but at least you have the satisfaction of knowing that, at least for now, the fine film of oil clinging to the metal parts is not heavily populated with tiny balls of water.

Fighting corrosion

After you lock the hangar door, the dry (well, reasonably so) oil film doesn’t last long on all those parts that are parked about the oil level. If your engine is a dry-sump type, none of the parts are parked in an oil bath.

If you can’t fly, you might consider using a pre-oiler once a week to rebathe all the parts in oil. The oil from the pre-oiler will reach all parts that see oil pressure during engine operation. It would require only turning on the master and the oiler switch for a short while, no longer than it takes to check the lights and flaps during a preflight, a few minutes at most. The oil should reach all the way up into the rocker boxes and then drain to form a brief pool over the tappets and cam, parts that are notoriously prone to corrosion pitting in all but the most active engines.

After the pre-oiling dose, you could get out and pull the prop through a few blades (normal direction of rotation, of course) to ensure all moving parts rotate through a couple of full cycles. Further, you will be giving all the rod bearings an oily trip through the sump reservoir, for wet sump engines. (Some people are queasy about touching the prop, so running the starter is an acceptable alternative, if you have confidence in the integrity of the battery.)

Cold dousing all oil-wetted parts isn’t nearly as good as an hour’s flight, but it seems far superior to the ground run-up, or the more often chosen “letting her sit.”

By Jim Stark|2024-09-18T13:48:37-04:00July 18, 2023|Aircraft, Articles|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|2024-09-18T13:48:58-04:00July 18, 2023|Aircraft, Articles|Comments Off

Aircraft Problems: Should I Be Worried?

One of the main purposes of oil analysis it to find problems that might be developing in an engine, and after doing this for a lot of years, I can say without a doubt that it works. However, some problems are more urgent than others, and part of our job is to determine if a problem is a major one or not. Most engine problems start out minor but if left unchecked can lead to major issues, which eventually result in an engine’s demise.

Minor problems

Abrasive Contamination

Dirt getting past the air filter will cause a lot of problems in an engine, and piston scuffing is the primary concern. Fortunately, most air filters do a really good job even when they are dirty. If you change your air filter on a regular basis, then this type of problem is pretty easy to avoid, but remember, it’s also important to check the whole air induction system down-stream of the air filter to make sure no cracks or other problems exist that could be letting dirt in.

Fuel dilution

This generally includes any fuel level between 1.0% and 3.0% that keeps showing up again and again. This is not a normal situation, but it doesn’t necessarily cause an engine problems in the short term. Still, since fuel is a contaminant, it will cause the oil to oxidize faster that it normally would. That typically causes problems like stuck oil control rings, which leads us to our next minor issue.

Oil Consumption

This one isn’t really a problem at low levels because all engines are designed to use some oil. What you really want to watch out for is a change in how much oil is being burned. If you always use 1 quart every 10 hours and it suddenly jumps to 1 quart every 3 hours, then you know something has changed. That’s part of the reason we ask about oil added between changes. If you’re not losing oil due to a leak, it’s either getting past the rings or the valve guides. Granted, you can buy a lot of make-up oil for the cost of a top overhaul, but there will probably come a time you’ll have to bite the bullet and fix the issue.

Corrosion

If you fly around 5 hours per month, that should keep this minor problem off you mind, though we all know that life doesn’t necessarily allow this. Still, if corrosion is minor it should easily disappear once the engine is back to flying regularly. If corrosion gets so bad that it causes pitting on the parts, that’s when the problem elevates to major status.

Major problems

Cam spalling

This one is often directly related to corrosion getting out of hand, though it can also be related to oil starvation on things like cold starts and high RMP starts. It takes time for that thick oil to get circulating through the engine and if it doesn’t get to the cam and followers fast enough, metal-to-metal contact happens. Problems of this nature won’t necessarily cause an engine to fail, but can lead to loss of some power, which might be needed to clear that 50’ obstacle at the end of the runway.

Excess Heat

This is really a pretty broad category and is often due to operational factors, though it’s almost always avoidable if you are paying attention to your cylinder head temperatures. If those are getting too hot, then maybe the cooling baffles aren’t quite working like they should. Maybe you have a crack in an air induction tube. That could allow abrasive dirt into the combustion chamber, but would also cause that one cylinder to run leaner than others (due to extra air being sucked in) and likely hotter. Excess heat causes the parts to expand more than they were designed for and that’s when wear starts getting heavy.

Stuck or Burned Valves

Abrasive contamination, fuel dilution, and oil consumption will all contribute to this type of problem. Sticking valves can be identified by things like morning sickness (not necessarily in the morning), intermittent rough running, and high mag drops (not due to a fouled spark plug). Burned valves are usually pretty easy to spot with a borescope though might not necessarily cause major operational problems until they burn to a point where compression has significantly degraded.

Detonation

This issue develops in an engine when the combustion process is not completed correctly, usually when an engine is under a heavy load and producing a lot of heat. It can easily burn a hole right through the top of a piston, resulting in all of the oil in your engine being pushed out the breather tube and oil starvation (see below). If your engine had a good muffler, you would hear a ticking or pinging noise, but since those don’t exist in general aviation, this problem can often go unnoticed without the help of oil analysis and/or engine monitor data. If this problem exists, running a richer fuel/air mixture to keep the engine cooler should help.

Instant failure

Oil starvation

Whether it’s caused by oil consumption left unchecked or severely worn bearings not letting oil get to all of the parts, this type of problem will cause an engine to fail in short order and it’s usually accompanied by the worst sound your engine can make — silence.

Spun bearings

When the babbit is worn off your bearings, either due to hard use, abrasive oil, or lack of oil, you will start to lose oil pressure. If the problem gets severe enough, the spinning shaft will actually weld to the bearing itself and spin in place. Once this happens, the engine is pretty much shot, though amazingly enough it might still run (but not for long).

Outside causes

Of course there are lots of other things that can cause instant engine death — see the first cartoon on this link for an example, or, although it’s not a plane, this picture of my flooded MINI. Unfortunately, outside factors probably take more engines down than anything else.

It’s pretty rare for engines to fail suddenly due to minor issues, so when we see something going on, that doesn’t necessarily mean you need to get out the wrenches or head straight to the engine builder and demand a repair. Usually, you’ll have some time to see if the problem persists or is getting worse. Once that has been established, then some action will likely be required to keep the engine going, but the cost should be minor compared to the hassle and expense of having to replace the whole engine. So test your oil every now and then. Chances are good your engine will look perfect, but if it doesn’t, you’re better off knowing about it sooner rather than later.

By Ryan Stark|2024-09-18T13:49:41-04:00July 18, 2023|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|2024-09-18T13:59:14-04:00July 18, 2023|Aircraft, Articles|Comments Off

EGT Myths Debunked

Reprinted with permission from the author, Mike Busch

Pilots still seem to have a lot of misconceptions about exhaust gas temperature (EGT). Let’s see if we can clear some of them up.

These days, pilots of piston-powered aircraft seem to have fixated upon EGT. Scarcely a day goes by that I don’t receive a phone call, email, or support ticket asking some EGT-related question.

Pilots will send me a list of EGT readings for each of their cylinders and ask me if think they look okay, whether I think their EGTs are too high, what maximum EGT limit I recommend, why their EGTs seem to be higher in the winter than in the summer, or why the EGTs on their 1972 Cessna 182 are so much higher than the ones on their friend’s 1977 model. They’ll voice concern that the individual cylinders on their engine have such diverse EGT readings, worry that the spread between the highest and lowest EGT is excessive, and ask for advice on how to bring them closer together. They’ll complain that they are unable to transition from rich-of-peak (ROP) to lean-of-peak (LOP) operation without producing EGTs that are unacceptably high.

Each of these questions reveals a fundamental misunderstanding of what EGT measures, what it means, and how it is interpreted. Let me attempt to clear up some of this confusion by asking you to forget everything you thought you knew about EGT and start at the beginning.

What EGT is not

The absolute values of EGT are not particularly interesting for a number of reasons. The most important is that indicated EGT is not a “real” temperature. To understand what I mean by this, I’d like you to conduct a thought experiment: Imagine that you’re an EGT probe, located in an exhaust riser between two and four inches from the exhaust port of a cylinders, and think about what you would see.

You’d see nothing much for two-thirds of the time¾during most of the intake, compression, and power strokes, because the exhaust valve is closed and so no exhaust gas is flowing out of the exhaust port and past the probe. During the one-third of the time that the exhaust valve is open, you’d see a constantly changing gas temperature that starts out very hot when the valve first opens but cools very rapidly as the hot compressed gas escapes and expands, and then ultimately is scavenged by cold induction air during the valve overlap period (at the end of the exhaust stroke and the beginning of the intake stroke) when both intake and exhaust valves are open simultaneously.

Now, all these gyrations are happening about 20 times per second, and you (the EGT probe) cannot possibly keep up with them. You wind up stabilizing at some temperature between the hottest and coolest gas temperature you see, and you dutifully report this rather arbitrary temperature to the panel-mounted instrument, where it is displayed to the pilot as a digital value accurate to one degree. The temperature you report to the pilot is not exhaust gas temperature (which is gyrating crazily 20 times a second) but rather exhaust probe temperature (which is stable but related to actual exhaust gas temperature in roughly the same fashion as mean sea level is to high tide).

To make matters worse, numerous factors can affect indicated EGT besides actual exhaust gas temperature. These include probe mass and construction (grounded or ungrounded), cam lobe profile, lifter leak-down rate, valve spring condition, and exhaust manifold topology, among others.

For example, the two front cylinders (numbers 5 and 6) on the left engine of my Cessna T310R always indicate lower EGTs than the other four cylinders. The exact same phenomenon also occurs on the right engine. This is not because those front cylinders produce cooler exhaust gas than their neighbors (they don’t), but because the exhaust risers for those cylinders curve aft while the other four risers go straight down. Thus, the gas flow past the EGT probe is different for the front cylinders than for the others, and their indicated EGT is lower. This temperature anomaly is quite obvious on my digital engine monitor¾and also quite meaningless.

What EGT means

Even if indicated EGT accurately reported actual exhaust gas temperature (which it doesn’t), it’s important to understand that exhaust gas temperature does not correlate with stress on the engine the way cylinder head temperature does. In fact, many things that increase engine stress (such as advanced ignition timing and high compression ratio) cause EGT to go down, while things that reduce engine stress (like retarded ignition timing and low compression ratio) cause EGT to go up.

Remember that CHT mainly reflects what’s going on in the cylinders during the power stroke when the cylinder is under maximum stress from high internal temperatures and pressures, while EGT mainly reflects what’s going on during the exhaust stroke after the exhaust valve opens and the cylinder is under relatively low stress.

High CHTs often indicate that the engine is under excessive stress, which is why it’s so important to limit CHTs to a tolerable value (no more than 400°, preferably 380° or less). By contrast, high EGTs do not indicate that the engine is under excessive stress, but simply that a lot of energy from the fuel is being wasted out the exhaust pipe rather than being extracted in the form of mechanical energy.

For instance, a 1972 Cessna 182 with an O-470-R engine will typically have indicated EGTs that are 100 degrees hotter than those seen in a 1977 Cessna 812 with an O-470-U engine. The -R has a relatively low 7.0–1 compression ratio because it was certificated for 80-octane avgas, while the -U engine has a much higher 8.6–1 compression ratio because it was certificated for 100-octane. Because the high-compression -U engine is significantly more efficient at extracting heat energy from the fuel, it wastes less energy out the exhaust and this its EGTs are cooler (despite the fact that the -U engine is much more highly stressed than the -R).

High EGTs do not represent a threat to cylinder longevity the way high CHTs do. Therefore, limiting EGTs in an attempt to be “kind to the engine” is simply misguided.

Diff versus Gami spread

Right behind the “high EGTs are bad” myth is the “identical EGTs are good” myth. Many pilots believe incorrectly that a flat-topped graphic engine monitor display (with all EGTs equal) is the mark of a well-balanced engine, and that unequal EGTs are a sign that something is wrong. This common misconception tends to be reinforced by digital engine monitors that display a digital “DIFF” showing the difference between the highest and lowest EGT indication.

As illustrated by the earlier anecdote about the front cylinders on my Cessna 310R, difference between absolute EGT values are both normal and benign. It is not uncommon for well-balanced fuel-injected engines to exhibit EGT spreads of 100 degrees, and carbureted engines often have spreads of 150 degrees or more. In fact, as shown in Figure 4, EGT spreads are usually smallest near or just rich of peak EGT (the worst place to operate the engine), and often significantly greater at leaner or richer mixture (that are much kinder to the engine.

The mark of a well-balanced engine is not a small EGT spread (“DIFF”), but rather a small “GAMI spread”– defined as the difference in fuel flows at which the various cylinders reach peak EGT. Ideally, we would like to see this peak be no more than about 0.5 gph (or 3 pph). Experience shows that if the GAMI spread is much more than that, the engine is unlikely to run smoothly with LOP mixtures.

It’s all relative

The only important thing about EGT is its relative value: how far below peak EGT and in which direction (e.g., 100 degrees ROP or 50 degrees LOP). Absolute values of EGT (e.g., 1,475 degrees) are simply not meaningful and are best ignored. There is no such thing as a maximum EGT limit or redline, and trying to keep absolute EGTs below some particular value¾or even worse, leaning to a particular absolute EGT value¾is simply wrongheaded. Don’t do it. If you must fixate on those digital engine monitor readouts, fixate on something important, like CHT.

By Mike Busch|2024-09-18T14:00:14-04:00July 18, 2023|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|2024-09-18T14:03:20-04:00July 18, 2023|Aircraft, Articles|Comments Off

Aircraft Oil: Go Ashless or Go to the Repair Shop

Any of you flying air-cooled aviation engines should be familiar with the phrase AD oil or Ashless Dispersant oil. These are special oils designed for aircraft engines and their use will help protect again pre-ignition and detonation. But what exactly does ashless mean?

Basically when you burn the oil, it will completely disappear and leave no significant ash behind. For non-ashless oils, most of the ash left behind when you burn the oil is from additives in the oil itself. Automotive and diesel engine oil designs for liquid cooled engines will contain a lot of additive that are is ashless and it’s not a problem in those types of engines because they don’t typically run hot enough to burn the oil.

Air-cooled aircraft engines are another story. In those engines, it is common for certain engine parts to reach temperatures at which the oil can burn. If a non-ashless oil was used, then deposits from the left-over ash could end up sticking to valves and ring lands. Those deposits could lead to hot-spots in the combustion chamber and those commonly cause pre-ignition.

The dispersant part of AD means there is additive present that is meant to hold solids in suspension so then can either be filtered out, or drain when the oil is being changes. If this additive is not present, it would be a lot easier for sludge to build-up in your engine during normal operation.

While the use of ashless oils won’t necessarily prevent all of the problems associated with pre-ignition and detonation, it is one easy way to help protect your engine from these dangers.

By Jim Stark|2024-09-18T14:04:07-04:00July 18, 2023|Aircraft, Articles|Comments Off

Fuel Contamination in Aircraft

Here in the northern latitudes autumn brings uncertainty about what to expect from the sky and wind each morning. Rain and overcast skies are frequent but counterbalanced by days when clear blue skies are accented with yellow sunlight that reflects the fall leaves and warms the spirit.

Those who fly in the winter months generally count the experience with mixed feelings. Cold toes and fingers are a certainty. So are hard-to-start engines and batteries that lack enthusiasm. But once the engine finally fires and the first BTUs of heat start filtering into the frosty cabin air, the whole experience can bring a smile to the face of the most determined pessimist.

The brakes may be stiff but they still work. Once you get to the run-up area, most of the breath-laden frost has cleared from the windscreen and you can stow the gloves. The sun streaming into the cabin does as much to warm things as the manifold heater. On lift-off, the rewards of winter flying come back to remind you why you do this in the first place. The prop bites the crisp air with authority. The dense air brings lift with a rush. Like the counter guy at the FBO said, “There’s a lot of lift going on out there today!” Indeed!

Understanding the gas

There are several reasons engines are hard to start in the cold. Parts are machined to operate easily in concert when they are at operating temperature. The further you get from that temperature—either hot or cold—the more interference there will be between interfacing parts. Poorer fitting parts increase internal engine friction.

Air-cooled aircraft engines typically run on SAE 50W oil when at operating temperature. Cold, the oil has the properties of molasses. The oil pump resists rotation. The oil resists being pushed around. The oil that starts out in the oil cooler may still be there when you land. If the surface air is cold, you know the air at altitude will be colder. It is not unusual to find the oil temperature at the bottom of the green arc, if it gets into the green at all. Pity the poor battery that has to coax all this reluctance into motion.

Mixture matters

Another reason cold engines are hard to start is the gas/air mixture is incorrect. The fuel system, whether carbureted or injected, is set up to operate at a given mixture for normal temperature operation, usually fifteen or sixteen parts air to one part gas. Cold cylinder walls condense gas from the mixture, causing the gas that’s left to become lean—far too lean to initiate normal combustion. Accordingly, engines of all types have an enrichment device to compensate when cold. For liquid-cooled land-based engines, a choke in the carb throat or an extra injector enriches the intake air. Both types of systems usually shut off automatically as the cylinder walls warm up, which usually doesn’t take long.

Air-cooled aircraft engines, on the other hand, have a primer. Even when carbureted, they won’t use chokes or have the acceleration pumps that are common for car and truck engines. Many aircraft engines that fire readily on zero or minimal prime on a sunny, warm day won’t even consider firing without prime in winter. If one squirt of prime works in July, it may take four to six in January.

If there is moisture in the first gasp of cold air that gets sucked into the cylinders, it can frost the plug electrodes. If this happens, no amount of priming or cranking (or swearing) will make any difference. The engine won’t fire until the frost is melted or otherwise eliminated.

Raw gas that condenses on the cold cylinder walls gets scraped down into the oil by the beveled oil control ring(s). It will mix perfectly with oil, so there is no good way to get it back out again unless you cook it out with heat and agitation (otherwise known as flying). If it is cold enough at the altitudes you fly, the gas from priming may still be in the oil when you land.

Problem or not?

But does gas in the oil really hurt anything? Hardly. It will cause a lower viscosity, but that may be an asset rather than a problem. There were WWII radial engines operating in the frozen north that were designed to inject gas into the oil before shutting down. The gas thinned the oil so that the engine could be cranked over in the morning with less resistance. After the engine got back to operating temperature, the oil returned to being an SAE 50 or 60W once the gas was distilled back out of the oil…sort of an automatic multi-weight oil before multi-weight oils were invented.

To say we find a lot of gas contamination in winter oil samples would be an understatement. We usually mention it because gas in the oil can show a fuel system problem. But that is rare in aircraft engines. We find a lot of moisture in winter oil samples too, and it goes into the same category as the fuel. Unless your aircraft engine is liquid-cooled, we don’t think the moisture is any more a problem than the gas.

When taking your sample, it’s ideal to have the oil warmed to operating temperature first, though if that’s not possible it’s best to just take the sample cold and not start the engine at all. Starting the engine but not flying it can introduce even more gas into the system. We realize an FBO mechanic won’t have the option of taking your airplane for a couple of turns around the pattern, even if he or she was qualified to do so, before draining the oil. Consequently, we turn up volatile gas and often moisture in your winter samples if you are flying in the cold. It only rarely points to a problem.

By Jim Stark|2024-09-18T14:04:47-04:00July 18, 2023|Aircraft, Articles|Comments Off