Your engine’s talking to you. Do you speak its language?
What happens under the cowl is a little mysterious to many pilots and owners. This is partly the fault of the airplane manufacturers who sometimes make cowlings that are hard to remove, thereby discouraging frequent checks of the engine compartment. In fact, the only time the cowl comes off of a typical airplane is when the oil is changed or during the annual. And at that point, it is likely to be only a mechanic who gets to see the engine in its birthday suit.
Luckily, there are several ways to determine the health of your engine through that little door that Mr. Cessna and Mr. Piper did put in their cowlings—the oil door. If you are an individual owner or the frequent pilot of a rental ship, the oil’s condition and the engine’s consumption of oil can tell you a lot about what’s going on inside.
Sight, smell, sound, and touch
Unlike auto mechanics, who simply plug a car into a computer and wait until it tells them what’s wrong, airplane mechanics and owners must use a sort of black art in troubleshooting aircraft engines. The human senses of sight, sound, smell, and feel all come into play here. The good news is that you don’t need much in the way of specialized equipment and expensive computer-diagnostic tools. The bad news is that it takes some talent to discern what’s acceptable and unacceptable.
When you check the oil, note its color. How long was it after an oil change? Oil that gets black soon after an oil change is typical of an engine with a lot of blow-by—the result of a poor seal between the piston rings and cylinder walls that allows combustion gases to "blow by" the rings and enter the crankcase. Combustion gases eventually mix in with the oil, which does its best to hold the mixture in suspension. This black mixture, which was golden motor oil a few hours ago, is becoming saturated with combustion by-products such as water and lead sludge from unspent fuel that blew by the rings. This acidic mixture is not good for an engine and can only be removed by changing the oil more frequently. These by-products can’t be caught by the filter, since it typically catches only particulates.
Ben Visser, the senior staff research engineer for Aeroshell Products, states that there are some caveats to judging an oil by its color. "In the old days when we were running straight mineral oil, the oil stayed clean and the engine got dirty," said Visser. "Today’s modern ashless-dispersant oils are designed to hold dirt in suspension so that the oil gets dirty and the engine stays clean." Although the oil may be black, it hasn’t necessarily lost its lubricating qualities.
Check for moisture on the dipstick. Moisture is a sign that either the oil isn’t getting hot enough or the crankcase breather system is malfunctioning. Visser states that oil temperature should be 170 to 190 degrees Fahrenheit so that the oil—by the time it has traveled all the way through the engine—is hot enough to boil away any water that has accumulated in the system.
Next, smell the oil on the dipstick. If it’s a pungent smell that resembles exhaust more than the usually mild smell of oil, then you have more evidence of top-end problems. Keep in mind that oil with 25 or more hours on it may smell this way too; it can probably be considered normal. But if oil with only five to 10 hours on it smells this way, you can check off the next box on the checklist for top-end problems.
To confirm your suspicions, a compression check of all the cylinders should reveal how good a job the rings are doing at sealing combustion gases out of the bottom end. Compression checks are done by filling the cylinder with 80 psi of air and measuring how much of that air is held in by the top end of the cylinder. The more air it holds, the better the seal of the rings and valves. If all of the cylinders score a low compression reading, it’s time to use your sense of hearing (and maybe even touch) to find out where the air is going. You might find your mechanic opening up the oil filler or dipstick and sticking his ear up to it. If he hears or feels air escaping out of it, he knows that the rings are doing a poor job of sealing against the cylinders. The oil breather is another air-escape path that you can stick your ear up to.
Check your belly
Speaking of the oil breather, it’s not a bad idea to glance at the belly of the airplane before every flight. Excessive oil streaming from the oil breather’s outlet can be another indicator of blow-by. Like the compression check but amplified greatly, a running engine suffering from blow-by pressurizes the crankcase. Think about it: If combustion gases are blowing by the rings into the bowels of the engine instead of going out of the exhaust pipe, the pressure has to escape from somewhere—usually through the breather. If your breather has become another exhaust pipe for the engine, you’ll notice that the belly of the airplane gets quite ugly only a few flight hours after a wash job. Continental Service Bulletin M89-9 details a method of using an airspeed indicator to measure crankcase pressure while the engine is running. In most airplanes, you can plumb an airspeed indicator via a hose to a point on or near the crankcase, such as the breather tube connection at the oil filler neck. If the registered "airspeed" is too high, you’ll probably be faced with some cylinder work. Contact Continental at 334/438-3411 or www.tcmlink.com for a copy of the bulletin.
Airplanes equipped with an air/oil separator can hide some of these symptoms, since the separator is designed to send oil from the breather back to the crankcase. In addition, the breather can work as a condensing unit, collecting water and oil and sending the mixture back to the crankcase—or, worse, freezing solid in extremely cold temperatures.
Of course, oil leaks also contribute to an ugly belly, and it may be hard to determine if the oil on the belly is the result of a genuine leak or if it’s from the breather. In this situation, a thorough washing and drying of the engine, the interior of the cowling, and the belly will hopefully reveal the source. Often a "leak" may be the result of a careless pilot who spilled some oil when adding the last quart to the engine. If it’s golden-colored oil that’s running out of the bottom of the cowling, you’ve probably found your culprit. If it’s dark-colored oil, it’s likely coming from within the engine. Remember that oil is like blood and only a little can look like a disaster. With this in mind, oil leaks generally have to be really bad before it affects the engine’s apparent oil burn.
The best sign?
Arguably, the most telling sign of an engine’s health is how much oil it uses. Typically this is measured in hours per quart in the four- or six-cylinder flat engines that most of us fly behind. Armed with this knowledge and the trend of what the engine has done in the past, you can give yourself and your mechanic a good look at what’s happening inside. If the trend takes a sudden turn for the worse, a more thorough investigation should begin.
How much oil is too much? According to Lycoming, the maximum oil consumption limit for all of its engines can be determined by the following formula: (0.006 x HP x 4) ÷ 7.4. The result should equal the max oil burn in quarts per hour. For example, if your Piper Arrow has a 200-hp Lycoming engine, the formula says that it can burn a maximum of 0.65 quarts per hour. That said, if your Arrow burns less than two quarts on a three-hour trip, Lycoming says your engine is within limits.
Continental’s formula is a little harder to work out: 0.006 x HP ÷ 100 = result x horsepower that you intend to use in cruise = pounds per hour of oil. An example would make this easier: Let’s say you have a 285-hp IO-520 and you typically run it at 65-percent power (185 hp). Plug the numbers into the formula, and you end up with 3.16 pounds per hour of oil. Since oil weighs roughly 2 pounds per quart, the net result is 1.58 quarts per hour. A Continental representative explained that the formula applies mostly to an engine on its first run after manufacture. In general, the company advises its customers that oil burn worse than one quart in two to three hours is grounds to start taking corrective action.
With the math out of the way, it’s important to know the baseline from which to start tracking oil consumption. A log that allows you to enter the amount of oil added—and at what time—is highly recommended. For an airplane flown by multiple pilots, set a policy to keep the oil level at a specified amount, say seven quarts. Service the engine with the same amount at the oil change or slightly more to account for what it takes to fill the empty filter, if equipped.
If not for your own information, the tracking of your engine’s oil burn will greatly assist your mechanic. If you can tell the mechanic that the engine has steadily trended from one quart in 10 hours to one quart in three hours, it would help far more than just saying, "It seems to burn more nowadays."
Cutting the filter
It’s a messy job, but someone’s got to do it, and it might as well be you. At each oil change, you should cut open the filter to see what’s circulating through your engine. There’s no better way to instantly find out whether or not your engine is making metal. Your mechanic can show you how to do it the first time but from then on, you’ll save a few bucks by doing it yourself.
First, you’ll need a filter cutter. The best we’ve used is made by Airwolf Filter Corporation (440/632-5136) and costs about $90. The filter cutter allows you to cut the metal can and remove the paper element from inside. Once the element is removed, you’ll need a set of snips to cut the metal seam where the pleated paper element is bound together and a serrated knife to cut the element away from the ends. Once these chores are done, you can unfold the pleats of the element and examine them with your eyes and a magnet. Bright, silvery slivers of metal are what you’re looking for among the other debris you’re likely to find. Confirm that the material is metal by using the magnet. If you find anything that looks fishy, bring it to your mechanic for further analysis.
Oil analysis
A more scientific way that oil can pinpoint internal engine distress is through spectrographic oil analysis. At the oil change, collect a small sample of the oil removed midstream (not the first or last oil out) from the engine, fill out a brief sheet of basic information, and send it out for analysis with a check for $12 to $25, depending on the company. Within a week or two, a report will return with a breakdown of what types of wear metals were found in the oil and in what quantity, measured in parts per million. The more savvy analyzers will even speculate as to the source of the wear metals based on their knowledge of aircraft piston-engine construction.
The key to oil analysis is the establishment of a trend. You can’t simply send in a sample when you think the engine is giving you trouble. A baseline of data must be gathered over a period of several oil changes. Once the baseline has been established, any deviations from it will be noted. It’s possible that oil analysis will pick up on trends before you’ll find pieces of metal in the filter, giving you warning of problems to come.
Sometimes oil analyses can be wrong and because of this fact, even purveyors of oil analysis recommend that you should cut the filter at each oil change. One oil expert says filter cutting will paint 50 percent of the picture and oil analysis will cover the other 50 percent. If you do both, then you’re doing about as much as you can to cover yourself.
Cross-checks
Of all of the ways that oil can provide clues to overall engine health, no one method can stand on its own. Oil consumption, compression checks, filter cutting, oil analysis, and overall oil condition must all be taken into consideration before you start tearing into an engine. If many or all of the signs are pointing to top-end distress, it would be a good idea to have the cylinders borescoped before committing to remove them, just to be sure you make the correct overhaul decision if an overhaul is needed.
Believe it or not, your engine talks to you, and one of the most obvious ways it talks is through the use of its lifeblood—the oil. Pay attention to all of the signs above, and it’s doubtful that you’ll ever have a problem that your engine didn’t warn you about.
Links to other articles about aviation oil and troubleshooting engine problems can be found on AOPA Online ( www.aopa.org/pilot/links/links0004.shtml). E-mail the author at [email protected].
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