Understanding Engine Guages

What those dials and lights are telling you

By Joe Castanza

You're en route to Lincoln Park Airport for that juicy hundred-dollar hamburger. You check the engine instruments. Wait—something is horribly amiss. You have normal oil pressure, but high oil temperature. What does this mean? Or perhaps you see indications of low oil pressure with high oil temperature. Would that make any difference?

If you've even noticed these indications, you're already ahead of the game. The ugly truth is that many pilots do not pay adequate attention to engine instruments, especially these days, flying brand-new airplanes with numerous electronic distractions. But flying new airplanes does not preclude a problem; airplanes and people are not infallible. If they were, airplanes would not come with warranties, and the term pilot error wouldn't be in the FAA's lexicon.

Even for the seasoned pilot, engine indications can be confusing. But they don't have to be. Some indications simply mean that everything is OK, and those are the ones with a lot of green on both sides of every needle. Other indications are signs of trouble, how much trouble to expect, and how soon to expect it. The key word here is expect, and it makes the difference between your finding trouble in time to address it, and trouble finding you after it's too late to do anything about it—in other words, the difference between a precautionary landing and a forced landing.

"You don't fly the airplane," Chuck Yeager once said. "The airplane flies itself, you just tell it where to go." How true. When you interact with the flight controls, you aren't "flying" the airplane, you're "communicating" with it. And just as you communicate your needs to your airplane, it uses its own language to communicate what it needs from you. It tells you when it needs more rudder, more airspeed, and more power; you tell it what to do. As long as both parties listen to each other, things go smoothly. In addition to the way an airplane sounds and feels to the pilot, the airplane tells you about its health through the engine instruments.

Here are a few things airplanes say: "I'm fine"; "I need a richer mixture"; "I'm going to be sick"; "I'm dying." What is your airplane telling you?

Two of the most important engine instruments are the oil temperature and oil pressure gauges. They are the vocabulary—the "airplaneese"—that your airplane uses to describe its health. Here's a quick lesson in the language:

High oil temperature with normal oil pressure

This is a tough one, and it presents a mountain of possibilities. There is really no way to tell what is going on without further investigation. The best policy is to head for the nearest airport—just in case. You can always try to work out the problem en route. This way, the worst-case scenario is that you erred on the side of safety. High oil temperatures in flight may indicate something as innocuous as a hot day or poor cooling during a long climb to altitude. Some airplanes just run a little hotter than others.

Something may be wrong with the electrical system. It sounds strange, but in some older aircraft, an electrical short may cause the oil temperature gauge to behave like an ammeter—the higher the electrical load, the higher the indicated oil temperature. Try turning off unnecessary electrical equipment, one item at a time, and remember to advise ATC, if appropriate. If this doesn't work, check the circuit breakers and recycle the master switch. On the other hand, there could be a serious engine problem. Ironically, indications of high oil temperature with normal oil pressure are common—and they are commonly the result of pilot error.

Maybe you missed something during the preflight inspection, such as failing to remove a cowl plug, or you made some sort of in-flight mistake, or something went awry during planning. When you performed the preflight inspection, was there oil leaking from the bottom of the cowling? There shouldn't have been. When you checked the oil, was there an adequate amount of oil indicated on the dipstick? There should have been. Did you even remember to check the oil? You better have, because in most reciprocating aircraft engines, high oil temperatures usually result from an inadequate oil supply. Engines rely on lubricating oil for controlling internal heat, and without it you're in trouble.

The airplane might be telling you that you have been using too much power with the mixture too lean—not enough fuel in the fuel—air mixture. That's also pilot error. Normally, you shouldn't lean the mixture when using more than 75-percent brake horsepower (BHP). This can cause detonation: an explosive ignition of the fuel-air mixture, and eventually, preignition. That's when parts inside the engine cylinder heat to an incandescent state, causing the fuel to ignite prior to normal ignition and forcing the cylinders to fire out of sequence. These conditions put a serious strain on the engine, will cause a loss of power, and may cause engine damage or even failure. Check in the performance section of your POH to find out what constitutes 75-percent BHP for your airplane and notice that this figure changes with pressure altitude and air temperature.

There are a variety of engine instruments that are useful in the mixture-leaning process. Every airplane has a tachometer. To use it for leaning an airplane engine turning a fixed-pitch propeller, set the POH-specified cruise power setting, lean the mixture until the rpm peaks, and then lean it just a bit more until they drop slightly—15 to 50 rpm, depending on airplane make and model. If available, exhaust gas temperature (EGT) and fuel flow gauges make leaning any airplane a breeze. Generally, to use the EGT gauge you would lean the mixture until the temperature peaks, then enrich it until the temperature is 25 to 75 degrees Fahrenheit lower. Fuel flow gauges are a little bit easier to use. You simply set the power as specified in the POH for your pressure altitude and air temperature and then lean the mixture until you have the POH-specified fuel flow indication. If you have a cylinder head temperature (CHT) gauge on board, it will make a valuable reference instrument; you'll see higher indications with leaner mixtures, and that's fine. In addition, CHT is great for confirming oil temperature indications; if both oil temperature and CHT indications are excessively high, the airplane is saying "Hey, I'm not kidding—I'm hot."

Use the proper leaning procedure for your airplane, as specified in section four of your airplane's POH. Just remember that, regardless of the leaning procedure specified or used, if the engine begins to run hot or rough, you've over-leaned the mixture. Here's a way to determine whether you are running your engine too lean: If your aircraft has a fixed-pitch prop and is equipped with carburetor heat, set your power and lean the mixture until you believe it's set properly. Then activate the carburetor heat and note the indication on the tachometer. If the rpm increased, you've over-leaned the mixture. The rpm increased because you have enriched the mixture—increased the amount of fuel relative to the amount of air—by heating the air entering the carburetor. When the air heated, it became less dense, resulting in less air for the same amount of fuel. This enriched the mixture to a point closer to the optimum mixture for that particular power setting. So, here the airplane is saying, "enrich the mixture." Try this technique on the ground first to get a feel for how it works.

Another common cause for high oil temperature indications with normal oil pressure is the use of too high a power setting at too slow an airspeed. This can happen during an extended performance climb at VX, best angle of climb airspeed. You are overworking the engine, the airspeed is too slow to provide adequate airflow through the engine cowling for cooling, and the oil is trying to compensate. The airplane is saying, "I need a break." If an engine runs too hot for too long, the oil pressure will eventually decrease. This happens because of thermal breakdown—when oil overheats, it begins to break down and lose its lubricating capacity. Now you've really got a problem: high engine temperatures can cause a loss of power and eventually engine damage or failure.

Treat any engine that is overheating, detonating, or preigniting in exactly the same fashion: Smoothly reduce the power, gradually enrich the mixture to full rich, and lower the nose slightly to increase the airspeed. Reducing power will reduce the amount of heat being generated. Enriching the mixture adds additional fuel that will absorb some of the heat. Lowering the nose and increasing the airspeed will allow more air to flow through the cowling for cooling. If there are cowl flaps, open them. Hold level flight—don't descend—for two reasons: First, you don't want to shock-cool the engine; that is, cool it too quickly by diving. This may cause the cylinder heads or engine casing to crack. Second, conserve your altitude just in case you actually do have a problem, because you will come to miss altitude very quickly when you really need it and don't have it.

Normal oil temperature with low oil pressure

This one is a littler easier to deal with. The airplane is saying, "I'm going to be sick." Low oil pressure is the problem here; it's a bad sign, a warning of the onset of a few possible problems, the least of which is a gauge malfunction. There may be an obstruction in the oil line. But here's the really bad news: An oil line, gasket, or seal may have blown, and the engine may be losing oil. This will invariably lead to a high temperature/low pressure issue, and that's the worst. Either way, there's an oil circulation issue and without that oil, engine failure is imminent. Here's the way to treat this one: Assume that your gauges are right and head for the nearest airport; if one gauge is wrong, the worst that will happen is that you will make a safe precautionary landing. If you see the oil temperature begin to rise further, or if this is accompanied by decreasing oil pressure, don't panic. But do expect a partial or full loss of power, because now the airplane is saying, "I'm dying." Look for a place to land safely, and go to it immediately.

Normal oil temperature with high oil pressure

This one is rare. It could be the result of an obstruction in the oil line, a malfunctioning oil check or regulator valve, a faulty gauge, or an electrical problem. The airplane is saying "Something's wrong, and I'm not sure what." This is probably not a life-or-death situation right now but there is still a circulation problem, so watch the gauges and proceed with caution to the nearest airport. Expect the oil temperature to rise.

High oil temperature with low oil pressure

Here are two gauges indicating serious trouble. The airplane is saying, "I'm dying." Don't waste time trying to work this one out because both gauges can't be wrong. The engine is definitely about to give up the ghost. Find a spot to land and treat the engine as though it has already failed. Get the approach right the first time—you may not have an engine for a go-around. There is a noteworthy pattern here: Low oil pressure or high oil temperature will eventually result in a high oil temperature combined with low oil pressure, and that's the really bad one. High oil pressure combined with normal oil temperature is a rare, usually erroneous indication, but don't bet your life or the lives of others on a gauge malfunction.

There's a common theme here as well. In each situation, the best advice is to land soon. There's a very good reason for this: Any engine indications that are not normal are just that—they're not normal. If you can address the problem, do so. You may have more options with certain indications than others; you may even have more time. Just don't count on it. A stitch in time may save you a lot of stitches when it's over. Remember to listen to your airplane—after all, it's been listening to you. If something doesn't look right, expect a problem, and you won't ever be caught off guard. Don't panic; you won't ever just fall out of the sky. If the engine is still running, then you still have an airplane. If not, then you at least have a glider. Continue to fly either one for as long as it will fly.

Meanwhile, remember to look outside. And remember those three little words that are even more important than "I love you": Aviate, navigate, and communicate. In that order. Check into any problems inside the cockpit, but your primary responsibility is for what is going on outside. Make sure that you are still flying the airplane. Your primary responsibility to yourself, passengers, loved ones, and strangers on the ground is to keep it in the air until otherwise necessary. Don't get caught in the same trap as the airliner crew that crashed into the Everglades in 1972 when the pilots were distracted by a panel problem. The gear was down and locked, but one hundred people perished because a $10 landing gear position light burned out.

Learn your airplane's language, and converse with it. A smart pilot on the ground is a safe pilot in the air, and a pilot who listens to his or her airplane will be a smart pilot on the ground again.

Joe Castanza is a professor of physics at the University of Medicine and Dentistry of New Jersey. He is a CFI and has approximately 1,500 hours of flight time.

Illustration by Cathy Gendron