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Engine Gauges

Form and Function

Paying attention to the powerplant
Of all the things a learning pilot must assimilate during those first hours, taking a close read on the engine gauges usually falls near the bottom of the list. Flying the airplane and acquiring healthy aviating skills are more important than eagerly watching the engine gauges.

Eventually, though, it seems that every pilot aspires to move from the basic trainer to something more sophisticated. And the more complex the airplane, usually, the more attention its powerplant requires. So it's a good idea to learn what the engine instruments are telling you (and how and why) well before you must cope with them and an airplane commanding more of your attention than a simple single.

Arguably the most important gauge on the panel indicates the engine's oil pressure. The integrity of the oil cooling system is crucial to the powerplant's health. Oil is used both as a lubricant and a cooling medium in air-cooled engines.

In most trainers, the oil pressure gauge is mechanical and is connected to a port on the engine by a small-diameter oil line. A restrictor inside the line keeps oil loss to a minimum and reduces the amount of oil inside the cabin in the event of a line break. Inside the instrument, you'll find a curled-up Bourdon tube resembling a watch spring. As the oil pressure increases in the engine and thus in the tube, the Bourdon tube tries to unwind itself. This movement is translated by a link to the needle on the face of the instrument.

Some airplanes use electric oil pressure gauges. This type has the advantage of not needing oil-filled lines in the cabin; it has the disadvantages of high cost and sometimes fragile pressure transducers at the engine's oil-pressure port. In the event of an electrical failure, the mechanical gauge will continue to function, but the electric gauge will not.

Oil temperature gauges are also required for air-cooled engines, and most trainers use electric gauges for this task. An electrical system with abnormal voltage (caused by alternator failure or an improperly set voltage regulator) can skew the accuracy of electric temperature and pressure gauges. Generally, however, electric gauges have proven to be quite accurate.

That's not true of the tachometer, particularly the purely mechanical models used in most trainers. Notoriously sloppy and prone to calibration drift over their lifetime, mechanical tachs can read high or low (usually low) and may actually be showing higher-than-true numbers at low airspeed and lower-than-true figures at cruise. This is one of many reasons to make conservative fuel calculations. Many a pilot has been surprised by abnormally high fuel consumption that resulted from incorrectly set cruise power, stemming, in turn, from an inaccurate tachometer. Here's a hint: If the airplane is moving faster than it ought to be, there's an excellent chance that the tach is out of calibration.

All of the main engine gauges are required to have limit and normal operating markings. A green arc denotes the normal range, with yellow arcs or lines indicating the caution ranges and red lines showing the limits. All of the normal operating marks are based on cruise flight. At idle power, an oil pressure indication near or below the green arc isn't cause for alarm; the same reading at cruise power certainly is. Markings on the tach indicate the normal range for cruise; there's no reason that you can't idle, taxi, and even perform the runup outside of the green arc.

What if the oil temperature needle is over the red line? If the oil pressure is in the upper part of the green arc and you have already lowered the nose in climb (or, possibly, you are in cruise), wait for a moment to see what happens. In some cases, an electric gauge or transducer can fail to the maximum-scale side. Again, if the temperature indication stays high but you don't see a resulting drop in oil pressure and can't so much as smell a hot engine, there's a good chance that the temperature gauge is wrong. In any event, if you are in a good position to return to the airplane's home base, do so to have the anomaly checked out by a mechanic.

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