Proficient Pilot: The outer limits

“Obviously correct,” he said. “But what about when oil pressure exceeds the upper redline?” I looked at him quizzically and had no answer. It was embarrassing.

“You should know,” he began, “that this can happen after starting an engine with very cold oil. Usually the pressure will decrease to normal as the oil warms at idle or very low power, but if it doesn’t, you might have other problems. These can be an improperly set pressure-relief valve [easily adjusted by a mechanic] or oil of the wrong viscosity. In either case, the problem must be resolved before takeoff.”

He also explained that excessive oil pressure can strain the weakest link in the oil system (often the oil cooler or a hose) and cause a rupture. At such time, the oil is pumped overboard—and you know what happens then.

Fascinating, I thought. I had never learned anything about the upper redline on an oil pressure gauge, only the lower one. This led me to do some digging, and I discovered that there was a lot about the oil system that I did not know.

For example, I learned that an indication of excess oil pressure in flight can be caused by an instrument malfunction, but not necessarily. It also can be caused by an oil restriction in the engine. The pilot should attempt to reduce oil pressure by operating at reduced power. If the problem continues, land at the nearest suitable airport. In the same vein, oil-pressure surges can cause engine damage, but these also usually can be held in check by operating at reduced power.

One of the most traumatic in-flight indications is the total loss of oil pressure. Although this indication can be caused by a failed gauge or an obstruction in the pressure-relief valve, it is most likely the result of a depleted oil supply. Oil pressure gauges rarely fail, so such an indication must be taken seriously. If flying a single-engine airplane, find a place to land as soon as possible. Most engines can develop oil pressure with as little as two quarts remaining, so if the gauge indicates zero, there likely is nothing left in the oil reservoir.

Some sources claim that a total loss of oil can be confirmed by a concurrent rise in oil or cylinder-head temperature. Engine manufacturers say, however, that such a confirmation will not be available if oil depletion occurs rapidly, such as when an oil line ruptures. There may be insufficient time for an oil-temperature rise to register prior to engine seizure. When all oil pressure is lost, there may actually be a decrease in oil temperature, as oil ceases to flow past the temperature-sensing probe in the engine.

In other words, a pilot should not rely on other indications to confirm the total loss of oil pressure, nor will he have much time to ponder the problem. An engine without oil and operating at cruise power will seize as a result of extreme friction in about 30 seconds. Although the propeller will come to an abrupt standstill, the shock of this should not damage the engine mounts or airframe (but do plan on buying a new engine).

Noticing a loss of oil pressure, a pilot should reduce power as much as practical to prolong engine operation. At idle, for example, the engine might run for five to 15 minutes, depending on engine condition. This might give the pilot of a single-engine airplane enough time to find a landing site—it does not need to be an airport—and possibly have a few seconds of reserve power available for emergency use during approach and landing.

Does a fluctuating oil pressure gauge signify an imminent loss of pressure? Probably not. I have been told that this usually indicates either a malfunctioning instrument or an improperly seated thermostatic bypass valve, an easily correctable problem (on the ground).

There is much to learn about our engine from the behavior of an oil pressure gauge. I have also discovered that the same is true about the information conveyed by other engine instruments. Unfortunately, we too often take these gauges for granted, and may not always know how to interpret their silent yet critical messages.

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