Surprise Surprise No Oil Pressure!!!
It’s a clear, cold day--five degrees Fahrenheit, to be precise. The ten-knot wind bites on your face as you go through your preflight. You hurry to get the inspection finished, so you can get into the cockpit, crank up the engine, and get some heat going.
Finished with the before-start checklist, you holler “Clear!” and hit the starter button. After a few seconds of cranking, your trusty engine roars to life. You stabilize it at a thousand RPM, then glance at the oil pressure gauge. The needle’s pegged at the bottom of the scale.
In your hurry to finish the outside preflight, did you remember to check the oil level? After a nervous half-minute, the gauge is still indicating zero pressure, so you disconsolately shut down the engine, get out, open up the engine cowling, and remove the dipstick. The oil’s almost up to the full mark. So why is there no pressure?
Many GA airplanes are equipped with a mechanical, as opposed to an electric, oil pressure gauge. In this type of gauge, a small capillary tube runs from the engine to the back of the instrument panel, where it’s connected to the gauge. This tube is filled with oil, and when the engine is running, the oil in the tube is pressurized. The gauge then measures this pressure and gives you an indication in the cockpit.
The problem with this type of gauge is that, in cold weather, the oil is very thick and viscous, and the engine’s normal oil pressure will likely not be sufficient to force oil through that tiny tube to the gauge. The engine has oil pressure--the gauge just doesn’t show it.
So, how do you deal with this problem? One way is to fill the tube with kerosene. Since kerosene is much lighter than engine oil, it will flow better in cold weather, giving you an accurate oil pressure indication. To do this, remove the tube from the airplane, and using a syringe or oil squirt can, pump kerosene in one end till all the oil is displaced and the tube contains nothing but kero. Reinstall the tube, run the engine, check for proper indication, then shut it down and check for leaks at the tube ends. Voila--now the gauge should jump right to life every time.
The other option is to replace your mechanical gauge with an electric one. Electric gauges utilize a sensor mounted on the engine which converts the oil pressure into an electric voltage, which is transmitted via wires to the cockpit. The voltage varies with the pressure, and the gauge’s needle moves proportionally, giving you an accurate indication. No tubes, no congealed oil, no worries.
So, the next time it’s five degrees outside, you can crank up your engine, watch the oil gauge spring happily to life, and get in the air!
By Ian Whittle. A&P