Oil pressure low on start up?

[francesco.doenz]

What is the explanation or significance of a low oil pressure on startup with a cold and dark cockpit? Sometimes, when I start up the oil pressure is below the green, but comes up when the throttle is pushed?
I understand the logic of a high oil pressure on startup, but a low...?

[Pistonpilot]

If your oil is warm, the pressure will be low at low throttle settings. The oil is less viscous and so flows easily with less pressure. So, if you shut down the aircraft for a few minutes after a flight, then start it back up, the oil pressure will be below the green arc until you push the throttle up and apply power.

If you have cold oil, such as if you stored the aircraft for a while in a cold area before starting it up, the oil pressure will initially be high even at low power settings. The cold oil is more viscous and thus more resistant to flow until it warms up.

Additionally, the weight/type of the oil you use affects this. If you are using a "winter weight" oil (which is less viscous at any given temperature than a "summer weight" oil) and the ambient temperature is high, it will flow too easily, and may consistently show unusually low pressure even when warmed up.

There are other factors as well (such as the diluting system), but all these factors mean that, depending on the circumstances of your flight, you may see different initial indications on startup and the initial application of power.

Additionally, using the 'cold and dark' function or re-filling your oil messes with the temperature of the oil sometimes. In some A2A birds, adding new oil adds pre-warmed oil, which means that even though the airplane is very cold, the oil is all warmed up before they put it in and thus flows well and shows a lower pressure at idle. It's common practice in below-freezing weather to pre-warm oil for big piston engines. Jets tend to be a little less sensitive to initial oil temperature, especially at idle.

Hope this helps!

-Ian C

[Scott - A2A]

It takes a while for the oil to pressure up when it first runs through the system.

Scott.

[Piper_EEWL]

What rpm is your engine running at what temperature is the oil when you notice this? Also is the oil pressure stable at a low value or is it still rising?

[francesco.doenz]

Thanks very much for all your inputs!
Yes, when the oilpressure is low at startup the oil temp is in the green already, and the oilpressure gets into the green also when the RPM augment, so I think it's like the engine remembers the temperature of the previous flight, and I guess it is expected!

[Piper_EEWL]

Yes probably. To test this you can hit Cold&Dark in the Shift+3 menu and "reset" the plane to a cold state. That should also bring the engine temperature down to ambient level. If you start the engine then the oil pressure should be rather high (unless of course the ambient temperatures are high).

Happy flying