Cessna 172: at which RPM should the oil pressure reach the green arc?

Question

I am part of a team developing a Cessna 172 P model for FlightGear, an open source free flight simulator. We are now trying to improve the oil pressure and temperature simulation, and we have a question concerning the behaviour of the oil pressure at startup. The oil pressure gauge in the model P ranges from 0 PSI until 115 PSI, with a green arc region between 50 PSI and 85-90 PSI. What we are trying to figure out is in which circunstances the needle should be in the green arc region.

I have checked many different checklists for the 172 and many state that the throttle should be at 1000 RPM for the oil check, so that the oil pressure should be at least 50 PSI. On the other hand, the 1982's POH does not state that the oil check should happen at a particular RPM; it only says that the throttle should be OPEN 1/8 INCH on p. 4-7. But the POH is quite confusing concerning this issue, since it simply says Oil Pressure -- CHECK but not that the oil should be on the green arc. To add more confusion to it, on p. 4-12 it says:

After starting, if the oil gauge does not begin to show pressure within 30 seconds in the summertime and about twice that long in very cold weather, stop the engine and investigate.

So nothing again about the green arc or 50 PSI, just about the gauge being alive. This is perhaps the reason those checklists include the 1000 RPM figure, perhaps the engine is not able to give 50 PSI or more of oil pressure when at idle (aprox. 750 RPM in the case of our model).

TL;DR

• in the Cessna 172P, at which RPM should the oil pressure be equal to or above 50 PSI?
• what is the procedure for oil pressure check, should it be done at 1000 RPM or at idle throttle?

Answer 1

Your answer will be elusive. The problem is that oil pressure is a function of the oil, oil temperature, time, engine pressurized oil draw (the "wear" of the engine), and RPM.

I am confident a model could be created using the above parameters, and possibly one or two more. However, you are probably not interested.

I have about 8000 hours of instruction given in C172M, which is a different engine. With 100 weight oil, at 20C or higher, I teach that oil must be moving up within 5 seconds, and with 1000 RPM it must be approaching or in the green within 10 seconds or there is an engine shutdown.

In contrast, with 80W oil, at 32F (we require preheat if engine temp is below 32F and the aircraft is at home base), I teach that the oil pressure must be moving within 5 seconds and in the green within 15 seconds.

These times are on our customized checklist, and do not address cold weather ops although they are quite common for us.

We will not start taxi until the oil is in the green arc. We always look for movement of the oil gauge within seconds after the engine start. We bring the engine above 500 RPM, but not necessarily to 1000 RPM waiting for the oil to rise up. A factor in not going to 1000 RPM immediately is that the engine wear without full flow on a journal bearing is the square of the surface velocity. On the other hand, the pump volume is nearly linear with respect to the rotational velocity.

Also, note that we use straight weight oil 90% of the time. Multivis oil will behave differently.

To address your two questions, I propose you use the following: -1000 RPM for oil check all the time

500 RPM idle typically yields 30 to 35 PSI

Answer 2

In my (unofficial) experience, Oil Pressure moves slowly, and definitely depends on the engine needing to warm up.

So the early checklist is just to make sure that the Pressure is alive and increasing, and appears to be headed for the green-arc. (If it isn't increasing, you are probably just about out of oil and need to shutdown immediately!)

Generally, I expect the pressure to be in the green arc by the time I'm in the run-up area, doing my runup checks at 1800 RPM. (Typically about 3-5 minutes after ignition). When I throttle back from 1800 RPM to an idle of about 700-1000, the pressure drops to the bottom of the green area, maybe a bit under it.

All of that is unofficial, and my instructors never really hammered it home much more than the checklist says.