First, the core problem — Page 56, Lesson 72

Let’s pick this up right where the oil system left off, because this passage is about what happens when the oil gets too viscous — too thick — and what that does to the engine. First, the core problem. When the oil is viscous, it does not flow correctly through the engine. Now, here’s the key point I want you to hold onto: an important task of the oil is to cool. So if the flow rate drops, the oil can’t dissipate the heat being generated in the engine. The result is that the oil temperature rapidly rises — but — and this is the subtle part — that rise is only local, at the point of sensing. The majority of the oil is still cold. So you have a hot reading at the sensor, but the bulk of the oil is cold and thick. Now, how do we fix that? To overcome this, the oil cooler flaps should be closed. That sounds counterintuitive — closing the cooler flaps will initially increase the temperature. But the reason is that closing the flaps improves the flow, particularly through the cooler, and then brings the temperatures down. So the sequence is: close the flaps, temperature rises briefly, flow improves, and then the temperature settles down. That’s the corrective action. Next, let’s talk about why the oil gets consumed at all. As the oil lubricates the moving parts of the engine, it comes into contact with the combustion gases. The sealing of the valves and pistons is not 100% — there’s always some leakage past them. As a result, some oil gets burnt, and the engine therefore has an oil consumption rate. Ignoring external leakage, that consumption rate varies between engines. For a light aircraft, a typical figure is around 1 pint per hour. And here’s the diagnostic point: a consumption rate greater than that would indicate wear in the engine. So that 1 pint per hour is your baseline — anything above it suggests the engine is wearing. Now, the checking procedure. The oil contents should always be checked prior to flight. But the timing depends on which sump system the engine has. If the engine has a Dry Sump system, the contents should be checked immediately after the engine has stopped — realistically within a few minutes of shutdown. Why? Because that ensures the tank contents are recorded accurately before the oil migrates under gravity down into the engine sump. Large piston engines have oil tanks fitted with a check valve, which is underneath the oil tank. That valve closes under spring pressure, or by an electrically operated actuator, on engine shutdown. The closing of that check valve prevents oil migration into the sump. So in a dry sump, you check quickly, before the oil drains back. The Wet Sump system is the opposite. Here, a period of at least 15–20 minutes should have elapsed before the contents are checked — in a similar fashion to motor cars. So you wait, let the oil settle, then check. In any event, the oil level is checked after a period of time. So the contrast is clear: dry sump — check within a few minutes of shutdown, because the check valve holds the oil in the tank. Wet sump — wait 15 to 20 minutes, because the oil needs time to drain back down into the sump before you get an accurate reading. That’s the whole passage — the viscosity problem and its fix, the oil consumption baseline, and the two different checking procedures.