Piston Engines - Lubrication — Page 56, Lesson 71

Right, let's pick this up with the operational side of the lubrication system. Up to now we've looked at how the oil gets around the engine; now we're looking at what the pilot actually sees and does with that system. The first thing I want you to take on board is that the oil pressure and oil temperature gauges aren't just there to tell you the oil is fine. They give you a direct indication of the mechanical integrity of the engine. That's the key phrase — mechanical integrity. If the engine is wearing, if a bearing is failing, if something is breaking down internally, it will show up as a change in oil pressure or temperature before it shows up anywhere else. So the pilot's job is to read those indications and, critically, interpret them correctly. The gauge is only useful if you understand what a given reading actually means. Now, before we even get to starting, there's a specific problem that applies to radial and inverted engines. These are engines where the cylinders aren't all upright — on a radial they're arranged around the crankcase, and on an inverted engine the cylinders point downwards. Because of that geometry, oil can drain down and accumulate in the lower cylinders, sitting between the piston crown and the cylinder head. This is called hydraulicing. Here's why it's dangerous. Oil is incompressible. So if you try to start the engine and the piston moves up on the compression stroke, it's trying to compress that trapped oil. It can't. Something has to give, and that something is the engine structure — you can bend a connecting rod, crack the cylinder, or worse. So the procedure before starting these engines is to pull the engine through the cycle by hand, using the propeller, to make sure no hydraulic lock has occurred. And I want to stress this — before you turn that propeller, you confirm the magnetos are OFF. That's a safety-critical step; you never turn an engine through by hand with the ignition live. Now, moving to the start itself. When you start the engine, you should see positive oil pressure within a specified time. For the Piper Warrior, that specified time is 30 seconds. If you don't see pressure within that window, something is wrong. There's a subtlety here about cold starts. If the engine is started from cold, the oil is thick and viscous, so the pressure can read excessively high. That is normal — as long as it drops back into its normal operating range as the engine warms up. The oil thins as it heats, and the pressure settles down. So a high reading on a cold start isn't a fault; it's expected behaviour. The fault would be if it stayed high. And this brings me to a fundamental relationship: correct engine operating pressure and temperature are dependent on each other. They're not independent. High oil temperature gives low oil pressure — because hot oil is thinner, it flows more easily, so the pressure drops. So the oil pressure should be within its operating range at the correct operating temperature. You can't judge the pressure in isolation; you have to judge it against the temperature. Now let's talk about abnormal readings. If you see fluctuations in pressure — the needle bouncing around — that could be the result of low oil levels, or it could be a system fault. But the serious one is this: low pressure at normal temperature. If the engine is warm, at normal operating temperature, and the pressure is still low, that indicates imminent engine failure. That's not a warning to monitor; that's a warning to act. A landing should be made as soon as possible. Finally, there's one more cold-weather starting problem called coring. This is caused by the fact that cold, viscous oil does not flow correctly through the engine. The oil is too thick to circulate properly. And here's the point I want you to remember — an important task of the oil is to cool. The oil isn't just lubricating; it's carrying heat away from the engine. So if the oil isn't flowing, it isn't cooling, and you can get localised overheating. That's why coring is a problem, not just an inconvenience. So to tie it together: the gauges tell you about engine health, hydraulicing is a pre-start danger on radial and inverted engines, cold starts give high pressure that should settle, pressure and temperature are linked, low pressure at normal temperature means land immediately, and coring is a cold-start flow problem that compromises cooling.