Right, let's pick this up with the engine intake and the breather system — Page 300, Lesson 382

Right, let's pick this up with the engine intake and the breather system. We've just been looking at how the oil gets around the engine, but now we have to deal with a problem: any air pressure that builds up inside the lubrication system has to be allowed to escape. This pressure comes from leakage past the seals, or from the de-aerator tray. Now, if we simply vented that air straight to atmosphere, we'd have a problem — the air is carrying oil mist with it, and that oil would be lost to the atmosphere. Over time, the oil contents of the system would quickly diminish. So, to prevent that loss, the oil mist is vented through a centrifugal breather, and that breather is positioned in the accessory gearbox. The centrifugal action separates the oil from the air, keeping the oil in the system. Now, let's move on to the Full Flow Lubrication System. This is a different way of delivering oil compared to the pressure relief valve system we saw earlier. The key idea here is that the oil pressure pump directly supplies the oil feed jets, without any pressure relief valve in the circuit. Because there's no relief valve spilling oil back to the tank, the volume of oil being passed is less than in the pressure relief valve system. And that's a real advantage — it means we can use smaller pressure and scavenge pumps. Remember, in the pressure relief valve system, at high engine speed a large amount of oil is spilled back to the oil tank, which is wasted work. The full flow system avoids that. Let's trace the flow in Figure 19.3. The pressure pump picks up oil from the oil tank through a suction filter. It then passes the oil through a pressure filter and on to the distribution galleries. Now, across that pressure filter we have an oil differential pressure switch. This switch gives a warning if the filter becomes blocked — it senses the pressure difference across the filter. That warning is usually indicated at the ground crew servicing panel, and it's sometimes duplicated by a warning light on the flight deck. From the distribution galleries, one gallery takes oil up to an oil pressure transmitter and a low oil pressure warning switch. These are primarily there to give a warning in the cockpit if there's a malfunction in the oil system. Other parameters we see in the cockpit are oil quantity and oil temperature — and note, the oil temperature is measured as the oil leaves the oil cooler. And it's from this same gallery that oil is taken to lubricate all of the bearings in the accessory gearbox.