Piston Engines - Propellers — Page 181, Lesson 235

We're in the middle of the propeller chapter now, and I want to pick up right where we left off — we've just looked at the single-acting propeller and its feathering spring. Now I want to walk you through what happens at the fine pitch stop, because there's a critical safety latch involved. When the engine is shut down, the feathering spring pushes the blades toward feather. But the blade angle is held at 5° — that's the fine pitch stop — and this prevents the blade from being pushed any further forward, into the feathered position, by that spring. So the blade rests at 5° of blade angle, held there against the spring. Now, when you start the engine, oil pressure builds up quickly. That oil pressure re-positions the propeller pitch-change piston onto the fine pitch stop, and the blades move to fully fine pitch. But here's where the centrifugal latch system comes in. As rpm is raised through 700, up to the warm-up setting of 1100 to 1200 rpm, centrifugal force disengages the latch system. So below that latch setting, the latch is engaged, holding the blades at fine pitch. Above it, the latch releases. And this creates a very important operational limitation. When centrifugal latches are fitted, it is not possible to feather a failing engine once rpm has fallen below the latch setting. So if the engine is failing and rpm drops below that latch speed, you cannot feather it. That's why it's critical to complete the feathering drill before rpm falls below the latch setting. You have to act early. Now let's move to the double-acting propeller. This is a different animal. A double-acting propeller has no mechanical assistance — no counterweights, no springs. All actuation must be hydraulic. So we need a protected source of feathering oil. Usually this is an isolated part of the main oil tank in a dry-sump lubrication system. That oil is sent to the propeller by an electrically driven "Feathering Pump." The pilot's basic control selection for feathering remains the same as before. You bring the rpm lever back to full coarse. Then you negotiate the feathering stop or gate, and take the lever further back into the "feather" position. This lifts the CSU/PCU control valve fully upwards. That ensures oil feed to the coarse pitch side of the pitch change piston, and drain from the fine pitch side. Then there's the "Feather" button in the cockpit. You push it in. This energizes a button hold-on relay, and in turn, the feathering pump relay, which drives the pump. So the pump runs, and feathering oil passes through the CSU/PCU, pushing the pitch change piston onto the feathering stop. Now here's the clever part — the automatic cut-off. Once the piston hits the feathering stop, oil pressure builds up. That pressure operates a pressure-operated cut-off switch, often called the POCOS. The POCOS interrupts supply to the button hold-on coil. So the feathering button releases, de-energizing the feathering pump relay, and the pump stops. The system shuts itself off once feathering is complete. Now, unfeathering a double-acting propeller. Here's the key principle: any aero-engine, if free to rotate, will unfeather itself by windmilling action. So all you need is to move the blades a few degrees away from the feathered position. To do that, you take the rpm lever out of the feathering gate and place it alongside the lever of the other, live engine or engines. This pressurizes the speeder spring and pushes the control valve down, arranging fine pitch supply and coarse pitch drain. Then you press the feathering button to run the feathering pump. Once windmilling has started, the button needs to be physically pulled out, overcoming — and that's where the excerpt cuts off, but you can see the sequence: the pump runs, the blades move off feather, windmilling takes over, and then you manually release the button. Let me make sure you've got the contrast clear. Single-acting propeller: mechanical assistance from the feathering spring, and the centrifugal latch prevents feathering below a certain rpm. Double-acting: no mechanical assistance, all hydraulic, with the POCOS providing automatic shut-off after feathering. And unfeathering relies on windmilling to do the work once the blades are cracked open a few degrees.