On multi prop airplanes there is an auto feather switch. Can someone please explain what this is used for and when do you use it/not use it?Thank you....

Prop feathering is used on twins to quickly feather the the prop on the side of a dead engine. It is used during engine criticle phases of flight; takeoff climb and landing. When an engine fails and the prop is still windmilling, it is causing more harm that good due to the fact that it is producing un-necessary drag. The prop pitch on a multi is controlled by a cylinder and piston inside the prop hub that is injected with oil from the running engine. For flat pitch, oil is pumped in, for fine pitch, oil is drained out. When the engine goes, the oil immediately flows out and the prop automatically feathers, reducing the pilot workload to secure the dead engine.

Just to amplify what Travis said, an engine failure in a twin can become a catastrophic event because windmilling and asymmetrical thrust can quickly lead to loss of control. It is particularly dangerous when it happens suddenly at a critical phase of flight such as takeoff or approach. To prevent this, the pilot must respond immediately with proper control inputs and by feathering the affected engine. However, experience shows that pilots often do not react appropriately, taking too long to diagnose the failure, identity the affected engine, and feather it. There are even many cases where pilots mis-identify which engine is at fault and feather their good engine leaving the plane without power. The extreme stress and high workload of an engine failure event makes pilot error a distinct possiblity.Auto-feather is designed to counter this. When RPM (or power?) falls below a certain level, the prop will feather automatically. This stops the windmilling and helps the pilot quickly identify which engine has failed. It has prevented countless accidents. I believe autofeather is usually turned ON for takeoff, climb and landing, and OFF during cruise and descent.

Thank you guys....This does explain it very well.Very useful information.Thanks again...