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Where exactly is the engine compartment, remarked in yellow? We know that normally the air going to the engine passes through the air filter. But when we open the alternate air valve, where does the air come from? When I examined the diagram related to this, I could not see a separate hose, pipe or transmission structure. The only thing mentioned is "from the engine compartment" and I don't know what it is. I'm also attaching the schematic found at 71-60. Our plane is DA40NG.

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The engine compartment is the entire area between the firewall and the propeller, which is normally covered by the engine cowling.

The alternate air valve opens to allow engine intake air to bypass the filter, and to be drawn from the ambient air surrounding the engine itself.

This would be done in case of blockage to the air filter system due to icing or other possible failure.

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They’re talking about the space inside of the engine cowling. In other words that valve can be set to feed ambient air into the turbo from an air scoop on the outside of the engine cowl. In the case of icing or some other emergency where the engine can’t breathe normally, draw in air from the alternate air source that’s open for air inside the cowl.

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