Alternate Air purpose?

  • Sorry for the stupid question, but what is the purpose of the Alt Air door? I assume it opens a flap or door that allows hot air from the engine compartment to heat the passenger compartment, but what is the purpose? Is it incase the built in hearer/fan breaks? Does it help with anti-icing?


  • Alternate Air controls the source of air entering the engine's intake manifold. If alternate air is closed, the engine draws fresh air directly into the intake manifold. If alternate air is open, the engine draws air from a different intake that warms the incoming air with waste heat from the engine. As I understand it, the intake can occasionally become constricted with ice buildup under certain conditions.

  • Thank you, so is it a cold weather procedure to use the Alt Air instead of the normal? Like if I fly into icing conditions

  • I am still learning about the airplane myself, so I'm not 100% sure what the standard procedure is. Alternate air causes a reduction in engine power because it draws warmer, less-dense air into the engine, so I suspect it would only be used if necessary. Since the arrow is not certified for flight into known icing, the main thing I can think of is unexpected icing. In the real world, it might be remotely possible for a bird strike to clog the primary intake, in which case the pilot would need to switch to alternate air to prevent the engine from sputtering out.

    For anyone who knows the plane better than I do, is it possible for intake icing to form even if ice is not developing on the wings? Theoretically, it seems possible. At low power settings, the intake manifold is at a partial vacuum. As the pressure of incoming air drops in the manifold, I would think condensation and even ice formation would be possible in humid air near the freezing point. Does that ever happen?

  • @MarcKC The Alternate Air is purely for abnormal situations were the main air intake is blocked. Whether it's ice formation or FOD. It bypasses the main air filter.
    You'd only use the Alternate Air if you have reason to believe there's a blockage.

  • @flightstrike just out of curiosity, has this clogging due to icing been modeled?

  • JF Staff

    Unfortunately the MSFS icing simulation is pretty basic. The alternate air lever does control the default system though.