Ask My Sky Mom. Aviation wit and wisdom.

Hey Mom,

Why does the tachometer drop when I check the carb heat during the run up?  I know it SHOULD, I even know HOW MUCH it should, I just don’t know WHY.

Thanks,

Pulling Out All the Stops

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Hi Stop,

When you pull that carburetor heat knob out (or pull the handle down, or deploy whatever variation your airplane happens to be equipped with), you’re routing warmer air through that carburetor than you were a moment ago.  Warm air is of course less dense than colder air, meaning that you just aren’t gonna be able to fit quite as many air molecules into the same square inch as you could before.  Less air for your engine to work with, less power.  You know what didn’t change though?  The amount of fuel you’re allowing to that fuel/air mixture.  The same amount of fuel and less air to mix with it will give you a richer mixture by default, making you more prone to spark plug fouling, or at the very least, wasting some of that rather expensive 100LL you’re flying around with!

Keep it lean and fly safe,

Mom

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~ by myskymom on September 15, 2009.