Density altitude takeoff procedure

1 Answers

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   Mark Kolber on Aug 20, 2016

   If you are operating for the first time in a high density altitude environment, local instruction is always your best bet because it’s not just about the effect on power. That said, this is the method generally taught at flight schools in Colorado.

   Target leaning for max power is done at runup this is for two reasons other than it just works for all but a very few make/models (yes, it definitely works in a 182). One is that leaning at full power can be hard on the brakes of higher performance engines; the brakes might not even hold in some. The other is that, a constant speed prop will act like a fixed pitch one when set at runup rpm.

   When you are ready to do the run-up:

   1. Enrichen the mixture (you should have leaned it for taxi, so you need to enrichen it for the run-up power demand). You don’t really need to go back to full rich at this point, but there’s no harm in doing so until you learn about where to set it)

   2. Go to run-up power.

   3. Lean. You will initially see a rise in RPM as you reach best power and then a drop. When you see the drop, enrichen back to peak.

   4. Enrichen more. On airplanes with a mixture vernier control, 3-4 twists will do it. Without a vernier, about 1/4-1/2″ tends to take care of it. The enrichment is for 2 reasons: to approximate the additional requirements for takeoff and for engine cooling.

   In most airplanes, this will be exactly what you need for takeoff (a lot of instructors I know stop here) but bear in mind that this is an =approximation= that needs to be cross-checked, at least until you have learned that the run-up technique works for your airplane.

   The cross-check should be done at full power. If the brakes will hold, you can do it while still at the runup area or before beginning the takeoff roll. But I usually do that final check on the takeoff roll. The exact check varies. It’s often simply getting expected rpm but you might have a table of fuel flow targets for takeoff to measure against. Personally I have never had to move the mixture more than a 1/2 twist or its equivalent – takes only a second.

   Finally, remember that this isn’t brain surgery. Look at your POH – that instruction to lean “above 3000′) is a pretty good indication of how much leeway is built in.

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