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Why can we use flaps in short field take off but not in high density altitude airports?

Why can't they be used in high density altitude to shorten the take off distance as in short field take off?

Why short field take off with flaps doesn't apply to high density altitude airports?

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With more flaps, you will lift off at lower speed, but will have worse climb performance due to the increased drag.

On short field you are limited by the field length, so you need to lift off early and therefore need to use more flaps.

But in the mountains you are limited by the climb performance. Therefore you need to use less flaps.

Of course, this is just general rule. You should look up performance for given density altitude and compare it with available runway length and required climb gradient.

Update: Extending flaps never improves rate of climb. Up to some points it does improve angle of climb though, because it reduces Vx and the engine can produce more thrust at lower speed. The setting for best angle of climb is higher than for normal take-off and is used on obstructed field.

However as excess power reduces (with density altitude), Vx increases towards Vy (at absolute ceiling, Vx = Vy and rate of climb is 0) and the flap setting for best angle of climb decreases. So even for best angle of climb you shouldn't use as much flaps at high density altitude as at low.

And most mountain airports are not obstructed. In most cases the valley ahead is large enough, but you want to get above the tricky winds fast and you want high rate of climb in case you encounter a downdraft. And that means no or little flaps.

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  • $\begingroup$ I was taught to use same indicated air speed to rotate in short field take off and normal take off. Are you talking about true speed here? $\endgroup$ – PMoubed Nov 14 '15 at 5:11
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    $\begingroup$ @PMoubed, no, indicated. And at low altitudes they are the same anyway. What you say is strange, because you achieve the same speed after the same distance—and if the field is short, you don't have that distance. On the other hand if the field is obstructed (runway is long, but then you need to climb steeply), this actually advocates rotating at higher speed (if paved, accelerating in ground effect if not). $\endgroup$ – Jan Hudec Nov 14 '15 at 9:34
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A cursory glance of this site reveals that the slight increase in lift is not worth the larger increase in drag when deploying flaps at high-altitude/low air-density conditions. http://www.newconthenet.com/mountainflying.html

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    $\begingroup$ Since the list gives, as a second point, a rule about X% of take-off speed at Y% of runway length, while as explained here no such rule can be given with any generality, I would not trust it too much. Many of the rules are right, but the performance ones tend to be over-generalized—your performance calculation might yield different result. $\endgroup$ – Jan Hudec Nov 12 '15 at 10:31

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