Good altitude to put flaps out

Is there a specific altitude that pilots extend the flaps? Or is it just speed?

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It’s usually speed, though flaps are almost never used above 10,000 feet.

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No specific altitude, just the speed and vertical speed that influence it (amongst a few other factors such as weight and …).

Obviously during a standard ILS approach it might often be a rather similar altitude where you extend your flaps due to being in a similar flight situation, but it is not the altitude which decides when to deploy/retract the flaps.

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What are the speeds to extend them?

Depends on type of aircraft weight and air density. If you type up Aircraft name followed by flap speeds they’ll be out there somewhere
eg: B787-9 Flap Speeds

Speed.
I heard you now have animated cockpits. So, for an A320, you should have marks in the PFD (artificial horizon). When you activate the approach phase the plane will decrease speed to the “green dot” mark in the PFD. You can set flaps 1. The speed will decrease to the S mark. You do your finale approach. At +2000 ft you can set flaps 2 and gear down, the speed will decrease to the F mark. Then flaps 3, full.
The orange marks in the PFD are the max speed to set the next flaps out.
For takeoff it is the same at F mark flaps 1 (even if you were in flaps 3). At S speed flaps up.
For go around you set flaps N-1, and now you act like a takeoff.

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For the A320,

Flaps 1: 230KIAS
Flaps 2: 200KIAS
Flaps 3: 180KIAS
Flaps full: 165KIAS.

Differs for other planes, obviously.

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As others have mentioned, speed is more of a factor than altitude. You can find flap limits for each aircraft at http://www.janpolet.nl/infinite-flight-aircraft-liveries-more/.

Some aircraft have a guide if you look inside the cockpit in detail. Next to your screens with altitude and speed etc. you can sometimes find a small sheet with IAS (speeds) and flaps.

Fun fact: in the a320 family under the autopilot panel there are some texts with flaps limit and speeds.

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