When is it appropriate to select flaps from 20 to 8 after takeoff?

Selecting flaps from 20 to 8 after takeoff is appropriate at V2+12 knots, provided that it is above the Flap Retraction Speed (FRA). This speed ensures that the aircraft has reached a safe margin above the stall speed with the gear retracted, allowing for a smoother transition and helping to maintain control.

Timing the flap retraction at this point contributes to the safety and efficiency of the climb-out phase of flight. It is important that the aircraft is operating confidently above the minimum airspeed required for safe handling characteristics, as pilots need to be mindful of the aircraft's performance capabilities during this critical phase. Lower speeds could lead to excessive drag or difficulties in maintaining climb performance.

In contrast, the other choices tend to either be too early to ensure adequate safety margin or may not provide enough airspeed to safely handle the aircraft while making this adjustment. Selecting flaps at V2 or V2+5 knots may still place the aircraft in a vulnerable position, whereas V2+10 knots is still not optimal for the transition required at flap retraction. Thus, V2+12 knots balances safety with the necessity of managing drag during the climb.