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After takeoff and upon achieving positive climb, why is flap retraction done in stages?

For example, in the Airbus there are characteristic speeds after which flaps can be retracted (S and F speeds) as shown on the PFD speed indicators.

Why is this the case? What implications does this have?

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    $\begingroup$ Are you asking why flaps aren't retracted continuously (or in smaller increments) as airspeed increases, or why they aren't retracted from the takeoff setting to no flaps in one step? $\endgroup$
    – Pranav Hosangadi
    Jul 13, 2022 at 20:34
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    $\begingroup$ @PranavHosangadi Aren't these the same? $\endgroup$
    – DeepSpace
    Jul 13, 2022 at 22:31
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    $\begingroup$ @DeepSpace I was wondering if OP meant retracting flaps slowly but continuously, starting at soon after positive climb vs. quickly from TO to 0 once airspeed is enough to sustain flight without flaps. $\endgroup$
    – Pranav Hosangadi
    Jul 14, 2022 at 1:03

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Why is flap retraction done in stages?

For takeoff, extended flaps allow an airplane to become airborne at a slower speed while achieving the necessary "lift" for its phase of flight (i.e., takeoff, per your question). But, with the flaps extended for takeoff "drag" is also increased.

As the airplane accelerates after takeoff during its climb-out its increasing speed creates more lift thereby eliminating the need for the increased lift that is being generated by the extended flaps.

So, the crew, at the appropriate speed, begins to reduce the "takeoff flap" setting, thereby reducing/eliminating the "drag" caused by the extended flaps.

This is sometimes done in stages depending on how much flap was extended for takeoff.

As the airplane continues to accelerate and reaches a performance speed that generates sufficient lift (for that airspeed) without the need for the extended flaps, flap retraction begins. Then, as the airplane continues to accelerate the remaining flaps (if any) are retracted at a speed that generates the required lift (without needing the lift generated by the extended flaps).

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Because flaps not only alter the shape of the wings, they alter the amount of lift, drag and the location of the center of lift on the wing. This means as you retract the flaps, the aircraft will have tendency both to sink due to insufficient lift until a new AoA and/or airspeed is achieved appropriate to the configuration, and the CL will shift forward on the chord line, causing the nose to pitch downward. This is very pronounced on jets with swept wings, as the wings in cruise configuration are designed for high-subsonic and transonic operation and don’t offer great handling characteristics at low speeds without the use of flaps and other high lift devices deployed. If flap retraction is done too rapidly, at too slow a speed, at too low an altitude and at too heavy a gross weight there is a danger that the aircraft could sink and impact terrain as a result before the flight crew can recover. In addition, abrupt changes in pitch attitude has a tendency to frighten timid passengers making for bad business for an airline. Consequently flap retraction schedules were developed for a smoother, more controlled transition between takeoff configuration and cruise climb.

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Main reasons for retracting flaps in increments instead of in one go, are safety, workload management and simplicity.

It would be possible to retract T/O flaps in one go, as T/O flap settings are usually quite moderate. This would however require careful pitch control and a lot of trimming and monitoring the airspeed. This would impose unnecessarily high workload on pilot flying the plane, and thus reduce safety.

Autopilot would manage this task quite easily, but then there would be a need for two procedures depending on manual vs. autopilot operation.

Slowing down the process of flap retraction would ease the workload some, but it would not fully solve the problem.

One "hidden" risk in single phase flap retraction is the failure of either wing flap mechanism. This would lead to a much more severe assymmetric lift if not caught quickly. And as the pilot is preoccupied with maintaining correct airpeed, trim etc, it is possible the situation would go unnoticed for a while (planes may have warning systems regarding assymmetric flap configuration, this would mitigate this problem).

So: as the departure phase of flight usually is quite busy, it is by far simplier, and safer to retract the flaps in a couple of separate phases rather than at one go. The crew (autopilot) has time to adapt to the new setting before the next one is selected.

Note: flap retraction during go around emphasizes the aforementiond safety and workload issues.

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    $\begingroup$ Retracting takeoff flaps at a speed slower than the appropriate speed schedule (not respecting the flap maneuvering speed) could be dangerous and reduce or eliminate the critical angle of attack protection. $\endgroup$
    – RTO
    Jul 14, 2022 at 19:43
  • $\begingroup$ Yes it would. The flap "speeds" are simply selected points between curves of minimum and maximum airspeed as a function of flap angle (or vice versa). It would be possible to fly within that region as flaps retract. But as you too stated: risky. $\endgroup$
    – Jpe61
    Jul 14, 2022 at 19:53
  • $\begingroup$ Also, as a professional matter, flaps are not purposely retracted at a speed below the flap maneuvering speed (for the flaps selected). I think workload management and simplicity are not part of the calculation. $\endgroup$
    – RTO
    Jul 14, 2022 at 20:05
  • $\begingroup$ I fail to see which part of my answer you are referring to, or are you? I suggest in my answer, that during a "continuous" retraction speed is maintained such that no limits are breached. I may need to edit if that is not obvious. $\endgroup$
    – Jpe61
    Jul 14, 2022 at 22:25
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    $\begingroup$ I'm not criticizing your answer. My comment is meant to focus on the OP's question and add the reality of flying the type of airplane involved (e.g. A320, B737, etc.). Depending on the airplane/conditions the flap maneuvering speed range from flaps 5 to flaps 1 then to flaps up can be significant. If you were to go from flaps 5 (in my example) to flaps up once reaching the flaps 1 speed, you risk stalling the airplane. I can only speak to my experience that adherence to the flap retraction speed schedule is absolutely imperative. I have never flown with anyone who did/thought anything else. $\endgroup$
    – RTO
    Jul 14, 2022 at 23:55
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It is not always the case, and since you didn't ask for airliners specifically, the Pilatus PC-6, for example, has something named "continuously variable flap system" you can find about it in the Pilot's information manual

CONTINUOUSLY VARIABLE FLAP SYSTEM (newer versions)

The continuously variable flap system has a switch labeled UP - OFF – DN. Depending on build status, the switch is either one which is spring-loaded to the OFF position or one which is manually selected to the OFF position.

If the spring-loaded switch is installed, the flaps move in the selected direction as long as the switch is held in the UP or DN position. When the switch is released it returns automatically to the OFF position and the flaps stay at the position selected.

If the other type of switch is installed, the flaps move in the selected direction when the switch is set to the UP or DN position (to stop flap movement before full travel is reached, the switch must be manually set to the OFF position).

The flap indicator shows the actual position of the flaps at all times. It must be used to set the flaps to the desired setting (UP, TO or LD) or any intermediate position.

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