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In the Airbus A320, why do we need to level-off for flap-retraction after a single engine failure?

Even if we maintain a climb, the airspeed will still accelerate and be enough to retract the flaps safely.

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    $\begingroup$ Are you sure you can still climb and accelerate at MTOW? $\endgroup$
    – Bianfable
    Commented Mar 25, 2023 at 10:54
  • $\begingroup$ @Bianfable the airspeed will increase for sure, why not $\endgroup$ Commented Mar 25, 2023 at 20:51
  • $\begingroup$ if the engines are working $\endgroup$ Commented Mar 25, 2023 at 20:51
  • $\begingroup$ You just said "after a single engine failure" in your question. Now you say "engines". Please clarify, how many of the two engines are working? $\endgroup$
    – Flo
    Commented Mar 26, 2023 at 10:36
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    $\begingroup$ @SachinChaudhary You say "the airspeed will increase for sure, why not", but how do you know? What is your source? Are you an Airbus test pilot? How many times did you have an actual engine failure on takeoff close to MTOW in a real A320? $\endgroup$
    – Bianfable
    Commented Mar 26, 2023 at 10:44

3 Answers 3

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In a nutshell: the aircraft has lost 50% of thrust capacity, and you can either choose to make a shallow climb, or accelerate in level flight to retract the flaps. Unless the aircraft is very light, you cannot do both at the same time. The performance is just not good enough.

You might even decide to increase the thrust on the remaining engine to TOGA, because when the aircraft is very heavy, it hardly climbs or accelerates if you keep it at FLEX/MCT.

Additionally, you need to accelerate to certain minimum speeds first (F or S speed), and then retract the flaps step by step, which then reduces drag and makes further acceleration easier.

You state that after a single engine failure, the aircraft would still accelerate while climbing. Where does this experience come from? A professional simulator, or a simple aircraft model in something like the Microsoft Flight Simulator? Could it be that your model is just not accurate enough, or you have made unrealistic settings like payload and fuel which makes the aircraft unusually light?

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The climb segments defined in the FARs or European equivalent are used for certification purposes. The different climb segments are designed to allow the engineers the ability to easily calculate performance during a climb.

  • First segment starts at rotation and ends at gear retraction. There is a requirement for a positive rate of climb during this phase of the climb.
  • Second segment starts at gear retraction and ends at 400' AGL. The requirements is to have a 2.4% climb gradient.
  • Third segment starts at 400' AGL where the aircraft levels off and accelerates to VFTO (final takeoff speed with flaps up).
  • Final segment starts after reaching VFTO and flaps are retracted and ends as the aircraft climbs to 1,500 FT AGL. The requirement is to have a 1.2% climb gradient.

Many aircraft procedures are designed around these requirements. Airbus aircraft will level off to accelerate to VFTO and bring the flaps up so they know what performance they have. It may be possible to lower the nose a little bit and start accelerating while still climbing but it would be unknown what sort of performance they would have. If there are no obstacles, it may be a good idea to do that. Leveling off, accelerating and bringing up flaps reduces drag and an increased climb rate may get the airplane to 1,500 FT AGl quicker than other procedures.

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Other types can make low rate climbs while accelerating on one engine. eg B767 SOP is Vertical Speed +200 at low and medium weights once terrain clearance has been achieved. At heavy weights VS +100 is used. I guess if near MTOW at high density altitudes level flight acceleration may be necessary. Pilot’s judgment was used, not specific weights at my old company.

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