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In the VNAN climb on B737, the FMC changes the target speed from V2+20kt at liftoff, to 230kt for acceleration at the preset accleration height, then to 250kt (below 10,000ft) as the leading edge devices are fully retracted along with the trailing edge flaps.

My question is: Why is the climb speed limited to 230kt on the VNAV mode until the leading edge devices are fully retracted, when there are only operating speed limits to trailing edge flaps as below?

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For your information, I've found a discussion on this topic on another aviation website but no one seems to have a definite answer to this.

https://www.pprune.org/tech-log/570676-737ng-led-s-speed-limitations-ops.html

Does anybody have a good answer to this?

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    $\begingroup$ That's a really good question. I would like to see an answer :) $\endgroup$
    – avionerman
    Feb 17, 2019 at 7:26
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    $\begingroup$ One of the reasons may be to leave a safety margin. For example, if you are with Flaps 10, you are not gonna fly at 210K IAS, but something less (or the maneuverability speed for Flaps 10). Same thought process for slats deployed. $\endgroup$
    – Afe
    Feb 17, 2019 at 13:06
  • $\begingroup$ Does it always target 230KIAS regardless of weight? In your question it’s not quite clear whether 230KIAS is upper limit of VNAV initial climb speed range or whether it’s a fix target. Also, FLAPS 1 limit speed implies slat limit speed, but is labelled only FLAPS in accordance with flaps handle (which operates the slats, too). $\endgroup$
    – Cpt Reynolds
    Feb 17, 2019 at 22:07
  • $\begingroup$ It's a fixed target regardless of the gross weight $\endgroup$
    – lemonincider
    Feb 17, 2019 at 23:30

3 Answers 3

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The non normal checklist QRH will answer your question. If the LE flap transit light remains illuminated I.e the leading edge flaps and slats haven’t retracted or disagree with the flap handle position, then the QRH directs you to limit the speed to 230 kits until you have rectified the issue. VNAV is honouring that restriction.

Hope that answers your question.

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  • $\begingroup$ The logical follow-up is, with flaps extended to 1 or 5, the barber pole limit is 250 knots. So as the leading edge devices retract, you're still restricted to 250, rather than 230. Given that VNAV will climb (clean) at speeds within a few knots of VMO (given a high cost index), why does it buffer the 250 knot limit by a full 20 knots? $\endgroup$
    – Ralph J ♦
    Jun 4, 2019 at 23:43
  • $\begingroup$ @RalphJ Presumably because the Le FLAPS TRANSIT light can also indicate a leading edge device asymmetry or skew according to the QRH. This would result in different aerodynamics compared to the normal flaps 1 or 5 settings. $\endgroup$
    – Bianfable
    Jun 5, 2019 at 11:11
  • $\begingroup$ @Bianfable I'd be okay with that answer if the operating limitation were "230 kts until LE FLAPS TRANSIT extinguished," but it isn't. We can fly the airplane at 250 knots; why such a different speed with VNAV? $\endgroup$
    – Ralph J ♦
    Jun 5, 2019 at 14:07
  • $\begingroup$ @RalphJ As far as I understand the QRH, you also shouldn't fly the airplane at more than 230 kt until the light extinguishes because you cannot know if there is an asymmetry. The 250 kt limit is for flap 1, 2 or 5 fully extended with light out. At least this would be my interpretation, maybe 737Pilot can shed some more light on this... $\endgroup$
    – Bianfable
    Jun 5, 2019 at 14:17
  • $\begingroup$ Taking into account in advance the possibilities of a future failure doe not comply with the rules of the art. That’s not flying. When a fault occurs you follow the respective procedure, no pilot is requested in advance to follow a hypothetical failure checklist, QRH or not! $\endgroup$
    – user40476
    Jun 10, 2019 at 9:23
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Because at flaps 1, the leading edges are deployed until the flaps are up therefore 250K speed restriction

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    $\begingroup$ True, but that would still allow 250KIAS. OP states the FMC flies at 230KIAS for Flaps 1. $\endgroup$
    – Cpt Reynolds
    Feb 17, 2019 at 22:00
  • $\begingroup$ In B737max The greater the speed the greater the pitch instability if not counteracted at low speed. If flaps are retracted at high speed it would be worse, but happily FMC does limit speed to 230 knots with flaps at 1 ( like 737 NG) for fuel reduction considerations, not structural. $\endgroup$
    – user40476
    Jun 5, 2019 at 8:01
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One of the FMC major objectif is to reduce fuel consumption, beyond this speed the lift to drag is too much expensive

Definitely it is neither a safety reason nor a structural protective reason, otherwise the barber pole won’t be at 250 knots.

None is obliged to follow the FMS, and the autopilot is not mandatorily on VNAV, it could be on manual selected speed, and in this case the selected speed could be as high as 250 knots.

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