B737 minimum maneuver speed amber band changes with speedbrake

[tae9141]

Weird thing I observed on my last flight was, the amber band I mentioned above, I thought it is meant for next flap placard speed in sequence of 1. 5. 15. 30, and 40 however during flaps 5 condition with S/B extended, amber bands moved inward(both bands) and became tighter. FCOM seems not explaining the reason, however I am wondering if min maneuver speed does change with S/B corresponding to flap placard speed?

[FlyingStone]

Answer lies in FCTM in the Maneuver Margins to Stick Shaker chapter:

Speedbrakes: maneuver margin decreases at any flap setting when speedbrakes are extended

[tae9141]

Thanks. I checked it out. Could you please explain in raw science why it is so?

[Aoyb]

Thanks. I checked it out. Could you please explain in raw science why it is so?

Stalling angle of attack is reduced with speed brake out (air flow separation occurs earlier due to the spoilers).
this means that the stalling angle (1g) will be reached at a higher speed. Hence why the margins decrease

[flown-it]

Probably because the "Speed Brake " is not a speed brake! It is a Lift-dump device!! When those boards come out you have to increase your angle of attack to maintain altitude.

[Too Few Stripes]

I think there’s some confusion in your initial post, the two different amber bands (upper and lower) were used interchangeably.
the upper amber band shows the flap limiting speed for the next flap setting using the logic you stated of 1 5 15 30 (40 if set on approach page). With flaps up the logic of the upper amber band is different.
the top of the bottom amber band represents minimum manoeuvre speed for the current config.

[tae9141]

Stalling angle of attack is reduced with speed brake out (air flow separation occurs earlier due to the spoilers).
this means that the stalling angle (1g) will be reached at a higher speed. Hence why the margins decrease

ok so, 1G is when it is level flight with no turn correct?

[tae9141]

I think there’s some confusion in your initial post, the two different amber bands (upper and lower) were used interchangeably.
the upper amber band shows the flap limiting speed for the next flap setting using the logic you stated of 1 5 15 30 (40 if set on approach page). With flaps up the logic of the upper amber band is different.
the top of the bottom amber band represents minimum manoeuvre speed for the current config.

however I saw upper band coming down tighter with speedbrake out and flaps5

[mustafagander]

It's not a new thing. AFIK the B747 had the same thing. Inconvenient when you cock up your descent!