777 maneuvring speed logic

[Niseko Pow]

Hello ladies & gents,

coming from a stone-age a/c, the 777 MMS (minimum maneuver speeds) logic got me confused. A 2 yr old thread about this didn't answer all my of questions so here we go:

Acc to our FCOM the top of the yellow bar on the PFD speedtape indicates minimum maneurving speed (below 20000' etc).
This MMS guarantees 1.3g man. capability to stick margin, which equals 40 degree bank capability in level flight.

-Clear so far-

Additionally, the flap maneuvering speeds are indicated by the green flap indications next to the speedtape, used for flaps retr./ext. reference. This also makes sense, but acc. our FCTM "full maneuvring capability or at least 40 degr bank is provided when the a/c has reached the recommended maneuver speed"

Now; if both minimum speeds provide the 40degr/full maneuver capability, where's the difference?

For instance: what margin does the green "20" give me with flaps 20 selected, if the yellow bar (MMS) already provides 1.3g?


btw not a newbie on pprune, just no clue where my old account is, forgive me

[NSEU]

I believe the manoeuvre margin floats on the dynamically computed red bricks (stall marker) and, as such, will vary with angle of attack (and other factors). The flap markers are simply Vref + xxkts. I assume the flap markers represent, well, not worst case scenario, but an airspeed value seen on test flights over a specific range of AOA's.


I've heard of instances on the 744 where a green flap marker has been been below the manoeuvre margin. A faulty Capt's or F/O's AOA vane has given a higher than normal stall value (and subsequently incorrect higher manoeuvre margin).

I stand to be corrected

[Spooky 2]

A little off the exact subject but I believe JAA certs speify a 1.2 margin, not 1.3? I'm just guessing that your in a JAA airplane?

[Niseko Pow]

JAA indeed...

Interestingly, during flap retraction one is flying below flaps maneuvring speeds for the new flapsetting as next flapsetting is set at the current flap maneuvering speed. According the FCTM, this still guarantees 30 degr bank margin during flap transition, which could very well equal the 1.2 g you mentioned.

NSEU, thanks for your comment. I found a chapter in the FCTM which confirms your story. Interestingly Boeing warns "one should not confuse flap maneuvering speeds with minimum maneuvering speed" in the same chapter

The way I see it now, the flap maneurving speed guarantees at least 1.3 stick shakermargin, but generally always a little more than that.


Cheers

[FullWings]

What I understand, As NSEU points out, was that the indicated flap speeds are static (vary only with aircraft mass) and the stall margin is dynamically computed using the prevailing conditions (AoA, mass, g-loading, etc.). If this is true, you could have a scenario where the yellow band goes past the flap speed, but you'd have to be pulling pretty hard to get there!

[Niseko Pow]

You're absolutely right, here's what I just found in our FCTM:

note:
During some non-normal conditions, the flap maneuver speed for current flap setting may be less than the minimum maneuver speed

Cheers

[Oakape]

during flap retraction one is flying below flaps maneuvring speeds for the new flapsetting as next flapsetting is set at the current flap maneuvering speed

The thing some people seem to miss is that the FCTM states -

"With airspeed increasing (my italics), subsequent flap retractions should be initiated:

* when airpeed reaches the maneuvering speed (number) for the existing flap position."

With increasing airspeed, the time you are exposed to any reduced maneuver margins during flap retraction, is quite small. However, I see a lot of pilots calling for flap retraction without a speed trend vector i.e. stagnant airspeed.

[bauduin_alex]

Unfortunately I cannot copy the content of the AMM here, but if you have a chance, look at the AMM SDS, ATA 31, page set 31-51-00-49 and 50 of a 777.
You will find an explanation about what the WES is doing on the speed tape and what the AIMS is doing to it too.
Happy landings.

[Sidestick72]

Unfortunately in that chapter there is no mention about MMS,

by the way the top of "amber foot" is independent from values inserted on the FMS and it is "self calculated" from the aircraft from 400' and above.
Boeing do this (similar to Airbus) in order to guarantee the margin on lower speed regardless of weight data inserted on FMS that sometimes are wrong.

What i should like to know is the source of the MMS, certainly the Angle of Attack, the air density & temp, the stabilizer trim settings and so on.

Anyone knows more about it?