747-400 Manoeuvre Margin

[NSEU]

I've been trying to figure out how the 747-400 computes the maneouvre margin. The question may have been answered before, but there are 101 ways of spelling manoeuvre :P

My manuals say that when the flaps are not up, the source of the manoeuvre margin is the Stall Warning Cards (SWCs). The SWCs feed data to the FMCs and the data goes straight through without interference to the EIUs/PFDs

Is aircraft weight required for the manoeuvre margin? If so, how do the SWCs get this information. All I see in the schematics is a Vbflo signal going from the FMCs to the SWCs (but this data doesn't seem to be used for manuoeuvre margins).

Are lookup tables and raw data, such as AOA, altitude, speed, etc, all that is required for manoeuvre margin computations with the flaps down? (not crew FMC weight entry)

[roundsounds]

I think you'll find it's from the crew FMC entries. If an incorrect ZFW or Manual Total fuel entry is made on the PERF INIT page the manoeuvre margin / stick shaker speeds are affected. An incorrect Vref on the APPROACH page will have the same result.

[NSEU]

FCTM reads:

"Minimum maneuver speeds (displayed as the top of the lower amber band) should not be confused with flap maneuver speeds. Flap maneuver speeds are based on airplane weight, while the minimum maneuver speed is based on airplane angle of attack and current airspeed."

With the flaps up, there is a comparison between the FMC calculated manoeuvre margin and the Stall Warning Card calculated manoeuvre margin and the higher of the two is used.

If you say that the manoeuvre margin changes with different ZFW's, I wonder if this is only a factor with the flaps up?

I wonder what would happen if no ZFW/GW is entered (FMC still powered, as the signal goes through the FMC to the EIUs). Would the margins still appear?

[JammedStab]

I'm not sure if this info helps but...the pilot enters the planned landing weight into the FMC on the Approach Ref page which will adjust the indicated flap maneuver speeds.

As you enter the landing weight(typically an hour or so before landing), you can see the Vref speeds for flaps 25 and 30 that have a default display for present weight on this FMC page adjust themselves(decrease) based on the newly entered planned landing weight. On occasion someone may make an adjustment to planned landing flap setting below 20,000 feet which is at a point that the minimum flap maneuver indications have come into view on the PFD airspeed tape. By simply changing the landing flap setting entered into the FMC, not only does the Vref change by about 5 knots which makes sense as there will be a greater landing flap setting, but so do all the other flap maneuver speeds even though the weight when they are to be selected will be the same. In other words using an example, you just saw flaps up maneuver speed just decrease by 5 knots but your weight is still the same(same thing will happen with all other of the initial flap setting min speeds).

I have seen someone do this temporarily from setting 25 to 30 and then later back to 25 in order to give a greater margin between our ATC assigned holding speed and the flaps up maneuver speed. Just a psychological thing of course.

[roundsounds]

The FCOM states something like:

Top of amber bar indicates minimum manoeuvring speed. This airspeed provides:
• 1.3g manoeuvre capability to stick shaker with flaps down
• 1.3g manoeuvre capability to stick shaker or VREF+80, whichever is less,
with flaps up at or below 20,000ft
• 1.3g manoeuvre capability to low speed buffet (or an alternative approved
manoeuvre capability as preset by maintenance) with flaps up to 20000ft

[NSEU]

@Jammed Stab

As mentioned in my previous post, flap speeds and manoeuver margins are not to be confused (The FCOM makes it sound like one appears to be based on pilot entry and the other based on (dynamic) sensed information)

@roundsounds

Top of amber bar indicates minimum manoeuvring speed. This airspeed provides:
• 1.3g manoeuvre capability to stick shaker with flaps down

Worded like this, it sounds like a fixed % is added to stick shaker speed which I thought it might be, but an observer reported that the top of the margin remained at the same position on the airspeed tape during manoeuvering whereas the stick shaker speed didn't (unconfirmed).

Stick shaker speed seems to be purely based on non-pilot entry. Inputs to the Stall Warning Cards include:

Pitch angle, body pitch rate, body longitudinal acceleration, body normal acceleration (IRU)
AOA, TAS, Mach, CAS, altitude, Vmo/Mmo discrete (Gear up) (ADCs)
Flap position (FCUs)
Gear position (PSEU)
Speedbrake handle position (the signal oddly goes via the MCP)

The AMM also states that Vbflo (Minimum Buffet Onset Speed) is sent to the SWCs from the FMCs. But what exactly is Vbflo, when is it used by the SWCs and for what purpose?

With the flaps out, the FMC chooses the higher of the two margin values generated by the Left and Right SWC's.

[FE Hoppy]

Don't know the aircraft in question but I would hope the manoeuvre speed doesn't move during manoeuvres. The idea being that it stays fixed at the speed that gives you the prescribed margin. As you bank the stall speed increases, thus using up some of the margin. Bank enough and the two will be aligned. And the stick shaker will be buzzing ;-)

[NSEU]

ah... Good points, thanks. Not sure what I was thinking

I assume the top of the lower margin and the stall "bricks" will meet at 40 degrees or 1.3g whichever arrives sooner.

[JammedStab]

I think 1.2 g is a company option. I remember seeing that gear position is factored into the calculation like you said. I wonder what the effect is?