744 maneuvre margin question
 

During a recent moment of in-flight boredom I checked the performance manual maneuvre margin speeds for our cruising level and compared them with those displayed on the PFD. The speeds shown on the PFD differed significantly (ie more than can be attributed to the thickness of my pencil) and seemed to represent a 1.2 margin rather than 1.3. This was on a RR powered 744. I tried it again on a later trip on a GE powered 744 and the 1.3 figures matched perfectly.

Would anyone be able to shed some light as to why this may be the case. Could it be to do with certification? Is the 1.3 margin a legal requirement or just a convention?

Thanks.

My understanding is that the 1.2g factor applies to aircraft certified under FAA rules whereas 1.3g applies to JAR certification. I stand to be corrected though.

Regards

csd

As already stated, the FAA requirement is Vs1.2 and the CAA requirement is Vs1.3. It is possible to insert your own numbers in the FMC, just not advisable!

To establish the edges of the envelope it is possible, on some software, to insert an impossible high speed, say .90 and the FMC will actually give you the maximum possible under the present circumstances, now put in an obviously too slow speed, say .70, and the FMC will give you the absolute minimum speed. Some companies inhibit some of the FMC functions.

Thanks guys. Appreciate the info.

Adding some questions
 

There are a couple of questions that came to my attention which I believe may be pertinent to this discussion with which, I hope, the experts on this forum can help. FYI, I am a sim pilot (767-300), but the person asking is real world, training on the 744. So here goes:

Yellow Band – Minimum Maneuvering Speed: What system inputs affect the maneuvering speeds or more to the point, what does the top of the band represent – besides margin to shaker or buffet? Obviously, aircraft FMC Gross Weight, and flap/speed brake configuration; but does MMS change with G-load and is there an IRS input? (I think NO, but can’t find a textual description.)

Red (brick) Band – Minimum Speed: What does the top of this band represent, besides – obviously – stick shaker speed? And is there an IRS/G-Load input to this band? Again, FMC Gross Weight, ADC(AOA) and configuration generate the limit, but what else? (I’m convinced that there is an IRS input and G-load that affect this, but again, the exact textual description eludes us at this point.)

Whatever the exact answer, it will be simple. We just have no reference for it.

Any direction you can point me to help him, I'd appreciate it.

Many thanks,
speedbird716

The description below is for the Stall Warning Management Computer on a 747-400. Speedbird716, you are correct - the IRS does input into the computation for minimum manouvering speed and it does exist in a textual form. (I found some old stone tablets out the back of the hangar with this info chiseled into them) The IRS provides a pitch acceleration value for the min manouv margin. The generation of the yellow pitch limit indicator on PFD is described in the paragraph on Arinc 429 outputs. It also drives the speed tape limits (max/min manouver speeds) I shall try and dig out a better description of this. Hope this helps.

Stall Warning Management Computer Inputs
The Stall Warning Management Computer (SWMC) cards gather
information from many sources to evaluate and determine stall onset.
These sources include information derived both directly and
indirectly through other units.
Input sources include:
• Proximity Switch Electronic Unit (PSEU) information:
− Nose gear position.
• Flap Control Units (FCU) information:
− Flap position.
• Master Monitor cards information:
− Gear and flap, position from PSEU and FCUs, the cards control
power-up BITE.
• Spoiler position information:
− Spoiler deployment.
• Inertial Reference Units (IRU) information:
− Pitch acceleration.
• Air Data Computer (ADC) information:
− Angle of attack, speed/mach.
• CMC test:
− BITE test initiated by use of the CDU.


Stall Warning Management Computer Outputs
The SWMCs directly control the operation of the control column
stick shakers. They also provide information to:
1. The EFIS/EICAS Interface Units (EIUs),
2. Flight Management Computers (FMC) and Central Maintenance
Computers (CMC) via the EIUs.
3. The Ground Proximity Warning Computer (GPWC).


ARINC 429 Output
In addition to generating the alpha-max value for stick shaker
operation, the SWMC computes two types of parameters for ARINC
429 outputs. They are:

• Pitch limit parameters:
− SWMC calculation of pitch limit parameters are used to
indicate the maneuver margins available for the airplane to
recover from windshear condition without encountering stick
shaker warnings. This information is transmitted to the Ground
Proximity Warning Computer.
The same information will also be transmitted first to the
[]EFIS/EICAS Interface Units (EIUs) and then on to Flight
[]Management Computers (FMCs) for display of the Pitch Limit
Indicator (PLI) symbol on the Primary Flight Displays (PFDs).

• Speed Tape Parameters:
− The SWMC also generates various speed tape parameters such
as Vmo and Vss(stick shaker speed). This information is
transmitted to the FMCs via the EIUs.

Further to the above. R.E minimum speed tape. The min and max manuever speed indications are those that the FMC may output commands to the auto-pilot system for aircraft control.

Speed Tape Data
The following FMCS data shows on the airspeed tape:
• Minimum Manuever Speed shows as an amber line which begins
at the minimum manuever speed on the tape and extends to the
stick shaker speed. Minimum manuever speed is the lowest speed
the FMC may use for control signals.
• Maximum Maneuver Speed shows as an amber line which begins
at the maximum maneuver speed on the tape and extends to the
maximum operating speed. Maximum Maneuver speed is the
maximum speed the FMC may use for control signals.
• Decision Speed (V1) shows as a green ’V1’ and digital readout at
the top of the tape when the value of V1 is off-scale. When the
value of V1 is within the range of the scale, ’V1’ shows next to
the decision speed on the tape.
• Rotation Speed (VR) shows as a green ’VR’ next to the rotation
speed on the tape, or as ’R’ when within four knots of the
decision speed.
• Landing Speed (V REF) shows as a green ’REF’ next to the
landing speed on the tape.
• Flap Manuever Speeds show as green digits next to the speed at
which a flap setting applies. Flap retraction to zero units shows as
’UP’.
• Selected Target Speed shows as a magenta readout at the top of
the tape and a magenta cursor on the tape. This data normally
comes from the AFDS, but comes from the FMCS when VNAV
is engaged.

Spanner,
This is exactly the information he was searching for...many thanks for your kind assistance.
All the best,
Dave/speedbird716

Huzzah! This is a discussion that happens to be unusually pertinent for me.

I have seen that different manufacturers seems to define the minimum manuever speed differently. For instance, I've seen Brand X always calculate Vmin as 1.2 times the 1.3G buffet speed, While Brand Y seems to give about a constant 10 knot margin over the 1.3G speed (I guess that's to gain access to higher altitudes earlier?).

Anyone know if there's a prescribed/advised method for calculating the minimum manuever speed?

Out of curiosity Dagger, did your lookup table include an adjustment for off-ISA temperatures?

Thanks for the information spanner!

Checkers, there is no ISA adjustment associated with this table.

An engineer advised that the FMC buffer margin speeds can be varied on the ground in the FMC, and that different operators use different margins based on their experience/preferences. Sounds risky.

It isnt, we do it all the time and on every fleet that we operate! We like planning flights with a 1.3G buffet margin. In some cases where we have a "cheap" FMC, it will decide to send the aircraft to an optimum altitude of FL410 on almost every sector based on a 1.1g buffet margin, so by increasing the buffet to 1.3g, we reduce the maximum altitude.

Mutt