Barkingmad and Aluminium shuffler, I and fellow readers must thank you for your knowledge and wisdom shared. FYI: I have and still do fly the old lady, yes, in fact for over 13 years gaining well in excess of 10,000 hours and still flying her today.
Now, please let me share some of the wisdom gained by not just reading the FCOM and flying a plane, quotes from the following document are made available to you by your airplane manufacturer, it's called the
FMC Supplementary Data Document. You can find this document through your performance department, or should be able to at the very least.
To all, please excuse the extend of contact quoted, as it has to be read in conjunction with each other. Alas the full manual is not available for sharing for obvious reasons.
Because ECON speed schedules are not available in either the Performance Engineer's Manual or the Boeing Operations Manual (References 1 through 3), they are provided in this document.
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In VNAV operation the FMC limits speed and altitude capability in order to ensure reasonable and safe operation. The VNAV operational envelope takes into account the maximum certified altitude, maximum operating speeds (VMO and MMO), buffet margins at an entered or default center of gravity position, and stick shaker margins. These limits are treated as hard constraints by the FMC guidance function.
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The data in this document are derived from the Boeing- developed data base used for all FMC performance calculations.
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The FMC defaults to the economy speed modes for VNAV operation. In addition, by default, a transition climb speed limit is observed below the speed transition altitude during climb (250 knots below 10,000 feet in the USA), and a similar limit, but 10 knots less, during descent (240 knots below 10,000 feet in the USA). The 10 knot margin was added to reduce the probability of exceeding the transition speed limit in turbulence. The FMC increases the transition climb speed at heavier gross weights to equal the flaps-up full maneuvering speed of VREF40 + 70 knots (minimum transition climb speed).
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The speed schedules provided in this document are a function of one or more of the following variables: airplane gross weight, altitude, cost index, wind, and temperature. No other FMC variables (i.e. drag factor, fuel flow factor) affect the speed schedules.
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The economy descent speed schedule provides the descent speed associated with the lowest total cost over a fixed distance for a given cost index and TOD gross weight. The method used to derive these speeds is similar to the one used for economy climb speeds. The FMC speed schedules were generated by iteratively computing cruise plus descent trajectories from a common cruise point to a common end of descent point. The descent CAS was varied and the trip cost for each resultant trajectory computed. The CAS value which minimized the cruise-plus-descent cost at each cost index and gross weight was selected as the economy descent CAS. This method allowed the cruise-descent interaction effect on trip cost to be included in the economy descent speed determination.
At a cost index of zero (minimum fuel), the economy descent CAS is near maximum L/D. This stretches the glide and allows an earlier TOD, which allows an earlier thrust reduction from cruise thrust (higher fuel flow), to idle thrust (lower fuel flow). This results in significant trip fuel savings despite the fact that the actual descent fuel burn is higher due to the extra time in descent relative to higher cost index cases. As cost index increases it is necessary to increase speed to reduce the time costs. This results in a steeper descent, more time in cruise, a higher overall fuel burn, but a lower relative cost.
The speed schedules are normally fixed CAS/Mach with the economy descent Mach set equal to the final economy cruise Mach. However, the application of more restrictive speed limits for the FMC descent path calculation may, at times, result in an economy descent Mach number somewhat lower than the final economy cruise Mach number.
Economy descent CAS is defined to not exceed the VNAV speed limit, which in descent is the airplane VMO (340 KCAS) reduced by a 10 knot speed. This limits the maximum economy descent speed to 330 KCAS. A minimum economy descent CAS of 250 knots has been defined to improve compatibility with the air traffic at low cost index values (software versions U10.6 and earlier).
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The FMC provides protection from operating near the region of airplane buffet. All target speeds are checked against a selectable maneuver margin to initial buffet. The FAA FMC will accept values of maneuver margin to initial buffet ranging from 0.2g to 0.6g, but defaults to 0.3g; the JAA FMC lower limit is 0.3g. If necessary, target speeds are adjusted to maintain that margin. Both the FAA and JAA FMCs accept CG values ranging from 5% to 36% MAC; the FAA defaults to 26.2%, while the JAA default is 5% (forward limit). Appendix B describes corrections for other values. FMC- generated speed targets may not be less than VREF40 plus a 70 knot increment.
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The VNAV operational envelope is defined by margins to initial buffet, stick shaker speed, and VMO/MMO. It provides margins to initial buffet and VMO/MMO speed as detailed below.
VNAV Operation Envelope Limits (VMO = 340 KCAS, MMO = .82 Mach)
• VMO/MMO - 5 KCAS for Climb & Cruise
• VMO/MMO - 10 KCAS for Descent
• 0.Xg margin to initial buffet, where 0.2 < X < 0.6 for FAA operators or 0.3 < X < 0.6 for JAA operators
VNAV operational envelopes for altitudes of 20,000 feet and above are shown in Figures 4.2.1.1 through 4.2.2.3. The low speed segment of the envelope contains data based solely on FMC computed, initial buffet derived data. An additional constraint is added to the buffet-based low speed limit to provide protection equivalent to a 14% speed margin to VSS, the stick shaker activation speed. This is accomplished by limiting the minimum maneuver margin to 0.3 g at Mach numbers less than 0.45. Above 0.45 Mach, this limit gradually washes out, varying linearly from 0.3 g at 0.45 Mach to 0.2 g at 0.78 Mach. This permits a minimum margin of 0.2 g at the peak of the envelope and higher Mach numbers. The effect of this added constraint may be observed by comparing the low speed portion of Figures 4.2.1.1 and 4.2.1.2. The two figures are identical in these areas.
The data in Figures 4.2.1.1 and 4.2.1.2 are based on a middle CG position of 26.2% MAC. This is the default value for FAA operation, but values forward or aft of 26.2% can be selected by the flight crew to adjust the VNAV operational envelope for expected cruise CG positions. The data in Figure 4.2.1.3 are based on a forward limiting CG position of 5%, the default value for JAA operation. Data are provided in Appendix B to determine the effects of CG position and maneuver margin to buffet for the flaps up VNAV operational envelope.
Below 20,000 feet the VNAV operational envelope is based chiefly on stick shaker derived data with the low speed limit based on 1.14 VSS.
For flaps down operation the low speed limit is defined by 1.14 VSS and the high speed limit is the appropriate flap placard less 5 KCAS.
This information must be used in conjunction with thrust- limited altitude data, Figures 4.2.1.1 and 4.2.1.2, to compute maximum altitude.