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amber band, any maneuvering increases the load factor and further reduce the
margin to buffet onset and stick shaker.
FMC fuel predictions are not available above the FMC maximum altitude and are
not displayed on the CDU. VNAV is not available above FMC maximum altitude.
Fuel burn at or above maximum altitude increases. Flight above this altitude is not
recommended.
Optimum Altitude
Optimum altitude is the cruise altitude for minimum cost when operating in the
ECON mode, and for minimum fuel burn when in the LRC or pilot-selected speed
modes. In ECON mode, optimum altitude increases as either airplane weight or
cost index decreases. In LRC or selected speed modes, optimum altitude increases
as either airplane weight or speed decreases. On each flight, optimum altitude
continues to increase as weight decreases during the flight.
For shorter trips, optimum altitude as defined above may not be achievable since
the top of descent (T/D) point occurs prior to completing the climb to optimum
altitude.
The selected cruise altitude should normally be as close to optimum as possible.
Optimum altitude is the altitude that gives the minimum trip cost for a given trip
length, cost index, and gross weight. It provides approximately a 1.5 load factor
(approximately 48° bank to buffet onset) or better buffet margin. As deviation
from optimum cruise altitude increases, performance economy deteriorates.
October 31, 2006
777/787 Flight Crew Training Manual
Climb, Cruise, Descent and
Holding
Copyright © The Boeing Company. See title page for details.
4.8 FCT 777/787 Preliminary (TM)
Some loss of thrust limited maneuver margin can be expected above optimum
altitude. Levels 2000 feet above optimum altitude normally allows approximately
45° bank prior to buffet onset. The higher the airplane flies above optimum
altitude, the more the thrust margin is reduced. Before accepting an altitude above
optimum, determine that it will continue to be acceptable as the flight progresses
under projected conditions of temperature and turbulence.
On airplanes with higher thrust engines, the altitude selection is most likely
limited by maneuver margin to initial buffet. Projected temperature and turbulence
conditions along the route of flight should be reviewed when requesting/accepting
initial cruise altitude as well as subsequent step climbs.
Recommended Altitude
787-8
Recommended altitude is the cruise altitude that accounts for forecast winds along
the route. To provide usable and accurate recommended altitude, the FMC
requires accurate forecast winds at multiple altitudes above and below the cruise
altitude.
Cruise Speed Determination
Cruise speed is automatically computed by the FMC and displayed on the CRZ
and PROGRESS pages. It is also displayed by the command air speed when
VNAV is engaged. The default cruise speed mode is economy (ECON) cruise. The
pilot can also select long range cruise (LRC), engine out modes, or overwrite fixed
Mach or CAS values on the CRZ page target speed line.
ECON cruise is a variable speed schedule that is a function of gross weight, cruise
altitude, cost index, and headwind component. It is calculated to provide
minimum operating cost for the entered cost index. Entry of zero for cost index
results in maximum range cruise.
Note: Thrust limits or maximum speed limits are generally encountered with cost
index entries of 5000 or more.
Headwinds increase the ECON CRZ speed. Tailwinds decrease ECON CRZ
speed, but not below the zero wind maximum range cruise airspeed.
LRC is a variable speed schedule providing fuel mileage 1% less than the
maximum available. The FMC does not apply wind corrections to LRC.
Required Time of Arrival (RTA) speed is generated to meet a time required at an
RTA specified waypoint on the FMC LEGS page.
October 31, 2006
777/787 Flight Crew Training Manual
Climb, Cruise, Descent and
Holding
Copyright © The Boeing Company. See title page for details.
FCT 777/787 Preliminary (TM) 4.9
Step Climb
Flight plans not constrained by short trip distance are typically based on
conducting the cruise portion of the flight close to optimum altitude. Since the
optimum altitude increases as fuel is consumed during the flight, it is necessary to
climb to a higher cruise altitude periodically to achieve the flight plan fuel burn.
This technique, referred to as Step Climb Cruise, is typically accomplished by
entering an appropriate step climb value in the FMC according to the available
cruise levels. For most flights, one or more step climbs may be required before
reaching T/D.
It may be especially advantageous to request an initial cruise altitude above
　
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