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cruise thrust will not maintain desired cruise speed.
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.
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.
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 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.
October 31, 2004
767 Flight Crew Training Manual
Climb, Cruise, Descent and
Holding
Copyright © The Boeing Company. See title page for details.
4.8 FCT 767 (TM)
Headwinds increase the ECON CRZ speed. Tailwinds decrease ECON 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.
Step Climb
Step altitudes can be planned as step ups or step downs at waypoints or they can
be optimum step points calculated by the FMC. Optimum step points are a
function of the route length, flight conditions, speed mode, present aircraft
altitude, STEP TO altitude (or adjacent STEP TO altitudes) and the gross weight.
The FMC computed step point provides for minimum trip cost for the flight,
including allowances for climb fuel. Initiate a cruise climb to the new altitude as
close as practicable to the step climb point.
Fuel for Enroute Climb
The additional fuel required for a 4,000 foot enroute climb varies from 550 to
1,300 lbs (250 to 600 kgs) depending on the airplane gross weight, initial altitude,
air temperature, and climb speed. The fuel increment is largest for high gross
weights and low initial altitudes. Additional fuel burn is offset by fuel savings in
the descent. It is usually beneficial to climb to a higher altitude if recommended
by the FMC or the flight plan, provided the wind information used is reliable.
Low Fuel Temperature
Fuel temperature changes relative to total air temperature. For example, extended
operation at high cruise altitudes tends to reduce fuel temperature. In some cases
the fuel temperature may approach the minimum fuel temperature limit.
Fuel freezing point should not be confused with fuel ice formation caused by
frozen water particles. The fuel freezing point is the temperature at which the
formation of wax crystals appears in the fuel. The Jet A fuel specification limits
the freezing point to -40°C maximum, while the Jet A-1 limit is -47°C maximum.
In the Former Soviet Union, the fuel is TS-1 or RT, which has a maximum freezing
point of -50°C, which can be lower in some geographical regions. The actual
uplifted freezing point for jet fuels varies by the geographical region in which the
fuel is refined.
October 31, 2004
767 Flight Crew Training Manual
Climb, Cruise, Descent and
Holding
Copyright © The Boeing Company. See title page for details.
FCT 767 (TM) 4.9
Unless the operator measures the actual freezing point of the loaded fuel at the
dispatch station, the maximum specification freezing point must be used. At most
　
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