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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.

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 and air temperature. 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.
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 airports, the measured fuel freezing point can yield a lower freezing point than the specification maximum. The actual delivered freezing temperature can be used if it is known. Pilots should keep in mind that some airports store fuel above ground and in extremely low temperature conditions the fuel may already be close to the minimum allowable temperature before being loaded.
For blends of fuels, use the most conservative freezing point of the fuel on board as the freezing point of the fuel mixture. This procedure should be used until 3 consecutive refuelings with the lower freezing point fuel have been completed. Then the lower freezing point may be used. If fuel freezing point is projected to be critical for the next flight segment, the wing tank fuel should be transferred to the center wing tank before refueling. The freezing point of the fuel being loaded can then be used for that flight segment.
　
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