.
deviate to a warmer air mass
.
increase Mach number.
Note: In most situations warmer air can be reached by descending but there have been reports of warmer air at higher flight levels. Air temperature forecasts should be carefully evaluated when colder than normal temperatures are anticipated.
It takes from 15 minutes to an hour to stabilize the fuel temperature. In most cases, the required descent would be 3,000 feet to 5,000 feet below optimum altitude. In more severe cases a descent to altitudes of 25,000 feet to 30,000 feet might be required. An increase of 0.01 Mach results in an increase of 0.5° to 0.7° C total air temperature.
Cruise Performance Economy
The flight plan fuel burn from departure to destination is based on certain assumed conditions. These include takeoff gross weight, cruise altitude, route of flight, temperature, wind enroute, and cruise speed.
Actual fuel burn should be compared with the flight plan fuel burn throughout the flight.
The planned fuel burn can increase due to:
.
temperature above planned
.
a lower cruise altitude than planned
.
cruise altitude more than 2,000 feet above optimum altitude
.
speed faster than planned or appreciably slower than long range cruise
speed when long-range cruise was planned
.
stronger headwind component
.
fuel imbalance
.
improperly trimmed airplane
.
excessive thrust lever adjustments.
Cruise fuel penalties include:
.
ISA + 10° C: 1% increase in trip fuel
.
2,000 feet above/below optimum altitude: 1% to 2% increase in trip fuel
.
4,000 feet below optimum altitude: 3% to 5% increase in trip fuel
.
8,000 feet below optimum altitude: 8% to 14% increase in trip fuel
.
cruise speed M.01 above LRC: 1% to 2% increase in trip fuel.
For cruise within 2,000 feet of optimum, long range cruise speed can be approximated by M.80. Long range cruise also provides best buffet margin at all cruise altitudes.
Note: If a discrepancy is discovered between actual fuel burn and flight plan fuel
burn that cannot be explained by one of the items above, a fuel leak should
be considered. Accomplish the applicable non-normal checklist.
Engine Inoperative Cruise/Driftdown
Performance of a non-normal checklist or sudden engine failure may lead to the
requirement to perform a single engine driftdown.
Engine inoperative cruise information is available from the FMC.
If an engine failure occurs while at cruise altitude, it may be necessary to descend.
The autothrottle should be disconnected, thrust reference set to CON and the
thrust manually set to MCT on the operative engine. On the FMC ACT CRZ page,
select the ENG OUT. This displays MOD CRZ calculated on engine out MCT and
maintaining the airspeed displayed on the EO SPD line.
Set the engine out cruise altitude in the MCP altitude window and execute the
ENG OUT CRZ D/D page. The thrust reference mode on the EICAS display will
display CON. The airplane descends in VNAV using the VNAV SPD pitch mode.
Once the descent rate has decreased to 300 fpm during driftdown it is held
constant by the FMC until altitude capture.
At altitude capture the ENG OUT CRZ page is displayed. Maintain MCT on the
operative engine and driftdown altitude until the E/O SPD speed is established.
Maintain this speed using manual thrust adjustments.
After level off at the target altitude, maintain MCT and allow the airplane to
accelerate to the single engine long range cruise speed. Maintain this speed with
manual thrust adjustments. Entering the new cruise altitude and airspeed on the
ECON CRZ page updates the ETAs and Top of Descent predictions. When the
ENG OUT VNAV mode is selected and an engine-out condition is detected, the
FMC computes engine-out trip predictions, guidance parameters, and MAX ALT
consistent with the detected engine-out conditions, the selected thrust rating using
the actual bleeds. Refer to Engine Out Familiarization, chapter 7, for trim
techniques.
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本文鏈接地址：757 Flight Crew Training Manual 機組訓練手冊(38)
