Flight Management, Navigation -Flight Management Computer 737 Operations Manual
With a derated takeoff selected, the thrust setting parameter is considered a limitation for takeoff; therefore, thrust levers should not be advanced further except in an emergency. A further thrust increase following an engine failure could result in a loss of directional control while on the ground. Use the takeoff speeds supplied by the FMC or specified in Chapter PI, Performance-Inflight, for the selected derate condition.
Derate takeoff ratings can be further reduced by using an assumed temperature.
Use the takeoff speeds specified in Chapter PI, Performance–Inflight, for the selected derate or variable takeoff rating condition.
Assumed Temperature Thrust Reduction Takeoff
A takeoff thrust less than the full rated thrust may be achieved by using an assumed temperature that is higher than the actual temperature. The desired thrust level is obtained through entry of a SEL TEMP value (prior to U10) on TAKEOFF page 1 or 2 or (U10) on the N1 LIMIT page or TAKEOFF REF page 2. Use approved sources for selecting the assumed temperature.
The maximum thrust reduction authorized is 25% below any certified rating. Do not use assumed temperature reduced thrust if conditions exist that affect braking, such as slush, snow, or ice on the runway, or if potential windshear conditions exist.
The assumed temperature thrust setting is not considered a limitation. The assumed temperature reduction can be removed. If conditions are encountered where additional thrust is desired, the crew can manually apply full thrust.
Reduced Thrust Climb
Two fixed climb thrust reductions can be selected on the N1 LIMIT page. CLB 1
provides a climb limit reduced by 3% (approximately 10% thrust reduction). CLB
2 provides a climb limit reduced by 6% (approximately 20% thrust reduction).
The reduced climb setting gradually increases to full rated climb thrust by 15000
feet. In cruise, the thrust reference automatically changes to CRZ. The reference
can be manually selected on the N1 LIMIT page.
Use of an assumed temperature reduced thrust takeoff or takeoff derate affects
automatic selection of reduced climb N1.
Use of reduced climb thrust decreases engine maintenance costs, but increases
total trip fuel.
Fuel Monitoring
The FMC receives fuel data from the fuel quantity summation unit. Fuel quantity values are displayed on the PERF INIT page and on PROGRESS page 1 as FUEL QTY. The FMC uses the summation unit for performance calculations.
Flight Management, Navigation -Flight Management Computer
737 Operations Manual
If fuel quantity data becomes invalid, the fuel quantity will be blank on the PERF
INIT page and on PROGRESS page 1. VNAV is not available. On airplanes with U7.5 and on, if fuel flow data becomes invalid after engine start, the CDU message VERIFY GW AND FUEL is displayed. The fuel value is replaced with dashes. VNAV operations may continue even after loss of the fuel data.The FMC uses the last valid fuel quantity for performance predictions. The flight crew should enter estimated fuel weight on the PERF INIT page. Periodic update of the fuel weight is required for the remainder of the flight to keep the gross weight value current.
The FMC monitors the total fuel load on board as detected by the fuel summation unit. The FMCS–CDU message INSUFFICIENT FUEL is displayed if the FMC predicts the total fuel quantity at destination to be less than 2,000 lb. (900 kg.). The USING RSV FUEL message is displayed if the fuel remaining at destination is less than the RESERVES entry on the PERF INIT page.
The CHECK FMC FUEL QUANTITY message is displayed if the FMC has detected a decrease in fuel quantity of greater than 1,500 lb. (675 kg.), and 120 seconds has elapsed since the decrease was detected. The flight crew must then manually compute the fuel quantity value to determine gross weight for the remainder of the flight.
FMC calculated fuel predictions are based on gear and flaps up during climb, cruise, and descent. Any prolonged flight with gear and/or flaps extended will increase fuel required, and will not be displayed correctly on the FMC fuel predictions pages.
Loss of FMC Electrical Power
The FMC requires continuous electrical power to operate. When the electrical power is interrupted for less than ten seconds:
.
LNAV and VNAV disengage
.
all entered data is retained by the FMC
.
the FMC resumes normal operation when power is restored.
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