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The operational flight program (OFP) continually monitors
computed performance targets against the airplane operation
limits. If any performance target is more than the limit, the FMC
will restrict the performance target to the correct value and
maintain both airplane and engine optimal performance.
FMCS - FMC FUNCTIONAL DESCRIPTION - PERFORMANCE SUB-FUNCTION
EFFECTIVITY
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FMCS - FMC FUNCTIONAL DESCRIPTION - PERFORMANCE SUB-FUNCTION
FMCS - FMC FUNCTIONAL DESCRIPTION - PERFORMANCE SUB-FUNCTION
EFFECTIVITY
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General
The guidance sub-function calculates data for the lateral and
vertical guidance functions and supplies guidance commands
to the digital flight control system (DFCS) and autothrottle (A/T)
system. The calculations start when the guidance sub-function
receives an active route (lateral flight plan) and an active
performance plan (vertical flight plan).
The DFCS and A/T use the commands to automatically guide
the airplane along a lateral path and to control airspeeds,
vertical speeds and N1 targets/limits. The LNAV mode must be
active to enable the lateral guidance function and the VNAV
mode must be active to enable the vertical guidance function.
NOTE: In path descents, LNAV must be valid and a valid end of
descent point must be defined in order to engage the
VNAV mode.
These are the major parts of the guidance sub-function:
* Flight plan management
* Lateral guidance commands
* Vertical guidance commands.
Flight Plan Management
The lateral route is a set of navigation legs. A navigation leg is a
path between two waypoints. The flight plan management
process calculates the great circle and turn segments for each
path. Navigation legs are great circle tracks between the flight
plan waypoints or constant heading legs.
A waypoint shows as latitude, longitude and altitude. The
waypoints come from the navigation data base or are put in by
the flight crew.
This data is calculated for each way point:
* Predicted altitude
* Time-of-arrival
* Speed
* Fuel remaining
* Distance between the waypoints
* Flight path course between the waypoints.
The performance plan (ZFW, cost index, cruise altitude,
performance modes) determines the performance speed
schedules and thrust that will be calculated by the FMC. The
path calculations then use the performance data to calculate
altitude, time, airspeed and fuel for each waypoint. If there are
any speed or altitude restrictions attached to the lateral
waypoints in the flight plan, these will be incorporated into
the vertical flight profile calculated by the FMC.
FMCS - FMC FUNCTIONAL DESCRIPTION - GUIDANCE SUB-FUNCTION
EFFECTIVITY
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Initially the flight plan management calculates the parameters
for the waypoints in the climb and cruise phases but the top of
descent (T/D) waypoint will be unknown.
The T/D is calculated using the last waypoint in the flight plan
and working backwards. The approximate T/D is estimated to
get a descent speed schedule and descent path gradient
together with fuel remaining. Range in descent is first
calculated from the altitude change in descent and an
assumed flight path angle. The T/D will be calculated by
subtracting range in descent from the total flight plan range.
Next the estimated fuel remaining at T/D and wind profile are
used to calculate descent range, speed schedules, path
gradient and final fuel remaining. Then the full descent leg is
calculated using the last set of data backward from the terminal
waypoint. This gives the T/D and a predicted fuel remaining
value. The descent phase of the flight plan is now combined
with the climb and cruise phases to complete the flight plan
predictions.
Lateral Guidance Commands
When a complete route has been activated in the FMCS and the
LNAV mode is valid, the lateral guidance function sends lateral
steering commands to the DFCS.
If the LNAV function is not valid and lateral guidance outputs are
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