C. Vertical Functions The main vertical function is the computation of time and fuel predictions along a computed vertical flight plan and guidance along this vertical flight plan. In order to achieve this, in addition to the vertical flight plan elements seen in para. A. above, the FM computes various speeds for flight envelope and optimization.
(1)
Weight computation The weight computation is performed by the FMS using the ZFW entry and fuel data. The fuel data is either a BLOCK fuel entered on MCDU before engine start, or a FOB value sent by the FQI (Fuel Quantity Indicator) after engine start.
(2)
Center of gravity The center of gravity computation is performed by the FMS using the ZFWCG entered on MCDU and the fuel data.
(3)
Various speeds Various speeds are computed by the FMS for prediction. First, the speeds determining the flight envelope are computed. They are based on stall/buffeting limits, engine thrust limits and VMO/MMO and the configuration assumed for the prediction point. Also, max. and min. operational speeds are computed ; they will limit the speed optimization research.
The optimum speeds are computed in clean configuration, in climb, cruise or descent from various parameters : Cost index, gross weight and CG, wind, altitude and temperature. The cost index is a parameter which takes into account both time cost and fuel cost optimization computation. Minimum fuel optimization is performed with 0 cost index. Minimum time optimization is performed with 999 cost index. This value is selected by the airlines.
The optimum speed may be corrected along the flight plan by a time constraint inserted by the crew. The obtained speed is called performance speed.
(4)
Speed selection The crew may manually select a speed or a Mach number on the FCU. In this case, predictions will use this speed instead of the optimum speed for the current phase. If the crew selects a speed in climb, the FMS will automatically select the corresponding Mach number when the aircraft crosses a predetermined crossover altitude. The same occurs in descent when the crew selects a Mach number on the FCU before crossing the crossover altitude.
(5)
Constant Mach Segment (CMS) The crew may select on the MCDU a speed for the cruise or only on a specific segment in cruise phase. All the flight plan constraints will be taken into account.
In addition to this selection, the crew may preselect a speed or a Mach number on the MCDU for any further flight phase. This preselection will become active when the considered flight phase becomes active.
(6)
Characteristic speeds The FMS also computes several characteristic speeds which are displayed on the MCDU and used for predictions. They are:
-Takeoff speeds (F, S, VFTO) and go around speeds
-Expedite speeds in climb and descent phase
-Approach speeds including VREF as a function of the landing
configuration preselected on the MCDU.
NOTE : Takeoff speeds (V1, VR and V2) are entered on the MCDU. V1 and
____ V2 are transmitted to the PFD. V2 is used by the FG portion in SRS mode.
(7) Predictions Having defined the vertical flight plan elements as well as the inputs parameters for prediction and guidance, the following shortly describes the prediction process.
The predictions never take the engaged autopilot mode into account. This means that the MCDU flight plan display represents the predictions along the flight plan assuming a full managed vertical control. The speed assumption is based on the selection as displayed on the PERF pages.
(a)
TAKEOFF In takeoff phase, the prediction profile starts with a first segment at V2 +10 kts and a Max takeoff thrust until the thrust reduction altitude is reached. This is followed by a second segment at V2 +10 kts and a Max climb thrust up to the acceleration altitude. These predictions (time, distance, fuel) are based on precomputed allowances.
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