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In case of the failure of one FMGC, both side receivers are also tuned on either MCDU
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In case of the failure of one MCDU, the other MCDU allows tuning of both side receivers
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In case of the failure of two FMGCs, the crew has then to tune the
frequencies on the RMPs. Manual tuning of the navaids is performed by selecting:
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An ident or a frequency, and possibly a course for the VOR (/DME)
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An ident or a frequency, and possibly a course for the ILS (/DME)
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An ident or a frequency, and possibly the BFO for the ADF. The autotuning selects the same data (except BFO for the ADF) following some internal logic (Ref. 22-72-00). It has to be noted that manual tuning always has priority over automatic tuning. For aircraft position the FMGS autotunes the DMEs and possibly the VOR as already described. The tuning (manual or automatic) can only be performed if no RMP is set in NAV mode. Having an active flight plan and the aircraft position, the system is then able to perform lateral guidance along the flight plan. The first task computes the various geometric parameters defining the current active leg according to its type (course, heading, great circle, arc, procedures, etc.) and the guidance parameters on this leg (desired heading/course, desired track, leg distance...).
Then the transition path between the current active leg and the next leg is defined by the same type of parameters and the system determines on which part of a leg the aircraft must be guided to. The following task consists in sequencing the flight plan as long as the aircraft progresses along it. This is done by evaluating the track distance to the termination point of the active leg (to waypoint or bisector between both legs).Sequencing occurs when this distance becomes zero. Then the system determines the type of control law which is required to guide the aircraft (heading, track, lateral path) as well as the targets (desired heading or course, crosstrack error, track angle error, precomputed roll angle). Some of these intermediate values are displayed on the MCDU or the ND (XTK, distance to go, ...). The control laws are located either on the FG part of the FMGC (heading and track modes) or on the FM part (lateral path). In this last case,a roll command is transmitted to the FG part. At last, the FMS computes, when the LAT AUTO control mode is armed, the capture zone within which NAV will be engaged. This zone varies with the type of active leg, and is defined so that the aircraft should not overshoot the lateral path for the geometric path case. In case of heading or course legs, the NAV mode, assuming all conditions of signals are valid, engages immediately.
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 FMS computes the aircraft weight and center of gravity continuously and various speeds for flight envelope and optimization. The weight computation starts when the crew has inserted a ZFW in the INIT page and one engine is started. At this time, the FMS reads the fuel quantity information which is used to initialize a filter. This filter determines the fuel on board from fuel flow indication and fuel quantity indicator. A reversion logic exists : when one sensor fails, corresponding data are ignored. The center of gravity is derived from the inserted ZFW CG during initialization and the fuel consumption. Gross weight and CG are displayed on the MCDU and the ECAM. Gross weight is also transmitted to the FAC before takeoff and above 15,000 ft./250 kts for speed envelope computation. The FMS uses FAC envelope speeds for guidance; these are computed with the FAC gross weight. Hence there may be discrepancies between real-time speeds and predicted speeds (which are computed according to the FMS predicted GW) if gross weights computed by the FAC and the FMS do not match.
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 translates the flight time cost in fuel flow units. 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 crew may manually select a speed or a Mach number on the FCU. This speed is taken into account by predictions 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. In addition to this selection, the crew may preselect a speed or a Mach number on the MCDU for climb or cruise flight phase. This preselection will become active when the considered flight phase becomes active. The FMS also computes several characteristic speeds which are displayed on the MCDU and used for predictions. They are :
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