-GPS for A/C position, if installed
-DMEs for direct distance to various stations
-VORs for bearing to a station
-MMR for localizer update.
These data are then mixed following various methods which are
selected by accuracy preference and signal availability.
(a)
ADIRS Normally, the three ADIRUs data are used and mixed together to provide the inertial position. If data from one ADIRU are not available, then each FMS computes the inertial position from only one ADIRU. If no ADIRS data are available, then the FMS does not compute any position:NO NAVIGATION warning is provided.
(b)
GPS (if installed) When GPS is installed, hybrid signals from IRS/GPS compute GPS/INERTIAL position. As long as GPS/INERTIAL mode is active, no DME/DME or VOR/DME radio update is allowed. However LOC update can apply on GPS/INERTIAL position.
(c)
DME The data from up to 2 DME stations may be used. These data are normally transmitted by the onside DME which is directly scanned by the FMS following an algorithm which researches the best DME station geometry as a function of the aircraft position. When 2 DME stations are correctly received, then the system computes a DME/DME position. If only one DME is correctly received, the system tries to use the offside DME interrogator.
(d)
VOR If still only one DME station is received, the system uses the VOR bearing associated to the DME (if any) to compute a VOR/DME position.
If no VOR is available, no radio position is computed. The radio position, when present, is then mixed with the inertial position through a complementary filter to provide the aircraft position which is used for display and guidance.
(e)
MMR When a LOC signal is available in approach, the FMS uses this LOC signal to update lateral aircraft position relatively to the runway axis.
(f)
Miscellaneous In addition to these computations, the FMS updates the aircraft position:
-At takeoff, on the runway threshold or intercept
-In flight, on demand by the crew.
As additional navigation data, the system provides a class of accuracy (HIGH or LOW) which depends on the algorithm used, on the nature of the signals and the flight area (route, terminal, approach). Various warnings are provided on the MCDU and ND when this class is modified.
Also the system computes the ground speed, the wind velocity/direction, the distance to the active waypoint and the bearing and distance to any pilot-selected waypoint or navaid.
(2) Radio navigation and tuning The second function consists of the radio navigation and tuning. The VORs, DMEs, ILSs and ADFs are normally tuned either automatically by the system or manually by the crew on the RAD NAV page. It has to be noted that, for manual tuning:
-
in case of 2 FMs valid, both side receivers are tuned on either MCDU.
-
in case of 1 FM failure, both side receivers are also tuned on either MCDU.
-
in case of 2 FM failure, the crew has then to tune the frequencies on the RMPs.
Manual tuning of the NAV AID is performed by selecting:
-
an ident or a frequency, and possibly a course for the VOR (/DME)
-
an ident or a frequency, and possibly a course for the ILS (/DME)
-
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. It has to be noted that manual tuning always has priority on automatic tuning. For aircraft position the FMS autotunes the DMEs and possibly the VOR as already described. The tuning (manual or automatic) can only be performed if the RMPs are not set in NAV mode.
(3) Lateral guidance 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 as a function of 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). Sequency occurs when this distance becomes zero.
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