Bearing/Distance to a Fix
 Figure 009

 Navigation Mode Selection
 Figure 010

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 (b)
 DME/DME/INERTIAL If a valid and reasonable radio combination allows the computation of a radio position, the DME/DME/INERTIAL mode is chosen. The logic to determine the availability of a radio position takes the DMEs of both sides into account. In particular, if one side DME has failed, the onside FMGC will use offside DME information. The navigation mode as displayed on the POSITION MONITOR page is N IRS/DME/DME (N = number of IRS in use: 1 or 3).

 (c)
 VOR/DME/INERTIAL The logic to determine the availability of a radio position will take the VORs and DMEs of both sides into account. In particular, if one side VOR or DME has failed, the onside FMGC can use offside VOR/DME information. The navigation mode as displayed on the POSITION MONITOR page is N IRS/DME/DME (N = number of IRS in use: 1 or 3). In all cases, VOR/DME/INERTIAL position mode can be selected only if DME/DME/INERTIAL position mode is not selected. Use of VOR and DME ident signals from VORs and DMEs will be made to avoid selection mistake in the position computation.

 (d)
 INERTIAL ONLY This mode of navigation will be used if neither the VOR/DME/INERTIAL nor the DME/DME/INERTIAL has been chosen and at least one IR information is valid. When selected, the mode displayed on the POSITION MONITOR page will be N IRS where N is the number of IRS's used to determine the inertial position: 1 or 3.

 (e)
 Localizer Updating When an ILS or LOC approach is selected and LOC deviation is available, the LOC signal is used to update the FMGC position. In this case, the navigation mode becomes N IRS/DME/LOC if it was N IRS/DME/DME, N/IRS/VOR LOC if it was N IRS/VOR/DME and N IRS/LOC if it was N IRS (N = number of IRS in use: 1 or 3).

 1_ En route and terminal area
 If the estimated error for the chosen rho/theta station is
 less than the estimated error for the INERTIAL ONLY mode and
 DME/DME/INERTIAL mode was not chosen, then the mode of
 navigation will be VOR/ DME/INERTIAL. If not, the INERTIAL
 ONLY mode is chosen.
 2_ Approach area
 The navigation mode is dependent upon approach type as
 follows:
 - ILS approach:
 If the DME/DME/INERTIAL mode is not selected, then
 VOR/DME/INERTIAL will be used if the error is less than the
 estimated error for the IR position.
 - RNAV, VOR/DME and VOR approaches:
 If the DME/DME/INERTIAL mode could not be chosen, the system
 uses the specified VOR/DME. If available, the
 VOR/DME/INERTIAL mode is selected. If not, the system will
 search for other VOR/DME to be tuned; then if the estimated
 error with this station is less than the estimated error in
 INERTIAL ONLY mode then VOR/DME/INERTIAL mode is selected.
 If not, INERTIAL ONLY mode is selected.
 - NDB or pilot created approach:
 Same as VOR approach except that there is no specified VOR. 

 (3) Class of navigation Two classes of navigation are defined. They reflect the fact that the system respects accuracy requirements expressed in the following. If these requirements are satisfied, the class is HIGH. If not, the class is LOW. The class is HIGH in the following cases:
 (a)
 En route area:

 -DME/DME/INERTIAL
 -VOR/DME/INERTIAL and INERTIAL if the estimated error is less than 2.8 NM for crosstrack and along track errors.

 (b)
 Terminal area:

 -DME/DME/INERTIAL
 -VOR/DME/INERTIAL and INERTIAL if the estimated error is less than 1.7 NM for crosstrack and along track errors.

 (c)
 Approach area:

 -When LOC update is active
 -
in DME/DME/INERTIAL

 -
in VOR/DME/INERTIAL with the specified VOR/DME, if estimated position error is less than the prescribed threshold.

 -When there is no specified VOR/DME, VOR/DME/INERTIAL if there is a VOR/DME for which the estimated error is less than 0.5 NM for crosstrack and along track errors.
 -in INERTIAL if the estimated error is less than 0.3 NM for crosstrack and along track errors.

 (d)
 GPS/INERTIAL mode: When a GPIRS position is available for navigation update, the integrity of this position is checked versus FM computed radio position (if available) and mixed IRS position. In order to reflect the confidence in the GPIRS position obtained by these integrity checks, the logic to display HIGH or LOW accuracy when GPS/INERTIAL mode is active is the logic related to the radio position used for integrity checks (if any), or to the mixed IRS position otherwise. When the class of navigation is not HIGH, it is LOW. The class of navigation is displayed continuously on the PROG page. (Ref. Fig. 011) When the class of navigation is downgraded from HIGH to LOW, the message NAV ACCURACY DOWNGRADED is displayed on EIS and MCDU. When the class of navigation is upgraded from LOW to HIGH, the message NAV ACCURACY UPGRADED is displayed on EIS and MCDU. If one of these messages is to be displayed while the other is displayed, then the previous one is deleted.
　
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