P R E D I C T I V E
<GPS
R EQ U IR E D  A C C U R E S T I MA T E D
0.15 NM LOW 0.32 NM GPS PRIMARY LOST

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5. The navigation and guidance system design

Other navigation alerts, far less probable may nevertheless be relevant as well:
.
FM/GPS POSITION DISAGREE

.
FM1/FM2 POS DIFF

If one of these alert occurs, the flight crew is expected to initiate a go around and try to determine the faulty system to navigate with the valid system.
5.2. FLIGHT GUIDANCE
For RNP AR procedures, the AP will be used in NAV or FINAL APP modes, which guides the
aircraft along a pre-planned lateral and vertical trajectory.
APP NAV is a lateral guidance mode and FINAL APP is a combined lateral and
vertical guidance mode using Baro-VNAV principle.

5.2.1. APP NAV ENGAGEMENT CONDITIONS :
Engagement conditions of APP NAV are: 
.. A Non Precision Approach is selected in the Navigation Database (NDB) and, 
.. The FMS is in approach phase and,
.. APPR push button is selected and NAV mode is engaged or the conditions to engage

NAV mode are met and, 
.. Aircraft is above 400 ft AGL. 

If HDG/TRACK is engaged, APP NAV engages when the intercept conditions are met (aircraft track line must intercept the flight plan active leg).

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5. The navigation and guidance system design
Getting to grips with RNP AR

5.2.2. FINAL APP MODE ENGAGEMENT CONDITIONS (EXCEPT FMS HWI R1A):
Engagement conditions of FINAL APP mode are:
.. APP NAV mode is engaged
.. FINAL mode is armed and
.. Aircraft is crossing (within 150 feet of tolerance), a descending leg of the approach profile (the deceleration point D has sequenced). Cases n°1 and n°2 and n°3 below:
E
n
g
a
g
e
m
e
n
t
o
f
F
I
N
A
L
m
o
d
e

.

Or,
.. OP DES, V/S or FPA is engaged toward an FCU altitude that is below the next approach path level, the deceleration point has sequenced and the aircraft position is less than 1.5 NM from the lateral profile. Cases n°4 or n°5 below:
E
n
g
a
g
e
m
e
n
t
o
f
F
I
N
A
L
m
o
d
e
(
c
o
n
t
’
d
)

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5. The navigation and guidance system design

FINAL mode engagement is not possible in the following situations:
6: Aircraft before Deceleration point D.
7: Aircraft trajectory does not cross the profile.

In FINAL APP mode, the aircraft V/S is limited to 1800 ft/min in CONF0 or CONF1 and 1200ft/min in CONF 2 or more. If the aircraft GS is too high the aircraft will not be capable to maintain the required vertical flight path angle. With strong tailwind this limit may be reached and if this happen, the aircraft will not be able to stay on the vertical flight path, the V/DEV will start to increase above flight path. This design characteristic must be taken into consideration in defining airspeed constraints and/or tailwind limitations. For example the 1200ft/min limitation will be reached on a 3°flight path with a GS of 226 kt. (These limitations have been modified with FMS HWI R1A)
　
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