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? If GPS PRIMARY LOST, navigation cross-check is required in climb, in cruise, about every 45 min, before Top Of Descent, reaching TMA and IAF and whenever a navigation doubt occurs.
? The crew will use, IRS only, LOW and NAV ACCUR DOWNGRAD messages as indications to trigger a navigation accuracy check.
NAVIGATION ACCURACY CROSSCHECK TECHNIQUE
The principle consists in comparing the FMS position with the RADIO position (aircraft real position).
 

  SUPPLEMENTARY INFORMATION
NAVIGATION ACCURACY
A318/A319/A320/A321
FLIGHT CREW TRAINING MANUAL 

navigation accuracy cross check technique 1
 
Two different techniques may be used:
? Either the crew will insert a radio ident in MCDU PROG page (which provides a bearing/distance relative to FMS position) and will compare with raw data received from the NAVAID which materializes the aircraft real position. This allows the error Epsilon to be quantified.
? On the ND, the flight crew compares: The position of the needle and its associated DME distance (the real position of the aircraft) with the position of the NAVAID symbol and its associated distance, indicated by the range markers (these markers provide a bearing/distance, in relation to the FMS position).
navigation accuracy cross check technique 2
 
 

  SUPPLEMENTARY INFORMATION
NAVIGATION ACCURACY
A318/A319/A320/A321
FLIGHT CREW TRAINING MANUAL 

OPERATIONAL CONSEQUENCES
The result of the navigation accuracy crosscheck dictates the strategy the pilot will apply for the use of the ND display, the AP/FD modes, and EGPWS.
 ND AP/FD mode EGPWS
 PF PNF  
GPS PRIMARY - Arc or Rose NAV with
raw data when required Lateral and vertical
managed modes ON
GPS PRIMARY LOST Or No GPS Cruise Navigation accuracy
check positive(≤ 3 nm) Arc or Rose NAV with
raw data when required Lateral and vertical
managed modes ON
  Navigation accuracy
check negative(>3 nm) ARC or ROSE NAV
may be used with care
and with raw data Lateral and
vertical managed
modes with care
with raw data OFF
 Approach (1) Navigation accuracy
check positive(≤ 1 nm) Arc or Rose NAV with raw data Lateral and vertical
managed modes ON
  Navigation accuracy
check negative(>1 nm) ROSE VOR or ILS as required Lateral and vertical
selected modes OFF

(1) A GPS defined Non Precision Approach must be interrupted if GPS PRIMARY LOST message is displayed.
POSITION UPDATE
In case of an obvious and major map shift noticed by specific messages such as "CHECK A/C POSITION, FM1/FM2 POS MISMATCH", the aircraft position may be updated on the MCDU PROG page. Two techniques are available:
　
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本文鏈接地址：A318/A319/A320/A321 機組訓練手冊 FCTM Flight Crew Training Manual(176)