Airbus A340 Series, Брошюра

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STL 472.502/90 Issue 6
A340 Automatic Flight System - Flight Management
10.25
Lateral navigation
• Position computation :
- Before flight, the three IRSs are aligned on airfield
or gate position (manually or via database).
- At takeoff, the position is automatically updated to
the runway threshold.
- In flight, position updating is computed using radio
navaids (DME, VOR, ILS and GPS when
available).
The FMGC position is a blend of IRS and radio
position. On a medium-term basis, the FM position
will tend towards the radio position, if any drift
occurs.
• Navigation mode selection :
- If the aircraft is equipped with GPS primary, the
FMGC uses the GPIRS position in priority (IRS-
GPS mode).
- If the GPIRS position is not available, or if the
aircraft is not equipped with GPS primary,
depending upon the availability of navaids and
sensors, FMGC automatically uses the best
navigation means to compute the most accurate
position :
- IRS - DME/DME
- IRS - VOR/DME
- IRS - ILS/DME
- IRS only.
• The FMGC position is associated with a high or low
criterion, which is based on an Estimated Position
Error
(EPE) .
This EPE depends upon the flying area (en route,
terminal, approach) and is permanently compared to
Airworthiness Authorities Accuracy Requirements
(AAAR).
- If EPE > AAAR, then LOW is displayed on MCDU
and the position must be cross-checked with raw
data (ADF/VOR needles, DME reading).
- Each time HIGH (or LOW) reverts to LOW (or
HIGH) the message NAV ACCUR DOWNGRAD (or
UPGRAD) is displayed on NDs and MCDUs.