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INITIALIZATION
Refer to the Refer to NO-020 ADIRS Initialization.
TAKEOFF
Each FMGC uses the MIX IRS position as its position, until the thrust levers are pushed forward
to TOGA. The FMS position is then updated to the runway threshold coordinates. The difference
between the MIX IRS position and the FMS position is referred to as the TO BIAS. The TO BIAS
is added to the MIX IRS position, for the subsequent FMS position.
FMS position updating at Take off
IN FLIGHT
The original TO BIAS is continuously updated with the current radio aid.
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Updating BIAS principle
If the radio position is lost, the system uses the updated BIAS to determine the FMS position
from the MIX IRS position.
NAVIGATION ACCURACY
The FMS computes the Estimated Position Error (EPE). The EPE is an estimate. To compute
the EPE, the FMS considers the immediately available navigation means in the FMS position
computation and applies defined tolerances for each of them. These tolerances assume that
the navigation means are working properly. They ignore any possible excessive IRS drift or
erroneous locations of navaids. The MCDU PROG page displays the HIGH/LOW indications,
according to the EPE. These indications reflect the probable accuracy of the FMS navigation
compared to the determined accuracy criteria.
WITH GPS PRIMARY
PRINCIPLE
The GPS interfaces directly with the IRS that outputs a GPIRS position. When a GPIRS position
is available, it overrides the RADIO position, if available. Therefore, the FMS position tends
toward the GPIRS position.
INITIALIZATION
Refer to the Refer to NO-020 ADIRS Initialization.
TAKEOFF
The FM position is automatically updated at the runway threshold. With FMS2, this automatic
position update is inhibited.
IN FLIGHT
The FM position tends to the GPIRS position as long as the GPS satellites are available.
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NAVIGATION ACCURACY
The GPS position is characterized by two parameters:
• Integrity
• Accuracy.
Integrity is a direct function of the number of satellites in view of the aircraft. If five or more
satellites are in view, several combinations of the satellite signal may be used to process
"several positions" and to carry out reasonableness tests on the satellite signals themselves.
Accuracy functions in direct connection with the satellite constellation in view of the aircraft.
If the satellites are low on horizon, or not in appropriate positions, accuracy will be poor. It is
provided as a "figure of merit".
If the GPS position fulfils both the integrity and the accuracy criteria, GPS PRIMARY is
displayed on the MCDU PROG page and the GPS position is the best raw data position
available.
SUMMARY
FM POSITION
Flight phase WITHOUT GPS PRIMARY WITH GPS PRIMARY
On ground before Takeoff MIX IRS GP IRS
Takeoff Updated at runway threshold (shift) (1)
In flight With RADIO Tends toward RADIO GP IRS
Without RADIO MIX IRS + BIAS GP IRS
(1)With FMS2, the FMS position update at takeoff is inhibited, when GPS PRIMARY is active.
USE OF FMS
Applicable to: ALL
The navigation accuracy is managed through several MCDU pages:
PROG PAGE
This page indicates the GPS PRIMARY.
The PROG displays the estimated navigation accuracy in green. This provides the EPE, if GPS
PRIMARY LOST, or is computed by the GPS, if GPS PRIMARY is displayed
The PROG page displays the required navigation accuracy in blue (this can be changed). The
required navigation accuracy thresholds are determined, depending on the flight phase, or can
be manually entered. These thresholds are used to change from HIGH to LOW accuracy, or vice
versa. These indications are used when flying within RNP airspace.
SELECTED NAVAIDS PAGE
The SELECTED NAVAID page is accessible from DATA/POSITION MONITOR/ FREEZE/SEL
NAVAIDS. It has a DESELECT prompt, that enables the flight crew to prevent the FMS from using
A330/A340 FLIGHT CREW TRAINING MANUAL
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the GPS data to compute the position, in the case of a major problem. GPS PRIMARY lost is then
displayed on MCDU and ND. The GPS can be reselected using the same page.
PREDICTIVE GPS PAGE (IRS HONEYWELL ONLY)
The PREDICTIVE GPS page is accessible from PROG page. The GPS PRIMARY criteria depend
upon the satellite constellation status (position and number) and this is predictable. The crew can
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A330/A340机组训练手册FLIGHT CREW TRAINING MANUAL(106)
