NORMATIVA PARTICULAR A CADA MEDIO A UTILIZAR: Bolsa porta herramientas

General

• The FIRST & ESSENTIAL function of the FMS is NAVIGATION; which is to compute the FMS position as accurately as possible.

The validity of all the other FMS functions depends upon the accuracy of the FMS position.

• Furthermore, the ACCURACY of the FMS position dictates the strategy which the pilot will apply in the use of Autopilot / Flight Director modes as well as on the type of display to be used on the ND.

Auto Pilot / Flight Director modes (managed or selected) FMS NAV ACCY

→→→→

EGPWS (ON/OFF)

FMS NAV ACCY CROSSCHECK is a PERIODIC DRILL to be achieved throughout the flight.
Reminder of key points in aircraft position computation

• Without GPS Primary

The FMS position is derived from the 3 IRS positions which are blended into a MIX IRS position and from a RADIO position whenever 2DMEs, or a VOR/DME, or a GPS supplemental are available. The GPS supplemental is considered as an additional kind of navaid; it is accepted if it falls within a circle around the radio or the MIX IRS position.

When a RADIO position is available, the FMS position tends towards the Radio position.

- FMS position principle -

Hence the FMS continuously computes:

- the FMS position out of MIX IRS and RADIO positions.

- the BIAS between MIX IRS and FMS position, so as to benefit from the latest update when the RADIO position becomes unavailable.

The ESTIMATED POSITION ERROR (EPE) is an ESTIMATE; this means that the FMS considers the instantaneously available navigation means used in the elaboration of the FMS position, applies specified tolerances to each one of them and processes the EPE. Those tolerances assume that the navigation means work properly; but they ignore possible excessive drifts of the IRSs, or erroneous locations of navaids within the Nav Data Base, for example.

Consequently the HIGH / LOW accuracy information provided on PROG page are INDICATORS to the crew of the POTENTIAL ACCY of the FMS position versus a specified accuracy criteria.

The EPE is an ESTIMATE

HIGH/LOW and thus NAV ACCY UPGRADED/DNGRADED messages are merely INDICATORS to the crew of the estimated accuracy of the FMS position versus a required criteria.

• With GPS Primary

* The GPS directly interfaces with the IRSs which output a GPIRS position.

When a GPIRS position is available, it supersedes the RADIO position if available, so that the FMS position tends to the GPIRS position.

The GPS provides two essential data, additionally to position (Lat/Long/Alt):

- The ACCURACY of the Lat/Long position: it is a direct function of the satellite constellation in view of the aircraft. If the satellites are low on the horizon, or if their respective position is unfavorable, the resulting accuracy will be poor. It is provided as a "figure of merit".

This accuracy can be computed with a high probability of confidence.

- The INTEGRITY, which is a direct function of the number of satellites in view of the aircraft, allows a defectuous or erroneous satellite to be rejected. If 5 or more satellites are in view, several combinations of those satellite signals may be used to process “several positions” and to carry out reasonableness tests on the satellite signals themselves.

Therefore if the GPS position (or GPIRS position) fullfills the INTEGRITY and ACCURACY criteria, this means that this position is the BEST RAW DATA position available.

GPS PRIMARY = ACCURACY + INTEGRITY CRITERIA met ➾➾➾➾ GPIRS = RAW DATA

* Some details on the information provided: - PROG page:

- Indicates GPS PRIMARY

- Indicates the value of the Estimated Navigation Accuracy in green:

- it is either the one as computed by the GPS when GPS PRIMARY is available, or the EPE when GPS PRIMARY is LOST.

- Indicates the Required Navigation Accuracy in blue:

to day it is either the Navigation accuracy criteria as required in cruise, TMA or approach area, or it can be manually inserted.

If the Estimated Nav Accy is less than the Required Nav Accy, HIGH accuracy is displayed (or LOW if vice versa).

These indications will allow the RNP concept (required nav performance) linked to FANS to be addressed.

NOTE:

- SELECTED Navaid page:

- DESELECT GPS prompt:

it is provided in order to allow the crew to prevent the FMS from using the GPS data for position computation, should a major problem occur with GPS. GPS PRIMARY LOST message is then displayed on MCDU and ND.

The GPS can be reselected later on the same page. - GPS PRIMARY prediction page:

- GPS PRIMARY allows the crew to carry out approaches down to MDA 250 ft AGL.

GPS PRIMARY criteria (INTEGRITY + ACCURACY) depends upon the SATELLITE CONSTELLATION status at a given time, in a given location; this is predictable (ephemerides). Therefore, it is most valuable for crews to know if GPS PRIMARY will be available at DESTINATION, or at ALTN.

This information is provided on GPS PRED page when the A/C is fitted with IRS Honeywell. - ND / MCDU messages:

- GPS PRIMARY: when GPS PRIMARY is again available – This message is clearable.

- GPS PRIMARY LOST: when GPS PRIMARY is lost. Clearable on MCDU but not on ND.

Summary

FMS POSITION