USEFUL INFORMATION ON FMGS

 

By Philippe Burcier

 

1. MODIFYING NON PRECISION APPROACHES STORED IN THE NAV DATABASE

 

A very frequent question is asked to Airbus:

 

Is it possible and permitted to modify NON PRECISION APPROACHES stored in the navigation database?

 

Originally the airworthiness authorities did not approve any modification of the SIDs and STARs entered in the navigation database of the FMGS.

 

These SIDs and STARs being considered as published departure/approach procedures just like paper approach charts.

 

But it occurs that in flight, the crews need to modify approach procedures for several good reasons.

 

Therefore Airbus requested, and obtained, a modification of the Flight Manual and

the following statement has been included in the A320/A340 Flight Manual and in the FCOM vol 3 "limitations" chapter:

 

"The modification of a published approach procedure or a missed approach or a SID or a STAR, once drawn from the navigation database should not be done unless the ATC clearance has been obtained and the implication of doing so is clearly understood by the flight crew. "

 

It is now possible to perform some modifications but flight crews must be aware of the internal FMGS limitations.

 

When the flight crew performs a F-PLN revision that modifies the final segment of a non precision approach, the FMGS may not be able to rebuild correctly the original vertical profile.

 

F-PLN revisions include

 

Late DIR TO (DIR TO the MAP). (Not reproduced on A330/A340)

 

Clearing the turning point (T-P) when the FAF (final Approach Fix) is the "TO" waypoint.

 

Any other modification while the final segment is being flown.

 

This may result in:

 

Incorrect vertical profile (V.DEV display and guidance)

 

Erratic engagement of the FINAL mode.

 

This concerns mostly approaches which terminate with a Mixed Approach Point (MAP), not a runway.

 

 

Example N°1

 

The NON PRECISION APPROACH terminates at a MAP located before the runway and the crew does a "DIR TO" the MAP or clears the turning point (T-P) while FAF is the "TO" waypoint.

 

 

 

Example N°2

The non precision approach terminates at a missed approach point (MAP) located beyond the runway and the crew does a DIR TO the MAP or clears the turning point (T-P) while FAF is the "TO" waypoint).

 

 

 

 

FINAL mode will engage to aim the MAP at MDA altitude

 

Example N°3

 

The final segment being composed of 2 slopes the flight crew modifies the FAF altitude (e.g. extreme low temperature encountered). May concern A330/A340 as well.

 

 

 

First segment of the final is replaced by an IDLE segment

CONCLUSION

 

Flight crew shall be aware that the FMGS may not be able to rebuild the original non precision approach stored in the database when a flight crew revision (lateral or vertical) modifies the final approach trajectory.

 

For example :

 

DIR to the MAP

Clearing the turning point (T-P) while the FAF is the "TO" waypoint

Any modification while the final segment is flown.

 

The A320 FMGS are more vulnerable than the A330/A340.

 

When the crew performs one of these modifications he shall revert to using FPA mode and disregard the VDEV indication on ND and MCDU. The lateral navigation is not affected, therefore NAV mode may remain engaged but the crew will not use APP NAV - FINAL.

 

Repercussion on the use of FINAL APP MODE

 

To perform Non Precision Approach using FINAL APP mode is an operational choice that the airline can make provided precautions are observed.

 

For each approach the Navigation Data Base coding has been verified in the simulator (in flight check for RNAV approach without GPS PRIMARY).

The airline with the Navigation Data Base producer have established procedures to ensure that the coding will not be modified without notice in a subsequent revision of the Data Base

 

The flight crews are instructed to use FINAL APP mode only for standard arrival procedures and approaches that have been validated by the airline.

No modification to the approach procedure including altitude constraint is permitted.
The approach trajectory must be intercepted before the FAF.

 

 

The vertical guidance must be particularly monitored. The crew should be prepared to revert to FPA or V/S if guidance is unsatisfactory.

 

 

2. REDUCED VAPP

 

Since the A321 entry into service, a new VAPP has been proposed to the crew on the PERF APPR page.

This new algorithm is basic on A319,A321,A330 and A340 and can be installed on A320 with a specific pin program.

The formulae is:

VAPP = VLS + 1/3 of the headwind component

1/3 of the headwind component being limited by:

5 knots as a minimum

15 knots as a maximum

 

2.1. Why a new VAPP?

To reduce the approach speed by several knots while remaining above VLS that is the limit fixed by airworthiness authorities.

 

2.2. Why VLS + 5 knots as a minimum?

To get a margin in the event of speed variation.

2.3. Is it possible to fly below VLS + 5 knots in approach?

Yes, at landing, the minimum required speed is VLS.

On short runway the crew may subtract the 5 knots. Airbus recommends to disconnect the A/THR and to maintain manually the target speed. Manual control is more accurate.

2.4. When are the 5 knots mandatory?

In actual CATII or CAT III automatic approach and landing.

When there is ice accretion.

In addition to the Flight Manual, Airbus recommends to keep the 5 knots when A/THR is used and for practice autoland.

 

 

2.4. Why on the A319/A321 PFD , the magenta triangle may indicate an approach speed which is less than VLS + 5 knots?

The VLS displayed on the PFD is computed by the FAC while the VAPP, computed by the FMGS, uses the VLS computed by the FM.

Both VLS may not be the same for the following reason:

Algorithms within FAC and FMGS are the same but the GW used for the computation is defined using different parameters.

The FAC estimates the GW using specific sensors such as AOA, aircraft attitude, indicated airspeed, center of gravity … while the FM uses the zero fuel weight entered by the crew and the fuel on board.

In CONF FULL, +/- 3 knots is an acceptable accuracy of the VLS displayed on the PFD (FAC computation).

 

 

 

 

 

 

 

 

 

 

 

 

A319/A321 PFD display

 

 

 

2.5. Why are the A330/A340 crews not complaining?

FE and FM parts of the FMGS use the same GW coming from the FCMC.

Therefore both information are coherent.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A330/A340 PFD display

 

 

3. AUTOLAND IN CAT I OR BETTER CONDITION

 

3.1. What are the restrictions to perform autoland in good weather conditions?

All A320 and 340 family aircraft are basically capable of autoland.

Airbus Autoland system is certified on airport equipped with CAT II or CAT III equipment:

ILS CAT II or CAT III capable,

specific visual aids and runway lights,

RVR measurement,

electro-magnetic protection of the ground installations,

activation of specific CAT II or CATIII procedures by the airport controllers.

Specific runway characteristics

maintenance procedures

 

Airbus found acceptable to perform autoland when some of the requirements are not fulfilled provided the weather conditions are CAT I or better.

Nevertheless, the airline operations should check the suitability of the selected runways used for the practice of autoland.

Tower Wind must not exceed autoland limitations.

Criteria to qualify the crews on this type of autoland should also be established.

Please refer to FCOM VOL3 "limitations" and also refer to the Airbus brochure

"Getting to grips with CAT II / CAT III operations ".

3.2. Is a minimum altitude recommended to disconnect the AP when practicing automatic approach or autoland?

For safety purpose, the autopilot can be disengaged at anytime during automatic approach or autoland.

During practice autoland, Airbus considers that the normal take over and the ability to fly safely the final flight path require a minimum altitude of 80 feet above ground.

Therefore, Airlines should consider an autopilot disconnection at or above 80 feet for practice autoland interruption.

Below this altitude a go around should be envisaged.