A slight navigation error...!

[744kid]

Watch your typing, guys! 

https://www.atsb.gov.au/publications/investigation_reports/2015/aair/ao-2015-029/

[United744]

Hmm... I read the report several times, but it doesn't add up.

The ATSB say that the GPS can't drive the ND map display, but that it feeds into the ADIRUs. This does not explain why the pilots didn't notice that neither distance to next was crazy, nor why the map would be slightly rotated when correctly aligned on the real runway.

Due to the massive position error, even the 640 NM range setting would not result in any of the flight plan being visible on the display.

Given the FMGC will turn towards the waypoint position based on ADIRU heading, the FMGC must think that it was in fact at Sydney. This information would have to come from GPS, and not the ADIRUs.

If the aircraft really thought it was near South Africa, as per ADIRU position, then the waypoint would be thousands of miles away, and not ahead of the aircraft, otherwise it would have not only made a slight left turn, but in fact a 90 degree turn to fly towards it.

IMHO the FMGC was using GPS position with bad ADIRU heading, until GPS PRIMARY LOST, where massive map-shift would have occurred.

Otherwise, the report about the systems doesn't match aircraft turn behavior. The FMGC also thinks it is some 120 NM off the coast of South Africa using ADIRU position, so the EGPWS alert makes no sense, either, as they would be over the ocean (so no terrain to hit, and no alert).

There was no mention if "TERRAIN ON ND" was used. If so, if the ND was displaying ADIRU position, they should see nothing but blue.

I seem to recall that when available, GPS isn't just used to supply full-time position updating to the ADIRU, but does in fact become a primary position reference in its own right.

[John H Watson]

This does not explain why the pilots didn't notice that neither distance to next was crazy,

The report does say that the F/O was cycling through the ND ranges suggesting that did have some concerns.

On the ground, is the distance-to-next based on FMC runway co-ordinates to first waypoint?

I seem to recall that when available, GPS isn't just used to supply full-time position updating to the ADIRU, but does in fact become a primary position reference in its own right.

This is only after the GPS position has been validated by IRU position. This is how it works on the 744 (within the FMC) and, reading the report, it's the same for the Airbus (within the ADIRU). These systems still live in fear that the GPS satellite network can be subjected to military imposed limitations, solar disturbances, etc.