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ADIRS Glitch when crossing 180° longitude


Jürgen Heuwetter

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Jürgen Heuwetter

Hello all,

While flying this superb bird around in the pacific a lot recently, I noticed a strange behaviour when crossing 180° of longitude. I encountered this behaviour each time on three different flights crossing 180° W/E:

Amber GPS PRIMARY LOST is displayed, a triple-clic sounds, the ND is shifting fast until it gets back to the current position. It takes about 3-5 seconds, then everything is back to normal, at least in two out of three cases. In one case, near the destination, the AP also disconnected. So far I did not record a video, but the situation should be easily reproducible.

It looks like the IRS/GPS is somehow glitching here...

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Jürgen Heuwetter

It happened on these routes:

- NSFA → NFFN: FA R453 NN
- NFFN → NLWW: CHARLI PACKO B577 PARZO MATAV
- NLWW → NGFU: 12W77 11W78 10W79 09W80 FU
- NGTA → PMDY: 04E74 08E76 12E77 16E77 20E78 24E80
- PMDY → PWAK: 27E80 25E76 22E71 AWK

Although I think the routing is pretty irrelevant, as it happens when crossing 180°...

If you want to test it, I'd suggest to take off in Funafuti (NGFU). It's closest to 180° longitude.

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I can verify this behaviour. Flying NZNR-NZCI (I might be the first FSL flight into the Chatham Islands?) via Q712, at waypoint NODAD (co located at the 180th) I get the same behaviour. Interestingly prior to crossing the meridian I selected the CI NDB on the PROG page and the distance showed as 15000-and-something NM. Evidently the system isn't calculating the great circle route until we cross the meridian. The fuel predictions and other calculations seem to be accurate prior to crossing the meridian as far as the true point-to-point distances are concerned.

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