Hi 76mike,
Our 320 manual says "If a high navigation performance is desired, (i.e. for long-range flights without GPS and without radio navigation updates, or if low RNP operation is expected), the crew should adjust the airport reference coordinates to the gate coordinates, provided that this data is published or available on board. In this case, the flight crew should use the slew keys successively for Latitude and Longitude, instead of inserting the coordinates on the scratchpad, (in order to avoid errors)."
Only our FM position is updated on take off by a "TO BIAS". The IRS positions remain unchanged. The TO BIAS is continuously updated by radio.
I think your "IRS NAV Only" message means you are not receiving radio position updates and the FMS will be using the last "Updated BIAS" it received + MIXED IRS.
If you then lost FMS (Back Up NAV) you would only be able to see your onside IRS POS.
When we had INS only, we could update the displayed position over a fix. It remembered that position bias until we either updated it again or "Flushed the Fix" to remove it. PFM
@ Flyboyike,
the 2-mile error would have been done and gone by the time you made it to TUL
It will have been done and gone whilst on the runway during take off.
@BOAC
Without radio updating, the FMC position will gradually 'blend' towards a position referenced to its alignment point, since up to then it has constantly been 'corrected' by updating. No sudden 'jump'. IE in Tom's language, the error vector will slowly reduce to zero.
Very strange indeed.
Which aircraft and do you have a reference please?