Iru position updating
– to update it’s own determination of aircraft position. In the event of a GPS failure which causes the FMC to re-consider (for example) the ADIRU/IRS as the new position source, it will take some time for the FMC position to “wander” across to the IRS determined position.It’s not instantaneous – which is a good thing, given that the aircraft would take a sharp turn to head back towards track if the position reference were to change quickly.Fortunately the GPS Checklist encourages the use of radio navaids as a navigation source – but it’s not exactly clear about the need for it.
It sure seems like the numbers (which starts at about 4 nm/hr and ameliorates out to about 20 nm after 10 hours) seems a bit excessive when I’m scarcely seeing more than about half a mile drift on the IRS when I shut down after 14 hours).They can all be seen on the POS REF page of the FMC.IRS L & IRS R Position: Each IRS computes its own position independently; consequently they will diverge slightly during the course of the flight.Today’s IRS’s are stunningly accurate – after 14 hours of flight, despite an ANP of 20 miles or more – it’s not unusual to see less than a mile’s difference between the GPS and ADIRU.
But that mile discrepancy is a significant impact on terminal navigation – were it not for radio updating of the FMC position.
This practically eliminates the need to enter a take-off shift in the TAKE-OFF REF page.