Iru position updating
But in the simulator – the IRS doesn’t drift and no matter how long you fly for, the IRS position is superimposed right over the GPS position – which is totally missleading. In preparation for RNAV RNP AR approaches – all radio nav updating in our aircraft has been disabled by default.Hence if the GPS fails, the FMC will not use radio aids to calculate position – instead it will default to the Inertial ADIRU/IRS position.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.GPS is indeed ubiquitous, as these days at any point in space it’s not unusual to be able to gain position information from between 6 and 9 satellites.This is a whole lot of redundancy and increased accuracy in position fixing.
The NAV STATUS page shows the current status of the navaids being tuned.
This practically eliminates the need to enter a take-off shift in the TAKE-OFF REF page.
Radio Position: This is computed automatically by the FMC.
Best results are achieved with both Nav boxes selected to AUTO (happens automatically on NG), thus allowing the FMC to select the optimum DME or VOR stations required for the position fix.
Series 500 aircraft have an extra dedicated DME interogator (hidden) for this purpose and NG’s have two.
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.