Originally Posted by
FlyingStone
This is a common misunderstanding. The IRS position is never updated.
The data from all the navigation sources (IRS, GPS, VOR, LOC, DME) goes into the FMC through Kallman filter, where FMC weighs reliability of each data (satellite coverage with GPS, geometry with VOR/DME, etc.) as well as compare outputs of data sources to each other (an IRS which shows completely different position would have lower weight) and calculate what it thinks is the position of the aircraft - so called FMC position, which is then displayed on the ND.
But the IRS itself never gets updated. On the B737, it is not unusual for IRS to be away a mile or two from GPS position after a long flight and Boeing recommends full alignment prior flight whenever possible.
totally agree.. thats what I always thought!