Originally Posted by
BOAC
An in-flight re-align would give a 'rough' attitude and heading but requires frequent re-entry of mag heading. At high latitudes this could be changing fairly quickly and around Alaska you are much closer to the magnetic 'pole' too.
Thanks,
The QRH for one type of aircraft says for an IRS message......
Place the Mode Selector to ATT(action not reversible). Then maintain straight and level constant heading for 30 seconds. Enter the current heading on the SET IRS HEADING line of the CDU position initialization page. The IRS heading may have to be updated periodically.
I suppose the periodically, depends on the actual amount that your actual heading is changing. That might be dependent on convergance and rate of change of variation.
I wonder if aircraft with GPS onboard still have proper nav capability. If Align or OFF is selected in flight, is that the equivalent of an IRU fault and then only ATT is subsequently available.