IRS selectors

[FS-chick]

Having just read an incident repport where th IRS selectors was left in align rather than NAV after takeof (classic) i Wonder what would happen if that was done in the (newer than 2006) NG?

And before you shoot me down in flames: keep in mind theres no such things as an stupid question, just dumb answers :ok:

BTW tis is about B737

[JammedStab]

A LOT 737 did this in London. Went back to Align while holding short of the runway and the forgot to go back to Nav. Seeing as the IRS's in align leave you without any ADI info in the modern Boeing aircraft I fly, I suspect it is the same for the NG as well.

No position, flight instrument displays providing little info.

[NSEU]

From an engineering perspective:

You should get attitude on your primary displays if you go to Attitude (although it may take 30 or so seconds to appear). However, the initial display will be affected by aircaft accelerations (so try to keep this to a minimum during that 30 seconds). I've noticed on the 747-400, for example, that the 30 seconds is extended if you do move about a lot. V/S will also be available in ATT mode.

You will get a compass rose if you enter a heading into the CDU (refer to your whiskey compass for this). You won't get boxes for a position entry, but you will get dashes for a heading entry. Update the heading periodically (to compensate for magnetic variation). The compass rose will only be available in HDG UP displays.

There will be other airplane systems affected in ATTitude mode. I'm sure your airline procedures will guide you in this area.

[Gryphon]

But if you don't switch them to NAV then they will stay in ALING mode, and in this mode any excessive movement/acceleration will force the Align process to restart in a way it will never finish if you take off, except some IRS models able to be aligned in flight. That means that perhaps you cannot get any info even in ATT reference mode (it will depend on the IRS model).
Finally, if you have them in ATT reference mode they are susceptible to gyroscopic precession as they are not navigating anymore. That's why you must introduce via the CDU the magnetic heading from other source.

Of course I'm just guessing as I cannot check in any manual what happens if you take off in Align :sad:

So any comments are welcome :ok:d

[vapilot2004]

As NSEU offers, a period of unaccelerated straight and level flight will bring the ATT functions of the IRS units online. 30 seconds is minimum, turbulent conditions or failure to fly straight and level can extend the time required.

This generally would mean a climb out of terminal airspace using standby AI and whiskey compass for attitude and heading reference. MAP info and FMC calculations are gone. Radio NAV systems are unaffected although presentation brings us back to basics.