Two A320 IRU questions
 

Hi all,

1. When aligning the IRU's, I've heard a popular theory/technique/procedure that they should be moved to NAV in the order 1-2-3. I've also seen 1-3-2. I couldn't find any specified order written anywhere- neither in my company manuals, nor the Airbus manuals. Is this just urban myth, or is there actually something to the order in which they're powered up?

2. Let's say we're sitting at the gate with the GPU supplying power, and the IRU's aligned, and the GPU gets disconnected. Would it be ok to leave the IRU's in NAV while starting the APU, or would it be better to switch them off until the APU is up? Either way, why?

Hi Check Airman,

The technique of switching 1,2,3 was taught to me also. Apparently it was to remind me that No 3 is the middle switch position to help avoid finger trouble when performing an ECAM / Checklist.
I would leave all 3 at NAV whilst the APU started. The internal battery was designed for short interruptions of power.

Case closed. :)

leave em on if you don’t mind the horn.

Yup, the horn usually gets drowned out by one of the two chaps up front shouting their heads off at the guys on the ground who pulled the GPU without prior clearance...

Makes sense. Thanks for the information.

IDIRU 1 will stay online, 2 and 3 will drop after 5 minutes on battery power.

On battery power even IR1 will be damaged that's why the horn to switch it off or attach a power source.