IRS alignment on Airbus

[captain_jeeves]

Originally Posted by 145qrh

If you have GPS primary it really doesn't need to be a full align as it updates itself on application of take-off thrust.

145qrh

I think I know what you are trying to say, but your response is slightly misleading (no disrespect intended).


True, with FMS1, the FM position is automatically updated at the runway threshold. However, with FMS2, this automatic position update is inhibited when GPS Primary is available.

Next, remember what GPS Primary actually does to the IRS system. When the GPIRS position is available, it overrides the RADIO position, if available. Therefore, the FMS position tends toward the GPIRS position.

However, without GPS Primary – but with GPS supplemental – the GPS position is considered to be an additional form of NAVAID, and can be accepted, if it falls within the radio position or MIX IRS position. In this case, the FM position is updated when thrust is advanced TOWARDS TOGA during the take-off.

Switch the IR’s off for at least 10 minutes to obtain a full alignment?

No idea why this is any companies SOP. You only need them to be off for about 5 seconds to get a full alignment (Maybe you company meant to print 10 seconds in the SOP, and you’re dealing with a typo).


On the other hand, they (IRS's) do continue to work for about 30 to 60 seconds after you select them to off, so maybe your company has found something in this that Airbus has not recognised.

Regardless, if you like your job and present company, DO AS THEIR SOP SAYS until such time as they change it, and then, DO AS THEIR SOP SAYS.

Cheers /cj