"Quick alignment"

[jonnevin]

I have heard a few times prior to departure a plane will ask tower for a short delay so they can do a "quick alignment", what does that mean?

[evilcuban]

It's to realign, check, or otherwise resync their IRS. 

[jonnevin]

I will ask the followup naive question...what is IRS?

[180MPH]

Inertial navigation, some aircraft have three separate units, some only have one, you feed in the Lat/Long to the unit and after it spins up it can detect what direction you are moving without any outside help from radio aids, in the old days a platform took about 30 minutes to spool up, a modern laser ring gyro can accurately sense a 1˚ change in angular displacement directly, and hence 7 minutes at mid-latitudes is sufficient to find the direction of true North.

An aircraft has to be stopped during this spool up process, an aircraft having an ir problem may attempt a quick alignment at the end of the runway to fix the problem.

180MPH

[goowe]

I always wondered what this was as well -- thanks!

[jonnevin]

thanks...that is very interesting