When The flight is completed and I am at the gate for the turnaround, if I check my position I often see that the GPS and IRS are not exactly the same. 

The difference tipically is very small.

the question is, in real world is there a maximum discrepancy after that you have to realign the aircraft position? Or is it done only in  the cold and dark startup?

thank you

7 minutes ago, Marenostrum said:

When The flight is completed and I am at the gate for the turnaround, if I check my position I often see that the GPS and IRS are not exactly the same. 

The difference tipically is very small.

the question is, in real world is there a maximum discrepancy after that you have to realign the aircraft position? Or is it done only in  the cold and dark startup?

thank you

Discrepancies are always present and in real life

After you shut down at the gate you turn off the IRS and realling them before the next flight. This is what most of the companies I know of do.

We realign the IRUs every flight.

Depending on previous flight length I sometimes use the Fast Realignment procedure as explained in FCOM1 SP11.3

Yep. Except in a certain batch of airplanes produced from 2016-2017, where you shouldn't realign IRU,up to an 18hr limit, its a fast or full re-alignment each flight.

Just turn the switches back to off, then NAV. Enter the airport I.D. and then go to page 2 to copy the GPS co-ordinates into the scratch pad, then back to page one and enter them.

2 hours ago, MarkJHarris said:

Yep. Except in a certain batch of airplanes produced from 2016-2017, where you shouldn't realign IRU,up to an 18hr limit, its a fast or full re-alignment each flight.

Just turn the switches back to off, then NAV. Enter the airport I.D. and then go to page 2 to copy the GPS co-ordinates into the scratch pad, then back to page one and enter them.

Thank you Mark,

This is what I usually do, but it takes time to realign. The fast alignement (that I did not use before) as Paul says, if I am not wrong requires to turn the switch to ALIGN (and not off) wait for the light ALIGN illuminate steadily and then it is possible to set the new position in the CDU. Then IRS mode selector back to NAV. This should be faster.

I wonder if the fast procedure is admitted only for a "small" realignment during a turn around (not a cold and dark start).

Thank you.

Edited June 18, 20187 yr by Marenostrum

The fast alignment does not null the effects of the Earth's movement through space on the inertial gyrorings, and only resets the location.  This is a good tool to have for those fast turn arounds on the Alaska North Shore where alignment can take much longer and it's only been a few hours since the last full alignment.

No turn is short enough that you would need to do a fast alignment.  You are better off just doing a full alignment every flight.

1 hour ago, JoeDiamond said:

No turn is short enough that you would need to do a fast alignment.  You are better off just doing a full alignment every flight.

I thought it was not so critical considering that, from my knowledge, the IRS     is the last system in term of priority that the FMC uses to define the aircraft position after GPS, 2 or more DME stations, 1 VOR-DME, 1 LOC-DME, 1 LOC and finally the IRS.

Nontheless in a "desert" area it could remains the unique alternative to the GPSsystem.

Edited June 18, 20187 yr by Marenostrum

Alignment takes a lot longer the further away from the Equator you are. Thus a WestJet or Air Alaska crew may consider a fast alignment, where I wouldn't bother in the UK.

There should be an option to fast align in the options on the FMC, so you do a normal one but it works quicker. That would be your best option I would think.

Typically, you finish the shutdown checks, and flip the IRS next, then do the tech log, and depending on who's flying start the FMC loading for the next leg, it's usually only a minute or so before you can enter the GPS co-ordinates and then just put everything else in. If you activate it, then put the winds in you should be about ready for the next pre-flight checklist by the time it's aligned and popped back up.

In reality, it's never a problem, then again, as I said, we have a batch of aircraft where Boeing have recommended we don't re-align until it's flown 18 hours. The fix for these should come soon, and our most recent arrivals don't have the part that requires this.

 

4 hours ago, MarkJHarris said:

There should be an option to fast align in the options on the FMC, so you do a normal one but it works quicker. That would be your best option I would think.

There is, in the PMDG SETUP > menu structure. FAST is a 30 second alignment. There's also INSTANT.

9 hours ago, MarkJHarris said:

Alignment takes a lot longer the further away from the Equator you are. Thus a WestJet or Air Alaska crew may consider a fast alignment, where I wouldn't bother in the UK.

 

It's Alaska or Alaska Airlines if you want to be fancy.

 

I've done Barrow and Deadhorse many times & never felt the need to do a quick-align. During the shut-down flow, I turn off the IRS and when we do the shut-down checklist, I turn them back on. They're off long enough to reset and aligned before it causes any problem.

The only *possible* exception I can think of would be if the wind is blowing so hard that the plane is shaking and it's giving enough motion to upset the realign. I've never experienced that. We run the hydraulics the whole time so the controls aren't flopping around and shaking the plane, too.