Suppose I start from an airport A ( where I align ),then fly to an airport B ( not stopping the engines,not switching align to OFF ),and then to an airport C

 

Do I have to realign at airport B ? It seems to me all data are in the IRS and therefore no alignment necessary at airport B.

 

Please correct me if I am wrong and explain me why .

 

HubertWerni

 

 

 

 

 

 

Normally you'd only turn it off at the end of the day.

Please correct me if I am wrong and explain me why .

 

There is no "correct" answer here. You could if you wanted to, but unless the flight from Airport A to Airport B was several hours, and the flight from Airport B to Airport C is also several hours, there's no real need. The IRU position is being updated by GPS and DME during the flights anyway.

Normally you'd only turn it off at the end of the day.

 

I think most operators at least run through a quick align between each segment, though I guess you're right that, in that case, they're never truly turned OFF until the end of the day.

I could be wrong but I believe Boeing recommends a full realign during each turn (stop). Set the switches to off, wait for the lights to go out, then turn them on and wait for alignment. Do to subtle equipment imperfections and natural variances, it's a good idea to realign the IRSs when possible. It's unusual, and probably unsafe, to not shut the engines down during a stop. Presumably one is unloading and loading passengers and/or cargo, best done with the engines off. And, as long as the airplane has power the IRSs would stay on anyway.

 

Boeing does provide a quick align supplementary procedure for those times where a full realign will take to long. But I think only running them all day with no realignments might lead to some significant drift error.

 

Dave

The IRU position is being updated by GPS and DME during the flights anyway.

 

Just to be clear -- the IRU position is not updated by the GPS, DME etc. The FMS position is updated. The IRU position will continue to drift.

 

Generally speaking though a "quick" alignment is sufficient. However, there are certain circumstances where a full 10 minute alignment is required (i.e. turning the selectors to OFF then NAV). First flight of the day, flights above/below a certain latitude, ETOPS flights are the main ones that spring to mind.

 

Of course, chances are it's going to take more than 10 minutes to turn the aircraft round anyway...

We perform a full alignment prior to every flight.  The IRS switches are put in the OFF position as part of the parking checklist.  They go back to NAV as part of the preflight for the next flight.

I have never used, or have I seen anyone else use, the quick align feature.  There is no real need to.  A full alignment only takes a few minutes.  You wouldn't have a turn short enough for the alignment time to be a factor.

Im going to be annoying to you Joe and say that some operators do the quick align so long as post flight IRS position drift and residual ground speed are in limits.

 

Hubert, if I took off from airport A, then landed at airport B (say, for landing practice), then took off from airport B without shutting the engines down to go to airport C, then, I would not re-align the IRS's - assuming everything is working ok.

 

I would quickly check irs position drift and residual ground speed before taking off from airport B if I were worried. With GPS and radio FMC position updating available, IRS position drift and residual speed checks should be in the 'OK' range.

Just to be clear -- the IRU position is not updated by the GPS, DME etc. The FMS position is updated. The IRU position will continue to drift.

 

Good catch. Thanks.

We perform a full alignment prior to every flight.  The IRS switches are put in the OFF position as part of the parking checklist.  They go back to NAV as part of the preflight for the next flight.

I have never used, or have I seen anyone else use, the quick align feature.  There is no real need to.  A full alignment only takes a few minutes.  You wouldn't have a turn short enough for the alignment time to be a factor.

 

This is personally what I do (because, like you said, why not, since there's plenty of time), but I'd heard someone on here say quick aligns were more common, so that's what I spoke to in my earlier post. Nice to see I'm not being overly cautious.

We perform a full alignment prior to every flight.  The IRS switches are put in the OFF position as part of the parking checklist.  They go back to NAV as part of the preflight for the next flight.

I have never used, or have I seen anyone else use, the quick align feature.  There is no real need to.  A full alignment only takes a few minutes.  You wouldn't have a turn short enough for the alignment time to be a factor.

 

Just out of curiosity : who is we ?

I've flown 737 for two RW airlines. One did quick align, the other full align. I never noticed any problems. The only advice I have is if you had kicking winds and the plane is shaking, to do a quick align. The full align might not be able to handle the shaking.

Im going to be annoying to you Joe and say that some operators do the quick align so long as post flight IRS position drift and residual ground speed are in limits.

 

There is nothing wrong with that if it's approved by the specific operator.  MIne just happens to prefer a full alignment for every flight.

Just out of curiosity : who is we ?

 

AA

The only advice I have is if you had kicking winds and the plane is shaking, to do a quick align. The full align might not be able to handle the shaking.

 

I always wondered about that, thanks for the tip.

 

With laser ring gyros in all modern equipment, I suspect the difference between quick align and full alignment is not significant, and since a full alignment can always be done in a normal turnaround I was pondering the effects of the airframe being jostled by the wind. Very good info.

I always wondered about that, thanks for the tip.

 

With laser ring gyros in all modern equipment, I suspect the difference between quick align and full alignment is not significant, and since a full alignment can always be done in a normal turnaround I was pondering the effects of the airframe being jostled by the wind. Very good info.

 

I've yet to see a normal alignment fail.  It's a pretty reliable system.  The only time I have ever had alignment issues was at KSNA, where they tow the airplane off the gate at night.  They towed it off with the IRSs aligned and towed it back in the morning with the IRSs off which caused a disagreement between the entered position and where the system thought it was.  Even that is easily fixed.