seven3seven From United States of America, joined Apr 2005, 296 posts, RR: 26 Posted (6 months 3 weeks 2 days 11 hours ago) and read 2038 times:
In all the pics of European airliner Boeings at the gate I notice what looks like a full inertial navigation system alignment everytime. At my airline we just do a quick align of the system between flights during the day. It takes just a few seconds as compared to up to 7 minutes with a full alignment, and we have GPS anyway.
Can anyone provide a justification for a full IRS alignment after every flight?
My views are mine alone and are not that of any of my fellow employees, officers, or directors at my company
strfyr51 From United States of America, joined Apr 2012, 488 posts, RR: 0 Reply 5, posted (6 months 3 weeks 1 day 19 hours ago) and read 1747 times:
a full alignment clears any errors that might have built up over time. Since Drift and radial position error are all monitored in the Maintenance manual with limits it just makes good sense to do a full alignment if that's what you need. . Since 2 alignments reported bad might cause the replacement of a $300,000 part needlessly? It makes darn good sense financially to take the extra 3-5 minutes. .
KAUSpilot From United States of America, joined Jan 2002, 1955 posts, RR: 37 Reply 6, posted (6 months 3 weeks 1 day 19 hours ago) and read 1741 times:
Obviously I'm not European but: We do a full alignment every time; granted our average leg length is about 8 hours and the fastest turn I've ever done is 45 minutes. Going from memory, the 744 can go no longer than 18 hours of flying time between full alignments.
Reasons we turn them off after the flight and usually do a full alignment prior to the next flight:
1. After a flight, we don't know how long the airplane will sit there between the time we leave and the time the next crew shows up (and the plane almost always stays powered up with APU or external). If there was a power interruption and the plane went into standby power while it was sitting there, the IRU's could overheat if there wasn't a mechanic or someone else around to notice (normally someone should be monitoring the aircraft all the time while the APU is on, of course). The airplane is rarely powered completely down between flights.
2. We have no real way of knowing how long the previous crew went before a full alignment. Fast alignment is still an option that can be used at the crew's discretion, but SOP is a full alignment before every flight. So it's possible that the aircraft might be close to it's 18 hour limit.....there's no way to know for sure without asking the previous crew, who we usually don't see.
3. True it doesn't matter very much given GPS, but IRU accuracy is tracked by the company at the end of each flight....we have to send IRU error out as part of the ACARS post flight report. Also much of Europen airspace is RNP 5 which is easy enough to maintain on IRU's alone, so it's nice to know you can maintain RNP even if GPS is lost for some strange reason. There is no such RNP requirement in the USA's domestic enroute airspace structure, to my knowledge, so it would make sense not to worry about it as much there.
AA737-823 From United States of America, joined Mar 2000, 5340 posts, RR: 11 Reply 7, posted (6 months 3 weeks 1 day 18 hours ago) and read 1730 times:
The catch is, if you're in WINDY conditions, the 737NG is notorious for refusing to align. For this reason, a lot of carriers won't do a full align if there's much wind at all. If you can feel the tail shaking, it's usually a bad idea, in my experience, to realign.
sandrozrh From Switzerland, joined Feb 2007, 3382 posts, RR: 51 Reply 12, posted (6 months 1 week 3 days 5 hours ago) and read 1248 times:
We do quick alignments during turnarounds if the residual groundspeed is less than five knots and if we're staying on the same aircraft for the next sector. In all other cases we will do full alignments. Also, the IRS positions need to be within 5NM from the airports reference position after realignment, or a new, and in this case full, alignment is necessary. This is our procedure on the A32S, A330 and A340 and is Airbus procedure, so im not sure why other European airlines would do otherwise.