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IRU Fast Align  
User currently offlineModesto2 From United States of America, joined Jul 2000, 2898 posts, RR: 5
Posted (15 years 5 months 1 week 1 day 23 hours ago) and read 4147 times:

Could someone please explain an "IRU fast align"? All I know is that it takes 30 seconds, not 10 minutes like a "normal align". Thanks and happy holidays!

3 replies: All unread, jump to last
User currently offlineBuzz From United States of America, joined Nov 1999, 697 posts, RR: 20
Reply 1, posted (15 years 5 months 1 week 1 day 14 hours ago) and read 4105 times:

Hi Modesto2, Buzz here. It's a 'refinement" of the present position instead of the full align. THe IRU's (Inertial Reference Units to answer the other guy's question) have to already be running, aligned.
In the 10 minute align that has to happen, the accelerometers are feeling the earth rotate. If it feels the rotation at a certain speed, it decided it's a a certain latitude. The longitude can be almost anything, but it compares previous position at shutdown to the presently entered positon. If it's more than 60 NM / 1 degree off it raises a question.
I don't use the fast align, i wake up the airplanes as the night draws to a close and see if they're in a good mood to go flying.
g'nite and Good Christmas
Buzz Fuselsausage: Line Mechanic by night, DC-3 Crew Chief by choice.

User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 24
Reply 2, posted (15 years 5 months 1 week 19 hours ago) and read 4078 times:

Fast align is just going to give you the aircrafts attitude and heading as opposed to present position. It takes thirty seconds of level unaccelerated flight in order to do a quick align. Once the 30 seconds is up...you will have attitude information. You have to set a heading - ususally thru the FMC, I'm talking Boeing here. Once the heading is set...you know know direction. IRS info cannot be used for Navigation.

User currently offlineFDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 30
Reply 3, posted (15 years 5 months 1 week 15 hours ago) and read 4074 times:

Cdfmxtech: I think what you're referring to is the IRS (or INS) in the ATT (attitude) mode. This is a degraded mode as you said supplying pitch/roll and heading as initially input by the crew.

As the inertial systems have matured, it seems like knob twisters like myself have less choices to choose from. Used to have OFF, STANDBY, ALIGN, NAV and ATT. Then we had OFF, ALIGN, NAV, ATT. Then OFF, NAV, ATT. The MD11 has OFF and NAV and I was told by a United mech that the IRU's aren't even switched off now on the 777.

You're only as good as your last departure.
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