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IRU Fast Align

Tue Dec 26, 2000 3:59 am

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!
Posts: 694
Joined: Sun Nov 21, 1999 11:44 pm

RE: IRU Fast Align

Tue Dec 26, 2000 12:27 pm

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.
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RE: IRU Fast Align

Wed Dec 27, 2000 7:59 am

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 will have attitude information. You have to set a heading - ususally thru the FMC, I'm talking Boeing here. Once the heading is know know direction. IRS info cannot be used for Navigation.
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RE: IRU Fast Align

Wed Dec 27, 2000 11:24 am

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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