Re: 747-400 Magnetic Detectors

From: (B367 80)
Organization: America Online, Inc. (1-800-827-6364)
Date:         18 Aug 96 20:13:36 
References:   1
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To the best of my understanding, once the present position is entered into
the IRU's they then go into the "Align" mode.  This mode determines
aircraft attitude by sensing the rotation of the earth and calibrates the
units to their position is space and consequently determines the heading
of the aircraft.  Anticipating the failure of one or more IRU's a method
to manually input heading into an individual IRU in flight is possible.
In this mode called attitude (ATT) the IRU can provide attitude and heading
information to the flight instruments but is unaware of it's LAT/LONG.
Perhaps this is the heading input information I think you heard referred

Gary Cieszynski
B757 B767 SA227