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Guest Jacob Lee

757 Navigation Platform

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Good day all. I was reading a pilots report (http://www.airliners.net/discussions/trip_reports/read.main/105607/) and in it the pilot said, "757 needs a few moments without moving before rolling onto the runway in order to align its navigation platform." Today I was listening to JFK Tower. Tower cleared an American 757 to position and hold, and the pilot responded, "We need a few seconds here to align." So why does the 757 need a little time to "align" itself? Thanks for the help!-- Jake

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Alignment is probably the inertial navigation system needing time to calibrate itself for navigational accuracy. There is a set of gyroscopes in the aircraft which can detect motion in all directions, so the airplane can be able to know where it is without having to rely on external measurements. The aircraft needs time to "align" or "zero" itself by measuring the gyroscopes while the aircraft is completely stationary. This gives it a solid base from which to compare movements.The part I am not sure about though is that AFAIK, alignment was usually performed at the gate with the engines off (because the vibration of engines can interfere with the alignment process) and it usually took about 10 minutes or more to fully align. Maybe there are other methods of aligning that I am not aware of.Norman

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Jake:i agree with norman that full alignment of the IRS system must be done parked with engines off.one can 'quick align' at the rwy = this give a more detailed start point for the IRS to 'know where it is' just as the 'in the air' portion of the flight begins. how long does 'quck align' take? i have no idea for real-world. (there is a reference to 'quick align' later in the airliners.net thread you linked.)if you want learn more about IRS alignment a great resource is the CIVA-INS for FS. a great 'old school' simulation of the IRS system used by aircraft in the 70s-ish. the manual for the CIVA-IRS explains in DETAIL all that is required to align the system and keep it 'close' to reporting current aircraft position (all of these functions or automated in current 'glass' FMS/FMGC systems). a great detailed panel to use with the CIVA-INS is the tinmouseII 737-200 freeware plane/panel.--

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I thought all the alignment was done at the gate too and found it strange that they needed to align before takeoff. The 757 is the only plane I've heard of doing this. 'Tis a mind-boggler!-- Jake

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