FMS - IRU/IRS Initialization

[UP2ZSKY]

I would like to know what you think about initializing an FMS (IRS/IRU & GPS Sensors) and using the standard airport center point lat and long rather than loading gate or FBO coordinates. For years I have been pulling out my Jepp 10-9 chart to map out the FBO Lat/Long to get the IRS/IRU the closest possible position. I was recently told that some training organizations were teaching that airport center point is easier and less prone to input errors.
Any Input? :)

[quid]

If you plot your actual position carefully, I don't think it would be any more (or any less) prone to input errors. By "input errors" I'm assuming they mean "keystroke" errors. I think most carriers want the most accurate position possible.

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[411A]

All IRS units precess to some extent and this limit is usually considered at a maximum of 2 NM per hour. With radio/GPS updating the nav computer is quite accurate. You are quite correct, the Airport Reference Point is..."close enough." Some will disagree of course, but I have found it quite OK.

[Roadtrip]

I think most, if not all IRUs are NEVER updated once initialized. The FMCs are updated via DME or GPS, but the IRS's simply provide pure inertial input to the FMS. I haven't used a GPS update before, so I don't know if the FMC allows a GPS update prior takeoff. I think it's best to be as accurate as practical. Also, you should always check to be sure the airplane symbol is in the center of the runway symbol prior to takeoff as verification that you've input a good initial position and that you're on the correct runway. That may not have helped the Singapore crew in TPE since the runways very close together -- but then again maybe it could have.

[This message has been edited by Roadtrip (edited 25 November 2000).]

[ooizcalling]

I have wondered about this one too, particularly as most data bases (if not all) have the runway threshold position included which updates the FMS at the press of the TOGA button. Seems to me that this makes the manual entry of a gate/stand position a little superflous as the aerodrome reference point would be within the limits of the last known position from the previous flight.