747 Question. Are there any EFIS experts out there?

From:         Tim Hills <Tim@cordle.demon.co.uk>
Organization: Timbo PLC
Date:         20 May 96 09:54:39 
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I've just finished the groundschool phase of the 747-400 conversion, and
I'm looking for an answer to a question that cropped up in classroom
discussion.

The Boeing computer based training package says that the sole source of
vertical speed information on the pilots PFD is the associated IRS.
Whilst I accept that all modern inertial systems calculate vetical
accelerations, I don't see how the VSI system can work without any air
data input.

My last aircraft but one used inertial reference for short term vertical
speed, and barometric sources for long term altitude *smoothing*. Does
the 747-400 do this in reality, or have they designed a system that can
sense its null point inertially?

Hope someone can answer my question, there is beer on it!

Thanks,

--
Tim Hills