Re: Vertical Speed Measurement (was Re: Peruvian 757 crash -- p

Date:         01 Dec 96 04:08:52 
From:         al@moyes.softnet.co.uk (Al Moyes)
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In message <airliners.1996.2519@ohare.Chicago.COM>,
Ed Hahn <ehahn@mitre.org> wrote:
> Now then, the reason the IRS supplies the Vertical Speed (VS) is
> that, while the vertical accelerometer CANNOT provide raw VS output,
> it CAN provide enough of a "smoothing" signal to the Altitude Rate
> signal to allow the cockpit instrument to not lag behind the
> aircraft.  (See Appendix below.)
>
> Note that the cockpit instrument is most properly known as the
> "Inertial-lead Vertical Speed Indicator", but is more commonly known
> as the "Instantaneous Vertical Speed Indicator".

As I understand it the Instantaneous VSI (or inertial lead VSI) is a
complete instrument in itself and does not rely on accelerometers from
the IRS but has its own. I may be way out of date but certainly I was
taught that the IVSI had two pistons on springs that were fed static
pressure. When the pressure changed the inertia displaces a piston
which produces a pressure differential which immediately registers on
the dial. As the effect from the piston fades the actual static
pressure change from the metering unit will take over.

--
Al Moyes                       al@moyes.softNET.co.uk

    "NEVER lost, just uncertain of my position"