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

Date:         01 Dec 96 04:08:52 
From: (Al Moyes)
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In message <airliners.1996.2519@ohare.Chicago.COM>,
Ed Hahn <> 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 I may be way out of date but certainly I was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             

    "NEVER lost, just uncertain of my position"