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

Date:         02 Dec 96 01:44:34 
From: (RD Rick)
Organization: Netcom
References:   1 2
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In <airliners.1996.2599@ohare.Chicago.COM> (Al
Moyes) writes:
>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 I may be way out of date but certainly I was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.

I disassembled an IVSI from a Cessna 411.  It has a pendulum that moves
the needle in response to the small g force until the pressure
differential can build up.

The 767 uses the very expensive pendulums in the IRU to achieve the
same thing.  :)