HSI with a suction failure?
 

Instructor chappies (and chappesses),

I was recently leafing through a book about flying on instruments - and was looking at the difference between a "Direction Indicator" and a "Horizontal Situation Indicator".

I get the impression that an HSI is still a gyro driven DI - but with a few extra goodies attached.

Question: if your aircraft suffers a gyro suction failure, which of the HSI components are usable/unusable?

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Hunting is bad!!
Support the right to arm Bears!!

First find out if the HSI is air or electricly driven.

Assuming it is air driven (which is pretty rare) then the DG protion fails, but the VOR presentation still works.

Checkboard...had a feeling that would be the case...but would it matter that the "direction" that you set the VOR CDI to would not be correct...and may be rotating?

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Hunting is bad!!
Support the right to arm Bears!!

HSIs in my experience are usually slaved to a flux valve at one of the aircraft's extremities. Don't think they employ gyros at all but without going and looking through my ATPL notes I can't be 100% sure.

The H.S.I. does indeed utilize a gyroscope (directional) and usually electrically powered, to provide a steady reference direction. Like the D.G. the reference direction is subject to precession and transport errors. The gyro must also be aligned either manually or automatically with magnetic north. A mode selector on the panel in the vicinity of the display (mechanical or EFIS) controls the source of magnetic input to the H.S.I. In the 'auto' mode directional information is provided from the flux detector. In the 'manual' mode the indicator may be slewed to realign with the standby compass system - a switched version of a D.G. Hence when used in conjunction with a C.D.I. the information is correct in either mode.

Some stand-by H.S.I's. have vacuum powered gyroscopes using the bleed air system to generate the vacuum.

One type of HSI that is found in light aircraft uses air to drive the gyro and electrics to sync the compass card it seems you are in for an interesting time what ever fails.