vsi zero error
 

dear friends ,
i am not an professional engineer , but am facinated by aviation. recently i acquired a VSI that belonged to a b 747 and was made by aerosonic. the problem is that when it is at rest on my level table it shows a small climb. i wonder why it has this zero error. i have seen photgraphs of vsi s , many of which seem to show a small climb error. is this some design error or is it simply somehting wrong with the unit that i had. furthermore , why don't they have a knob that can correct for the error located outside of the instrument ?

loads of questions ....and merry christmas to all !

there should be a small adjusting screw on the back of the instrument or maybe on the face to adjust the cam to centre the needle.
If the instrument is on a desk, there will be the same atmospheric pressure outside the capsule as inside it, so the needle should be at zero.
The instrument works on the principle that a change in altitude will cause a change in pressure. The pressure around the capsule is allowed to leak into it at a controlled rate to show as a change in barometric pressure, hence a change in altitude.

I forgot to add, there is also compensation internally for temperature, using bi-metallic springs, so this could cause a slight error outside of the aircraft. I would guess that there is a small adjuster though.

[This message has been edited by CONES R US (edited 26 December 2000).]

thanks ...i will look for it ...though it isn't obvoiusly visible. i am new to london and have bought a tool set ...oncce i do ...it will be opened ....and i will let you know ...

prasanna

thanks ...i did find an nicely hidden adjustment screw on the front . it looked just like the ones holding the front glass ..so i didn't notice it at first ...i thought it would be labelled !

prasanna