My VSI is broken

pcorman

Pre-takeoff checklist
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Apr 8, 2005
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Paso Robles
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Phil Corman
My VSI, on my Mooney M20C, seems to have failed.

It reads 600fpm on static air, and 150fpm on alternate static.

Does anyone have thoughts on what may have failed?
1. The VSI itself
2. The feeder into the VSI clogged?

Is this something I can troubleshoot before going to the mechanic?
 
pcorman said:
My VSI, on my Mooney M20C, seems to have failed.

It reads 600fpm on static air, and 150fpm on alternate static.

Does anyone have thoughts on what may have failed?
1. The VSI itself
2. The feeder into the VSI clogged?

Is this something I can troubleshoot before going to the mechanic?
Maybe failed. How about moisture in the static line making a plastic blockage, esp. given the weather you just had....A mech can blow it out easily enough....

You didn't just wax it, did you?!
 
I think if you disconnect the static line from the back of the instrument (easy in some a/c, harder to reach in others) and it zings back to zero, it was probably the line. If not, I'd suspect the instrument. Also I believe the regs say you void the 2yr static test if you crack the system open like that though.

You can troubleshoot, remove, etc all you want, but if you do it, you need to have replace under supervision and have a&p sign off before flying again. And the 2yr static check (aka 'pitot-static test') before flying ifr. PS- an A&P cannot repair an instrument though.
 
pcorman said:
My VSI, on my Mooney M20C, seems to have failed.

It reads 600fpm on static air, and 150fpm on alternate static.

Does anyone have thoughts on what may have failed?
1. The VSI itself
2. The feeder into the VSI clogged?

Is this something I can troubleshoot before going to the mechanic?

The VSI has failed. Chances are that the seal between the glass face and the bezel has deteriorated.

I posted a similar reply at AOPA but maybe they went down before you read it. Your problem is a leak in the case of the VSI into the cabin. A VSI indicates the pressure drop across an orfice between the static line connection and an air chamber (which is usually the case of the instrument). If that chamber is vented to the cabin (via a leak or if you bust the glass) the needle simply shows the difference between the cabin pressure and the static line pressure. In flight with the alt static valve closed, there will be a nearly constant indication on the VSI because the the cabin usually ends up at a slightly different pressure than the outside ambient pressure. When you open the alt static valve you mix cabin pressure with the pressure from the static port which brings the pressure in the static line much closer to cabin pressure. That causes your leaky VSI to read less than it does when the alt static valve is closed.
 
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