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VSI and Altimeter showing random drop

When in the cruise, every 4-5 mins the VSI will drop by 500 fpm and slowly come back to zero. Simultaneously the Altimeter will drop by 100ft. I have avionics work to do, but we are just troubleshooting the C206 doing confidence flights and this one of the little things I need to iron out.

What is this? Is it a leak or a blockage?

Buying, Selling, Flying
EISG, Ireland

Wild guess: maybe a leak towards the cabin and the pressure in the cabin changes for some reason? Maybe the behavior can be provoked by opening the pilot’s window?

EDFM (Mannheim), Germany

It sounds like possibly a cracked hose. You see exactly this if you open the alternate static port.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

You see exactly this if you open the alternate static port.

I tried the alternate static but it didn’t seem to make the issue any better. We did leak test it maybe 8 months ago, but we may get the test kit back and see what’s going on…

Buying, Selling, Flying
EISG, Ireland

Maybe water in the static line? This is then pushed up into a vertical section of pipe as the air pressure changes, and at some point in time a bubble of air makes its way through the water column to equalise things? 100 feet of air is equal to the pressure of about 3 cm of water.

Most aircraft have a water drain at the lowest point in the system, but otherwise you might need to unplug some hoses and possibly even force the water out by blowing the lines.

And on that latter, DO NOT blow out the lines (manually or with compressed air) when any part of the line is still connected to any instrument (ASI, ALT, V/S, TXP encoder, AHRS, …). Even the pressure from a gentle blow will totally destroy the instrument.

I’ve seen similar behaviour when a drop of water has got into the system and moves around as the pressure changes.

Andreas IOM

Yes – Sounds like getting the avionics guy to blow air back through, with the lines disconnected at the panel, and then leak test it again is a worthwhile endeavour.

I will ask him for a day!

Buying, Selling, Flying
EISG, Ireland
7 Posts
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