VSI shows descend in flight

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Magisterol

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Hello. I have a small ultralight and in flight, VSI shows 1700-2000ft/min descend. I took the instruments off, one by one and tested on a vacuum system. ASI has no leaks, in fact is the only one working properly, VSI works normal: increase the vacuum, it climbs, decrease it, it descends. Altimeter has leaks, so it is not good. I blocked the altimeter line and I am using GPS altitude for now. Still in flight, actually once the ASI starts to increase, VSI starts to show descent. Other than buying a new one and retrying, don’t know what else. Don’t understand why the ASI is working normal. I checked it with the gps over the field at low altitude where the wind doesn’t shift or varies much from ground and it shows ok.
 

TFF

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A VSI is an altimeter with different gears inside to give a different reading. It’s hooked to the static line right?
 

Basil

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Those symptoms could happen if the VSI case is leaking and you have connected it to pitot instead of static.
 

Magisterol

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The VSI is hooked to the same line that goes in the static port of the ASI, so if the ASI is working ok, I am thinking VSI should work ok too. I used a serine to these the integrity of the instruments and only the altimeter was leaking. ASI and VSI were ok and responding app to the pressure variations in the servings.
 

TFF

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Pull it and Find someone with a pitot static box and give them a $20 to see if it works right. No need for full calibration. If it’s hooked up to a good static line, it’s broken. Only real test equipment will tell that. If it matches the box, the static lines are crossed. If it doesn’t work , lines are good instrument is dead.
 

Magisterol

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I just got a new VSI from a friend at the field. I will put it on and if the wind dies down today, I will try it, see what it does.
 

Dan Thomas

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A VSI has a calibrated leak in its bellows. It ONLY reads when there is air moving in or out of its static port, with a bit of lag in the reading. If it's going to 1700 FPM down and staying there, that calibrated leak is plugged. Period. And it's getting positive pressure, too, somehow.

1634502800424.png

The air leaves the case at B as the airplane climbs. It's sucking the air out of the VSI case. There's a tiny orifice in the bottom of the bellows, and as long as air is leaving the case, the air from the bellows can't get out fast enough and it swells and moves the needle to show a climb. Once the static pressure stabilizes, the pressures in the case and bellows equalize and the needle goes to zero.
 

Magisterol

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I just got a new VSI from a friend at the field. I will put it on and if the wind dies down today, I will try it, see what it does.
A VSI has a calibrated leak in its bellows. It ONLY reads when there is air moving in or out of its static port, with a bit of lag in the reading. If it's going to 1700 FPM down and staying there, that calibrated leak is plugged. Period. And it's getting positive pressure, too, somehow.

View attachment 117058

The air leaves the case at B as the airplane climbs. It's sucking the air out of the VSI case. There's a tiny orifice in the bottom of the bellows, and as long as air is leaving the case, the air from the bellows can't get out fast enough and it swells and moves the needle to show a climb. Once the static pressure stabilizes, the pressures in the case and bellows equalize and the needle goes to zero.
I just took the lines attached to the static and pitot side and the lines were ovalized. So I am think it was leaking through there. I sealed everything and I am waiting for the wind to die out. it is gusting to 16kts and straight across.
worst case I will come in tomorrow
 

drwet

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My best guess is your ASI is leaking internally, allowing pressure from the pitot tube to enter the static system. I think this would explain your symptoms.
 

djmcfall

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I was getting ready to say that a VSI has an intentional calibrated leak, but as I continued to read I saw Dan Thomas beat me to it. Behind the glass is static air, that is why they teach you if your static port plugs up due to ice or some other reason, you break the front glass on the VSI to allow static air into the static system, from the cabin. Not exact but close enough to get your altimeter and ASI back to being usable. This will only work in non pressurized aircraft.
 

wsimpso1

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The VSI is hooked to the same line that goes in the static port of the ASI, so if the ASI is working ok, I am thinking VSI should work ok too. I used a serine to these the integrity of the instruments and only the altimeter was leaking. ASI and VSI were ok and responding app to the pressure variations in the servings.
VSI is designed as an altimeter with static pressure on one side of the aneroid and a calibrated leak on the other side, so it responds to how fast static pressure is changing.
 

drwet

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I used a serine to these the integrity of the instruments and only the altimeter was leaking. ASI and VSI were ok and responding app to the pressure variations in the servings.
I'm usually pretty good at deciphering the gibberish created by autocorrect, but I have to admit you got me on this one. :confused: What were you trying to say?
 

Victor Bravo

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If it is reading 1700 FPM down in level flight, then the pressure at the static port is far higher than "static", or there is a clog in the calibrated orifice downstream from the bellows or aneroid. The air is not "leaking" out of the aneroid fast enough to equalize the pressure on the static side of the bellows.
 

Magisterol

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If it is reading 1700 FPM down in level flight, then the pressure at the static port is far higher than "static", or there is a clog in the calibrated orifice downstream from the bellows or aneroid. The air is not "leaking" out of the aneroid fast enough to equalize the pressure on the static side of the bellows.
I was thinking at what are you saying, but when I test it with the syringe, it works normal. It shows climb and if I stop the plunger, slow gets back to zero. When I push the plunger back in, it shows descent and then returns to zero. Looks like no plugs. Don’t know what to say. Did not fly it yet. I hit a pothole on the ramp and my nose gear strut is bent backwards. The faring pushed into the bottom cowling..... Few days of work....
 
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