Erratic VSI

[Nark]

I've been chasing ghosts in my plane for the last week.
Originally I replaced the airspeed indicator because it was full of water (heavy condensation on the face plate). Switched it out with a working one.
Quick test flight, no joy.

Blew out both the pitot and static lines, which had fair amount of water.

Another test flight, the airspeed works as it should, however the VSI was "frozen" upon landing. Pulled it out, and a little bit of water came out.
Put it in a dehydrator over night, seemed to do the trick (it returned to "0"). Test flight this afternoon and was bouncing all over the place. The Altimeter was a little jumpy as well.
Took this quick video.


You can see the face plate on the Airspeed fogged up, and the VSI & alt bouncing around.

The A&P seems to think there is still moisture in there. So we pulled all 3 pitot/static instruments to sit in a bag of rice overnight.
The static lines are airing out to dry as well.

Any other suggestions?

[Big Pistons Forever]

Definitely water still in the system. If you are pouring water out of the instruments they may not come back to normal without being disassembled and fully cleaned out.

Good luck

[black hole]

I agree!

BH

[cgzro]

Most likely water still in the altimeter.
Only takes a bit shaking about anywhere in the system to cause that.

[Nark]

Thanks gents.

I'm hoping the overnight rice bath will wick the remaining moisture out.

I'll report back tomorrow.

[Colonel Sanders]

Nark: to stop this from happening,
you need to run the line from the
static port down to the floor, install
a 1/8 quick drain, tee it, then run it
back up again.

This "drain loop" will give the water
(which is likely entering via the static
port, when the aircraft is parked outside
in the rain) someplace to go, and a way
to get rid of it.



You don't want the system draining into
the flight instruments, which I suspect
is the arrangement you have now.

I'm sure AC 43-13 has something to say
about this.

I used to have a Mooney M20J with a setup
as pictured above.

[Nark]

I went out there today. The 3 pitot/static were in a big bag of rice. The ASI was still a little moist on the glass, so I'm going to give it another 24 hours.


Col, We installed a drain tube over the weekend since it did not have one originally. It's unpluged in order to vent any residual moisture out while the instruments are getting a rice bath.

[Jungle Jim]

Nark,

How are you getting away with the non-standard control yokes on this Stinson? I remember wanting to do this with my 108-3 and the response was that if I changed them it would not conform with the airworthiness certificate and there was no STC for them.

Just wondering,

Jim

[DonutHole]

Leak check after install?

[Colonel Sanders]

non-standard control yokes

Shot in the dark: field approval 337.

I doubt it came with a six-pack panel layout, either.

[iflyforpie]

Land of the free..... I wish we could do that in Kanada without getting three readings in Parliament and Royal Assent....

[Nark]

God Bless the FAA err USA...

Dried it out for 3+days in the bag of rice. Seems to do the trick.
Thanks for the insight.

[PilotDAR]

If you have had visible water (condensation) in the static instruments, I would not expect that 3 days in the bowl of rice has entirely returned them to service. They all contain rather precision steel pinon gears, which are in the static areas of the instruments. Rust in the teeth of these gears impairs their operation. Similarly there are small coiled springs whose coils can rust and stick together, resulting in wrong readings.

The rice bowl trick works for some electronics because the circuit board has a large surface area to be dried, and the moisture can migrate from the board to the rice across a large volume of air, which is conducting the moisture. The instruments, on the other hand have the tiny static and pitot ports. There is little incentive for large volumes of air or moisture to migrate through these tiny ports. Everything helps of course, but consider sending them to the instrument shop* if you value them.

Owners of non certified aircraft have been known to open them up. If you are considering this, use caution as they might be the older ones with face markings containing radioactive paint. (from your photo, it certainly appears that your T&B and suction are), so I'd be suspect if the static instruments are the same vintage. This would also probably *cause the instrument shop to decline working on them. The personal hazard resulting from the ingestion of minute amounts of this paint are real and very long term - do not expose yourself, or your workshop to this stuff. Inside the instrument, it is not a hazard.

If the instruments begin to malfunction again, and you have solved your moisture problem, consider replacing the instruments (rather than spending hours of your valuable time chasing unsolvable problems). There are some things which simply will not last forever....

[DonutHole]

In a vsi there is static source on both sides (bellows and case) through a calibrated orifice so Iif there is moisture in the case there is probably moisture in the bellows.

[Nark]

Thanks pilotdar. Should the problem manifest itself again, it would be quickler and much less time consuming to send them to an avionics shop for overhaul/replacement.

The humidity gets around 1000% in the summer here,so it won't be too long before they are fully tested for residual moisture.

Thanks again. I've learned quite a bit.