Menu Sign In Contact FAQ
Banner
Welcome to our forums

VSI showing deviations while altitude is rock solid

I noticed this yesterday. Both altimeters didn’t move even a fraction of a mm, but the VSI was showing +20 to -20fpm. The GPS altitude varied a little too, by a few feet, but it does that anyway.

Is there a lot of damping on an altimeter?

Administrator
Shoreham EGKA, United Kingdom

Mechanical altimeters tend to stick. In gliders you need to tap them from time to time and they’ll suddenly move by 50 feet. In power, usually vibration from the engine/prop will do the job.

Andreas IOM

Peter wrote:

Is there a lot of damping on an altimeter?

In a mechanical altimeter, yes. Sometimes the programmers of electronic gadgets could adopt that damping, some apps are way too nervous.

mh
Aufwind GmbH
EKPB, Germany

On my non-certified instruments, I find the vario more sensitive than the altimeter.

EBZH Kiewit, Belgium

Peter wrote:

I noticed this yesterday. Both altimeters didn’t move even a fraction of a mm, but the VSI was showing +20 to -20fpm. The GPS altitude varied a little too, by a few feet, but it does that anyway.

Is there a lot of damping on an altimeter?

There is a noticeable delay in a basic VSI because air has to physically move through the choke, so many (most?) instruments are “IVSIs” (Instantaneous VSI) that include a vertical accelerometer — a simple air pump. This can lead to small fluctuations of the kind you noticed unless the air is absolutely smooth.

Last Edited by Airborne_Again at 24 Aug 09:24
ESKC (Uppsala/Sundbro), Sweden

It’s not an iVSI?

@Peter,

this is the price you pay for your crank and prop balancing…. Just take a file to the prop tip and shorten one blade a bit, and the incremental vibration will sort it…

Some of the high-precision, RVSM mechanical altimeters have small vibrators built in to ensure they don’t establish this hysteresis.

Biggin Hill

So do people think the issue is that the altimeter is not responding to small changes?

Administrator
Shoreham EGKA, United Kingdom

Possibly that too, but mainly the sensitivity of the VSI to turbulence.

ESKC (Uppsala/Sundbro), Sweden
9 Posts
Sign in to add your message

Back to Top