Sticky altimeter

I just received a new altimeter from Vans and wanted to see it work so I drove up the nearest hill in my car. Overall the instrument seems to indicate correctly, but appears to stick a bit every 50 feet or so. Giving it the slightest little shake or tap unsticks it, but I'm just wondering if this is normal. Should I send it back and ask for a non-sticky one? I'm assuming that in an operating aircraft it'll be shaking all the time, but I don't want it if it's actually defective.

Steve Zicree

Interesting experiment!

Giving it the slightest little shake or tap unsticks it, but I'm just wondering if this is normal.

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I have to say that is a very interesting experiment! I've never tried it, and I don't have an altimeter outside of an aircraft, but next time I do I will give this a go.

Steve,
This sounds normal to me, although I've never performed a similar test.

There's just a certain amount of friction (stiction) in the bearings and other moving parts inside.

I believe they call this hysteresis.

I don't think you'll have a problem while flying.

Mark

Had the same problem

Steve

I had the same problem with mine. I fixed it by putting an instrument vibrator on my plane. I found one at an antique shop. The tag on the lower left side says: Lycoming O-320, 160hp.

The A/F Bomb/Nav systems, when moved from
the B-29 and B-36 to the all jet B-47 and B-52,
had the same problem. The engines were too smooth.
They put a unit in the syncro driven analog computers
called a dither motor. It provided the same function
that we get by tapping on our altimeters as we set them.

Some things never change,
Tom Webster
RV-7A (FWF)

All systems normal

On modern jets the backup altimeter has a shaker to keep it from sticking. Go in the cockpit of a Jet on the ground with electric power on, you can hear the drone of a muffled buzz/clacking. No worries with the engine running. There is plenty of Vibs to get the job done. George

A slightly sticky altimeter is pretty normal. Remember there are very, very small forces involved to move the pointer for a 50 foot change. When calibrating an altimeter it's normal to have to tap the instrument to remove friction. A 50 foot change is not uncommon, for a new altimeter it's not the best but I don't think it is worth worrying about. I think you will find that when fitted to the aircraft the vibration from the engine will free it up.

So, with a smooth-running engine (i.e., rotary, subaru), an electronic atlimeter will be more accurate/reliable?

Even a smooth running engine will give enough vibration to free up the altimeter unless it was totally stuffed. As for the electrical altimeters. I can't comment on the non certified types as I have no direct experience with them but, electric altimeters such as the King KEA 346 are in general more accurite than their mechanical counterparts. However that is not to say the mechanical versions are inaccurite. As for reliability, there is no question. Mechanical altimeters are far more reliable than electronic versions. We maintain a fleet of 10 PC-12's, a bunch of C-441's and a few Jetstream 32's. Most of these aircraft have one electronic altimeter on the pilots side and a mechanical one on the co-pilots side. I can only remember replacing one mechanical altimeter in a King air. Last month we have changed 2 electronic altimeters in PC-12's and one each in a C-441 and a J-32.
These are also very, very expensive. I have read somwhere that the list price for a new KEA 346 is $22,000 us .

Give it a shake

cobra said:

So, with a smooth-running engine (i.e., rotary, subaru), an electronic atlimeter will be more accurate/reliable?

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I think you will find there is plenty of vibrations in a Subie or Rotary engine RV to shake things loose. George