The following RV-List message has been reposted with the permission of the author:

--> RV-List message posted by: Bill Marvel

For those of you in the market for altimeters, you are probably going to choose between Aerosonic and United. I chose Aerosonic. A year later, I now have a United altimeter. You may be interested in why.

The short story -- Aerosonic altimeters have an internal mechanism to diminish needle vibration that United instruments don't have. This means that they also have more internal friction than does the United version. My initial pitot static check was fine, but a year later I had to troubleshoot a mode C problem. In the investigation process we did another pitot static check and the altimeter flunked the friction test-- by a lot. It was not yet two years old. What I learned from three instrument repair shops and one technician who does nothing but pitot static checks caused me to change over to the United altimeter and putthe other on the shelf. It is not uncommon for the newer Aerosonic altimeters to fail the friction test, sometimes every two years. This results in repeated repairs and overhauls. United altimeters don't have the same problem.

Read on for more.

The longer story. When this happened, the tech doing the test asked me what type of altimeter I had (you could not tell from the front). When I told him it was an Aerosonic, his eyes rolled. He then told me that he had been having problems with their new units for several years, sometimes failing right out of the box and thereafter, mostly due to excess friction.

I Fed Ex'd the instrument to Aerosonic and then called them to ask what was going on. After all, this unit was almost new and had failed a friction test. I queried them as to manufacturing problems, etc. but they said they were not aware of any and had received no feedback on this problem. They also said they had confirmed the problem, that their warranty was only good for one year and since the unit was out of warranty, I would have to pay to have it repaired. I told them I would be happy to pay for them to tell me why it failed and that I was surprised they weren't interested themselves to find out what had happened so early in its life. They started into the work.

In the interim, I called the technician back and he gave me the numbers of three major instrument shops -- one in Wichita, one in Van Nuys and the third in Huntington Beach. The one in Wichita said they'd had this problem for a good 10 years and had repeatedly told the factory about it. He also explained to me the design feature Aerosonics had to address the needle vibration seen in United altimeters and how this is the cause of the friction problem. The other two shops said the same thing and that they recommended United altimeters for that reason. Aerosonic had told me nothing of this design matter and only that they had no problems reported from the field.

I ordered, installed and starting flying the United instrument, which passed the pitot/static check fine. I called Aerosonic back and told them what I had learned about the design and the problems that had been experienced in the field. I gave them names and phone numbers. They stuck to their story -- no problems with their instrument. The president of the company later called me and said that they were going to fix mine under warranty and send it back, and that they would call me to keep me advised of what they learned. I got the altimeter back without the phone call. I called and left a message. No return call. I called again today and finally got through. All they did was disassemble, clean, lube and reassemble the unit and it tested fine. It is now sitting on my spares shelf.

I have no ax to grind with Aerosonic and no kudos to toss at United. I have no involvement with either of them, either directly or indirectly. But this is what happened to me and what I learned about it. Your mileage may vary.

Bill Marvel