Testing an Altitude Encoder on the ground

The place to raise issues, ask questions, swap ideas and discuss anything related to aircraft engineering, maintenance and building.
NB Any opinions expressed in this forum are not necessarily those of LAA Engineering

Moderators: John Dean, Moderator

Post Reply
Simon Clifton
Posts: 91
Joined: Sun Feb 24, 2008 5:59 pm
Location: Worcestershire

Testing an Altitude Encoder on the ground

Post by Simon Clifton » Sat Oct 02, 2010 6:10 pm

We have an Ameriking altitude encoder connected to a Garmin GTX328 transponder.

It works extremely well, but there is a problem. Above 6,200 ft (ish from memory), it begins to read 52,000 ft ish (again ish for poor memory reasons).

This gives ATC people a bit of a shock, so we want to fix it. I have deduced it is a problem with one of the lines between the encoder and transponder (A2 line, again from memory, the encoder link is arcane to say the least!).

What I want to do is play about with the cables and do some substitution engineering to find the culprit, but without having to do it at 6,000 ft.

Is there an established method of safely reducing and controlling static pressure feed into an instrument for test purposes?

Thanks

Simon C
~~~~~~

PS. I know we can switch to Mode A, but that is not fixing it!

matherp
Posts: 12
Joined: Tue Apr 07, 2009 10:41 am

Post by matherp » Sat Oct 02, 2010 6:43 pm

I use a 20ml syringe. Push the plunger full in then connect it to the static system. Withdraw very gently and watch the altitude rise. It's also interesting to see the impact on airspeed. If it looks like the airspeed is going to hit the stops disconnect the ASI from the static system before further testing.

Best regards

Peter
026807

Kevin Stewart
Posts: 15
Joined: Thu Nov 27, 2008 1:20 pm

Post by Kevin Stewart » Sat Oct 02, 2010 10:02 pm

Interesting problem. If it occurred at 6800ft that would imply that the A2 line was somehow shorted to somewhere it shouldn't. This may be of some assistance to work out what is going on:

http://www.airsport-corp.com/modecascii.txt

Don't forget that the Ameriking unit takes time to before it outputs a code.

The manual for the standard AK-350 is here:

http://www.ameri-king.com/pdf/01%20AK%2 ... Manual.pdf

and page 10 shows the connection to a Garmin GTX327 which appears to have the same pin out as the 328. Here's the manual if needed:

http://aviation.vortex.is/install/Garmi ... TX-327.pdf

Good luck!

Simon Clifton
Posts: 91
Joined: Sun Feb 24, 2008 5:59 pm
Location: Worcestershire

Post by Simon Clifton » Mon Oct 04, 2010 12:19 pm

Gents

That looks like sensible advice, I will follow it and report my findings.

(I should be able to find a spare syringe outside my local high school, judging by the local newspaper)

Regards

Simon C
~~~~~~

Rob Swain
Posts: 393
Joined: Tue Jan 08, 2008 2:11 pm

Post by Rob Swain » Fri Oct 08, 2010 12:13 pm

I have created a wiring loom for between an altitude encoder and a transponder - and it worked first time. Lucky me!

However, if memory serves, a number of the encoder lines form what is basically a parallel interface with the height info passed across it. Sounds like either two (or more) of the lines have become shorted together, or the loom has a couple of wires reversed. My guess is that when the 6400 feet line switches on it is sending this signal to one or more of the more significant lines.

This theory is utterly speculative, but does seem to fit the facts as I read and recall them. :) Please feel free to shoot holes in it as necessary. :wink:

As regards the syringe, useful trick, but I'd start with the syringe halfway up its scale and then pull - that ought to give better control at lower 'altitudes'. You can always start from an emptier position if that doesn't 'go high enough'.
Rob Swain
If the good Lord had intended man to fly, He would have given him more money.

Kevin Stewart
Posts: 15
Joined: Thu Nov 27, 2008 1:20 pm

Post by Kevin Stewart » Sun Dec 05, 2010 8:01 pm

This thread coupled with a similar one on the BMAA site has created a mini project for me. There is more info that I have posted here on the Gillham code:
http://www.ccsinfo.com/forum/viewtopic.php?p=140960

and here on my decoder circuit:
http://forums.bmaa.org/default.aspx?f=22&m=86558&p=1

Post Reply