altitude encoder error

johnM · Post by **johnM** » Tue Jul 15, 2008 7:10 pm

I have just installed a second hand King KT76A transponder and a new Ameri-King AK-350 encoder.
Today I asked Farnborough radar for a test whilst at 2000ft on the local QNH. They reported having me at 2500 ft.
Is this likely to be due to a bad wiring connection, or does the encoder need calibrating? It seems unlikely to me that a new encoder would be 500 ft out as I assume they are calibrated at the factory.
cheers
john

steveneale · Post by **steveneale** » Tue Jul 15, 2008 10:11 pm

John,

We did the same a while back with the same combination.
Assuming you have double checked your connections there are two ways to do this.

First the hard way. Find a friendly radar with time to spare. (Filton down our way is often quiet) You will need to fly above 3000ft so they can report FLs then you are comparing apples with apples. Then climb and ask them to watch you. The trouble with this method is you need to get to FL68 to test signal A2 and FL148 to test signal A1. Depends upon your ceiling.

We gave up and went to RGV aviation at Glos. They wheeled out £30k of kit and calibrated ours for us. They found a wiring fault we had and fixed it then calibrated everything. They will also check your static system and altimeter calibration as part of the process. They will do the work at hourly rates if you are on permit. We paid £110 total for a couple of hours per diem and a nice bit of paper for our aircraft log book.

hope that helps

Steve

G.Dawes · Post by **G.Dawes** » Tue Jul 15, 2008 10:30 pm

All transponders work on 1013 and therefore is not giving th altitude which is the QNH, the difference will be the actual pressure figure.
Work it out and I think you find your setting is OK.

steveneale · Post by **steveneale** » Tue Jul 15, 2008 10:51 pm

This is not simple. for example he could have an altimeter error. The encoder may be correct. It may be wrong and his altimeter is correct. His may have a static leak or other problem. Radars do the QNH QNE sums. If he is on the QNH the radar gave him his altitude and that reported by ATC should be the same. Trouble is he needs FL for his signal lines and your brain can explode working it out.

johnM · Post by **johnM** » Wed Jul 16, 2008 12:02 am

Do certain wires transmit a signal at certain altitudes? Can I establish that because the reading is 500 ft too high then the blue wire for instance is a bad connection?

I would like to replace the connectors that go in the back of the rack but do not know what they are called or where to get them. Does anybody know a source please?

Frank Parker · Post by **Frank Parker** » Wed Jul 16, 2008 1:52 am

I had a problem with my ancient AK30 when an avionics shop adjusted it without giving it time to warm up. I was told by the shop that corrected it that it needs to be allowed to warm up before being adjusted. And guess what, after the incorrect adjustment, it read 500 feet high! That may be coincidence, and it is an old unit, so more modern technology may not have this problem.

I believe that the wires send a binary coded signal in 100 foot increments to the transponder, so it would take more than one incorrect wire to produce a 500 foot error. I assume that you have the installation manual and can check for incorrect installation?

steveneale · Post by **steveneale** » Wed Jul 16, 2008 7:48 am

johnM wrote:Do certain wires transmit a signal at certain altitudes? Can I establish that because the reading is 500 ft too high then the blue wire for instance is a bad connection?

I would like to replace the connectors that go in the back of the rack but do not know what they are called or where to get them. Does anybody know a source please?

See:

http://www.rst-engr.com/rst/articles/kpapr98/index.html

Best of luck

A friend of a friend supplied this information about the connectors when he was doing his:

"The housing is a Molex EDP# 09-50-6125 (Eng#4338-12). The terminal (loose gold plated crimp contact) is a Molex EDP#08-05-0302 (Eng#4366-P55L) good for Gage 20 to 18 (0.5sqmm to 1.0sqmm). To find the datasheets look for www.molex.com <http://www.molex.com/> then you find a search window. Enter the part numbers without a preceding Zero and without a dash i.e. 9506125 for the housings and 8050302 for the terminals. The series is a so called 3.96mm pitch double sided edge connector system"

I think Farnell or RS sell them (at a price)

Steve

johnM · Post by **johnM** » Wed Jul 16, 2008 11:15 am

Steve
thanks for that info. The crimp terminals really need replacing before I go any further. If anyone has any for sale I would be very interested.

Pete · Post by **Pete** » Sun Jul 20, 2008 1:34 am

you don't actually need anything very sophiiisticated to test a blind atimeter encoder. all you need is a 10 quid B&Q multimeter.

Download and print out the following web page

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

Grab your encoder handbook, which hopefully has a pin-out diagram

switch on your transponder and set it to standby.

the encoder probably gets it's power from the transponder.

set your altimeter to 1013

grab a paper and pencil and measure the voltage on the pins of the encoder ( or transponder )

the output of the encoder is DC, a voltage of + something relative to the ground pin of the encoder is a zero in altimeter speak so if the pin is at 0v, record a 1 on your piece of paper, it it's > 3v or so record a 0.

now all you have to do is work your way through the chart of encoded values until you find the row that matches your row of zeros and ones.

If you have a loose connection, then it could be difficult to spot, you need to check that all the pins currently showing 0v go to 9v and visa versa, you can do that by sucking air gently out of the air pipe, I leave that to you to work out how to safely do that.