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altimeter encoder problem

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davidjoyce(at)doctors.org
Guest

Posted: Sun Apr 21, 2013 8:48 am Post subject: altimeter encoder problem

I would be grateful for any expert avionics view out there 

on the question of the likely fault in my altitude 

encoding on a Garmin GTX 328 transponder. This produces 

entirely sensible altitude read outs as I climb to FL60 

and then starts to show gently diminishing read outs as I 

go higher, so may be reading something like FL50 when I 

get to 10,000ft. There are 10 wires connecting encoder to 

Transponder (or at least they should be connecting!). 

Would you experts feel it is most likely that I have a 

wiring fault or that I have a defective encoder? It has 

done this since fitted new.

Regards, David Joyce G-XSDJ

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budyerly(at)msn.com
Guest

Posted: Sun Apr 21, 2013 12:13 pm Post subject: altimeter encoder problem

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]-->  David,

 Not that I am an expert on the 328, but we have a number flying  successfully.  It is pretty much a plug and play piece of equipment.

  It sounds as if your altitude encoder is separate and not part of an  EFIS.



 If it is encoding properly at low altitude or sea level, but not at  altitude, then your wiring is sound, it is the encoder.  You can use your Z  component of your GPS to verify that the rest of your static system is sound in  flight.

  

 If the altitude encoding is in error as altitude increases only, then it is  in the encoder static pot itself.  However, to be safe, check your  pitot tube for the W34 black static port attached well and secure to the lower  pitot tube also.  Any leak in the system (especially in the wing) will  cause errors with speed and angle of attack.  Of course if the W34 is  missing, your static system is hooked to a pitot source which is really  bad.

  

 Ameriking encoders as well as ACK are quite reliable, but if they fail,  which it sounds as if it has an internal static leak, you will need a proper  pitot static check by a qualified avionics shop to verify.  

  

 A crude check is to disconnect the static line from the encoder and run it  to a very light suction hand pump to check pressure vs readout.   Of course you will need the temp and atmospheric tables also to establish the  proper QNH.  For the very brave, only connect the altimeter and encoder  together with a static line and use a light duty vacuum hand pump to increase  the altitude and verify the readout on the 328 display with the altimeter.   If you pull too much suction or leave the airspeed attached, you will break  costly equipment.  So as you can see, it is best to just go to an avionics  shop and get a proper checkout as they have the calibrated equipment to do the  job quickly and correctly...  If you built your panel for easy removal,  your avionics shop will appreciate it.

  

 Note to all, it is prudent to have the pitot static system verified prior  to first flight, and every two years after that by a qualified avionics  shop.  Altimeters, encoders, and EFIS systems do fail.  It is best to  have them checked periodically.  For years, I have simply pulled one inch  of vacuum on my static system and checked the altimeter at 1000 feet increase,  and the airspeed increases to 135 KIAS, the Vertical Speed Indicator is checked  for rate of climb and the entire system checked that it will hold for 60  seconds.  However, at altitude, our absolute indicators may have small  leaks and lie to us.  I still do my 1000 foot check just as a security  blanket, but insist on a proper certified pitot static check prior to first  flight.

  

  

  

 Best Regards,

 Bud Yerly  

 [quote]   ---

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davidjoyce(at)doctors.org
Guest

Posted: Sun Apr 21, 2013 12:33 pm Post subject: altimeter encoder problem

Bud, thanks for that. I do a lot of group flying including 

trips like going down over Spain at altitudes of 10,000ft 

or more. This has given me the chance of checking my 

conventional altimeter and EFIS altimeter not only with 

bits of passing scenery but also with my friends in 

formation. This has shown that both altimeters are on the 

ball but that the transponder gives false readings. They 

are all on the same static line so I am reluctant to 

suspect the 'plumbing'

     As I said there is close correspondence up to around 

6000 ft and then transponder read out goes down as height 

on altimeter or GPS or observing scenery goes up. It is a 

separate encoder and from what you say it sounds a though 

it needs replacing, or is there some way of sorting out a 

duff encoder static pot?

         Regards, David Joyce, G-XSDJ

On Sun, 21 Apr 2013 16:11:50 -0400

  "Bud Yerly" <budyerly(at)msn.com> wrote:

[quote] David,

 Not that I am an expert on the 328, but we have a number 

flying successfully.  It is pretty much a plug and play 

piece of equipment.

 It sounds as if your altitude encoder is separate and 

not part of an EFIS.

 If it is encoding properly at low altitude or sea level, 

but not at altitude, then your wiring is sound, it is the 

encoder.  You can use your Z component of your GPS to 

verify that the rest of your static system is sound in 

flight.

 

 If the altitude encoding is in error as altitude 

increases only, then it is in the encoder static pot 

itself.  However, to be safe, check your pitot tube for 

the W34 black static port attached well and secure to the 

lower pitot tube also.  Any leak in the system 

(especially in the wing) will cause errors with speed and 

angle of attack.  Of course if the W34 is missing, your 

static system is hooked to a pitot source which is really 

bad.

 

 Ameriking encoders as well as ACK are quite reliable, 

but if they fail, which it sounds as if it has an 

internal static leak, you will need a proper pitot static 

check by a qualified avionics shop to verify.  

 

 A crude check is to disconnect the static line from the 

encoder and run it to a very light suction hand pump to 

check pressure vs readout.  Of course you will need the 

temp and atmospheric tables also to establish the proper 

QNH.  For the very brave, only connect the altimeter and 

encoder together with a static line and use a light duty 

vacuum hand pump to increase the altitude and verify the 

readout on the 328 display with the altimeter.  If you 

pull too much suction or leave the airspeed attached, you 

will break costly equipment.  So as you can see, it is 

best to just go to an avionics shop and get a proper 

checkout as they have the calibrated equipment to do the 

job quickly and correctly...  If you built your panel for 

easy removal, your avionics shop will appreciate it.

 

 Note to all, it is prudent to have the pitot static 

system verified prior to first flight, and every two 

years after that by a qualified avionics shop. 

 Altimeters, encoders, and EFIS systems do fail.  It is 

best to have them checked periodically.  For years, I 

have simply pulled one inch of vacuum on my static system 

and checked the altimeter at 1000 feet increase, and the 

airspeed increases to 135 KIAS, the Vertical Speed 

Indicator is checked for rate of climb and the entire 

system checked that it will hold for 60 seconds. 

 However, at altitude, our absolute indicators may have 

small leaks and lie to us.  I still do my 1000 foot check 

just as a security blanket, but insist on a proper 

certified pitot static check prior to first flight.

 

 

 

 Best Regards,

 Bud Yerly  

  ---

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Remi Guerner

Joined: 14 Dec 2010
Posts: 284

Posted: Mon Apr 22, 2013 3:43 am Post subject: Re: altimeter encoder problem

Hi David,

I propose to share with you my personal experience on that subject :

My aircraft was originally fitted with a Garmin GTX320 and a Terra AT3000 encoder. After a few years flying, I started to experience altitude reporting problems. ATC was occasionally seeing me at 6000 ft or below ground level while I was flying  at 2000 ft AMSL. I was once wrongly accused of having busted the class A London TMA while I was flying well below the max allowed altitude. Then I reworked the wiring without success, then I had the encoder tested by an avionic shop and it was found to be ok. Then I borowed another GTX 320 from a frend for a few flights and the system was working ok. So the culprit was clearly the transponder which was intermittently reporting an altitude different from the one transmitted by the encoder.

I have since installed a mode S GTX328, keeping the Terra encoder and it has been working fine for the last five years.

Conclusion : in most of the cases the culprit is the altitude encoder but it could also be the transponder itself.

I hope that helps.

Regards

Remi

F-PGKL

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davidjoyce(at)doctors.org
Guest

Posted: Mon Apr 22, 2013 7:27 am Post subject: altimeter encoder problem

Remi, Many thanks. Helpful as always! David



On Mon, 22 Apr 2013 04:43:12 -0700

  "Remi Guerner" <air.guerner(at)orange.fr> wrote:

 	  Quote: 	
		   

<air.guerner(at)orange.fr>

 

 Hi David,

 I propose to share with you my personal experience on 

that subject :

 My aircraft was originally fitted with a Garmin GTX320 

and a Terra AT3000 encoder. After a few years flying, I 

started to experience altitude reporting problems. ATC 

was occasionally seeing me at 6000 ft or below ground 

level while I was flying  at 2000 ft AMSL. I was once 

wrongly accused of having busted the class A London TMA 

while I was flying well below the max allowed altitude. 

Then I reworked the wiring without success, then I had 

the encoder tested by an avionic shop and it was found to 

be ok. Then I borowed another GTX 320 from a frend for a 

few flights and the system was working ok. So the culprit 

was clearly the transponder which was intermittently 

reporting an altitude different from the one transmitted 

by the encoder.

 I have since installed a mode S GTX328, keeping the 

Terra encoder and it has been working fine for the last 

five years.

 Conclusion : in most of the cases the culprit is the 

altitude encoder but it could also be the transponder 

itself.

 I hope that helps.

 Regards

 Remi

F-PGKL

 

 

 

 

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budyerly(at)msn.com
Guest

Posted: Thu Apr 25, 2013 11:04 am Post subject: altimeter encoder problem

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]-->  David,

 Sorry but the encoders are normally hooked to the rest of the static  instruments.  However, it would be prudent to have an avionics repairman  check the whole system.  The encoder analog systems can be reporting  properly but the Transponder itself is squirting out the wrong input due to  internal problems, wiring problems such as corrosion etc.

  

 Bud

 [quote]   ---

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