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
----- Original Message -----
From: David Joyce<mailto:davidjoyce@doctors.org.uk>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Sunday, April 21, 2013 4:33 PM
Subject: Re: Europa-List: altimeter encoder problem
<davidjoyce@doctors.org.uk<mailto:davidjoyce@doctors.org.uk>>
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
"Bud Yerly" <budyerly@msn.com<mailto:budyerly@msn.com>> wrote:
> 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
> ----- Original Message -----
> From: David
Joyce<mailto:davidjoyce@doctors.org.uk<mailto:davidjoyce@doctors.org.uk>>
> To: europa
list<mailto:europa-list@matronics.com<mailto:europa-list@matronics.com>>
> Sent: Sunday, April 21, 2013 12:47 PM
> Subject: Europa-List: altimeter encoder problem
>
>
><davidjoyce@doctors.org.uk<mailto:davidjoyce@doctors.org.uk<mailto:david
joyce@doctors.org.uk%3Cmailto:davidjoyce@doctors.org.uk>>>
>
>
> 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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