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>
To: europa list<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>>
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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