Expert: Marc Ebelini - 4/13/2006

Question
I had a local avionics shop install a Garmin 300XL GPS/Comm in my plane for VFR use, with later intentions for upgrading my plane systems to be certified for IFR. However, I was told that the GPS cannot acquire altitude data from my blind altitude encoder (ACK A-30) because my transponder (King KT-75) uses a strobe line to control the altitude encoder. The solution recommended is, either to replace the transponder with a newer model that does not make use of the strobe line, or install a second blind altitude encoder to provide barometric altitude information to the GPS. My question is, why can we not simply disconnect the strobe line from the transponder, and hard wire the altitude encoder strobe input to airframe ground to work with the GPS? What will this do to my transponder? Will this work?

Thanks,

Leo Pascual

Answer
Please bear with me as my KT75 training certificate dates back to the early 70's and I no longer have the manuals to get exact details. The strobe line was to synchronize the encoder to Mode “C” in the transponder. Radar interrogations were normally mode “A” for your assign code. When interrogated in mode “C” the information transmitted came from your encoder. Not having a manual for your encoder, can only make a suggestion. A transfer switch can be installed to connect the encoder output to the GPS when needed including the necessary solid control line. If you are required to transmit mode “C” continuously while needing altitude data to the GPS then this would require another encoder. Wish I had the manual for your GPS because I need to see why it needs external altitude data when the GPS system has it's own. Must assume that the GPS altitude data is not accurate enough to satisfy the IFR requirements.  My suggestion would be if a second encoder be needed that you replaced your altimeter instead with an encoding type. Using a newer transponder without a strobe line would require paralleling the data from your encoder. This may present a problem of impedance matching that I would avoid.
The switch would actually operate a sealed multi contact relay to transfer the data. Used this method when the aircraft had two transponders and one encoder.

As I see, it if you need encoded data going to different devices simultaneously then go for the second encoder. It would be the cheapest and more reliable method. Do not change the transponder and parallel the data.