wrote:
: Ok, you avionics wonks, here's one for you. KNS-80 problem...

: Around the time of installing the GPS, the groundspeed on the
: KNS-80 began reading 00 KTS under all conditions while flying
: BUT when pinged on the ground--in the aircraft--immediately reads
: the correct value even with the test signal turned so low that the
: DME can barely lock on.

: This has happened in two different late sixties Bonanzas;
: one 28v one 14v.

: The 14v model had a KLN-90B installed, the 28v a 530W.
: Both airplanes have TCADs, both have the KNS-80 talk-only
: suppression line properly connected.

: The DMEs both have great range, often seeing 80-100 mile locks
: below 10,000 feet altitude MSL.

: Things tried in flight that have no effect:

: 1. Stack of 4 KNS-80s rotated into the two airplanes: all
: have same symptom. BTW, RNAV works on all.

Distance displays in both VOR and RNAV mode, but no GS, right? Does OBS work correctly?

: 2. Tune in VOR-DME stations in various directions, various
: distances, various frequencies -- all lock but display 00KTS gs.
: No Y channels 40us pings (.05 channels) tested.

: 3. RNAV tried, GS still 00KTS

: 4. All other radios turned off; no effect

: 5. All other radios REMOVED from stack... still 00 KTS

: 6. All other electrical loads shut off; Alternator shut off;
: still 00 KTS

: 7. Various rpm and attitudes.

: 8. Can't listen to the DME audio in either installation.

Is that normal or has it changed?

: Yet to try: Run the airplane while being pinged on the ground.
: Expected result: Will indicate correctly.

: The DME portion of the box has A+, an antenna, and the
: suppression line coming into the box. How complicated
: can this be? The DME sends serial distance pulses
: internally to
: the main 6800 processor (running at 0.95 MHz) where gs
: is computed along with the trig for RNAV. (There are a lot
: of external lines related to the VOR).

: So what the heck can it be? Get rid of the Bonanza?

: My plane is the 14v one; I've chosen to ignore the problem for
: the last 5 years; the 28v one is being worked on by a local
: shop whose customer is far more anal than I and wants it
: fixed.

: I called King and they are clueless. This one will take
: a lore-based solution. I've written it up in detail, investigated
: in detail but am missing something.

: Am I being persecuted by MI5?

: Bill Hale, Loveland CO

Just a thought, but is it possible that somehow the encoder lines to the OBS are messing with
things? The GS is computed as the arrival/departure speed to/from the waypoint (or VOR). If you're
flying parallel to the waypoint, it'll read 0GS. Is the newly-installed GPS using the same OBS head via
a switch/mux box?

My installation has a slightly flaky connection to the back of my KNI-520 head. One of the
resolver lines occasionally loses connection and then the head doesn't work right. It's impossible to
center the needle as it swings from full one side to full the other, 90 degrees off from what it
*actually* is. I've never noticed if the GS goes to hell, but I'd suspect it would. I think the
distance still reads right.

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
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