Altitude encoder anomaly

I mentioned in another thread that I had been experiencing incorrect
mode C reports when the airplane was cold. Since a few people offered
suggestions, I'm following up with a report.

Installation: PA28-180, ACK A-30 mod 1 encoder, KT 76A transponder.
Observation: Mode C reports in error in winter time but ok in summer.
In winter reports became accurate after 15-30 minutes of operating
time. Mode C appeared, as monitored by MRX PCAS, to be correct if
encoder was preheated.

I built an encoder test box and tested the encoder output, both on the
aircraft, and in a bench test. For the bench test the encoder was
cold soaked overnight in the freezer at approx +5 deg F. It was then
powered and the current draw and altitude code were monitored.

Time zero current 0.37 A. No altitude report.
4 minutes 0.40 A. No altitude report.
10 minutes 0.35 A. No altitude report.
11 minutes 0.30 A. 22,100 ft.

After about 13 minutes the current stabilized between 0.24A and 0.26A
and the reported altitude continued to reduce. (Current includes
monitor LED current so varies with altitude)

21 minutes 18,200 ft
27 minutes 9,500 ft
33 minutes 6,000 ft
39 minutes 4,400 ft
46 minutes 3,300 ft
59 minutes 2,200 ft
77 minutes 1,700 ft
100 minutes stabilized at 1,500 ft / 1,400 ft (as expected for my
elevation)

The data seem to indicate that the heater is working but shutting off
before the sensor is heated to working temperature. I suspect the
problem is in the temperature feedback circuit.

Anyway this kept me amused for a while but now I have to send it back
to be fixed or buy a replacement. Either way no airplane driving for
me for a few days. (unless someone in SW USA would like to lend me an
ACK A-30 for a week or two)

Just because you have a transponder with a current certification don't
assume it's sending good data. Spot check with ATC, PCAS, or some
other monitor.

Andy

Andy wrote:
I mentioned in another thread that I had been experiencing incorrect
mode C reports when the airplane was cold. Since a few people offered
suggestions, I'm following up with a report.

snip


Anyway this kept me amused for a while but now I have to send it back
to be fixed or buy a replacement. Either way no airplane driving for
me for a few days. (unless someone in SW USA would like to lend me an
ACK A-30 for a week or two)

Would using Mode A be adequate?

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

On Dec 27, 1:55*pm, Eric Greenwell wrote:

Would using Mode A be adequate?

Fraid not. I live under the Phoenix class B.

BTW additional current measurements with monitor LEDs out of circuit
showed post warm up current of 0.15A with strobe open and 0.20 with
strobe enabled. Bench power supply set to 12v.

Andy

On Dec 27, 11:17*am, Andy wrote:

The data seem to indicate that the heater is working but shutting off
before the sensor is heated to working temperature. I suspect the
problem is in the temperature feedback circuit.

Good work collecting your observations and putting your data together!

Bob K.

Good documentation. I assume the altimeter was reading normally, so no
icebergs in the lines.
Andy , do any other aircraft on your field have an ACK on board? If
so, try hooking your encoder to another transponder (one connector and
you won't even need to move the hose for this test). It will probably
do the same thing.
The tech who certified my transponder installation referred to the
A-30 as a "disposable" encoder. They're cheap, and easy enough to
change. You could spend more on repairing the old one.
Jim

JS wrote:

The tech who certified my transponder installation referred to the
A-30 as a "disposable" encoder. They're cheap, and easy enough to
change. You could spend more on repairing the old one.

It's worth calling them. My ACK had a problem about 3 years ago with the
output reading being off as much as 1000'. It was easy to notice, as my
Becker shows the encoder reading. ACK repaired and calibrated it for
$66, including shipping, and it didn't take very long.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

On Dec 27, 6:48*pm, JS wrote:
Good documentation. I assume the altimeter was reading normally, so no
icebergs in the lines.
* Andy , do any other aircraft on your field have an ACK on board? If
so, try hooking your encoder to another transponder (one connector and
you won't even need to move the hose for this test). It will probably
do the same thing.
* The tech who certified my transponder installation referred to the
A-30 as a "disposable" encoder. They're cheap, and easy enough to
change. You could spend more on repairing the old one.
Jim

The results provided are for a bench test of the encoder with the
static port vented to ambient pressure. There was no aircraft,
altimeter, or transponder involved. I designed the test box so it
could be inserted both "in- line", and to interface with the encoder
alone. My first test was on the aircraft as I wanted to see how the
installed system behaved. Once I knew it behaved the same with or
without the transponder in circuit I knew the problem was the encoder,
not the wiring or the transponder. I then moved to the comfort of my
living room instead of working on the aircraft floor.

After the first test results were posted I did additional testing with
a fridge cold soak (less severe than freezer) and also a room
temperature soak. For the fridge test, soak at +37 deg F, it took 40
minutes to stabilizes at correct altitude. For the room temperature
soak (67 deg F) it still took over 30 minutes from power on to
stabilize at the correct altitude. Only an engineer would understand
the quest for data

I can replace the A-30 for $150 + shipping but I think it can be
repaired for under half that. Maybe I'll do both.

Andy

Andy,

A question about the current draw. Eric measured the A-30 current a
few years back and reported a maximum draw of up to 0.4 amps, as you
also got. But Eric found that once things stabilized, the current draw
dropped to 0.090 amps. Your measurements seem to show a heater off
current minimum of 0.200 amps, more than twice what Eric found.

Do you think this 2x current draw is more because your A-30 is
malfunctioning, or do you think this might point to variability in the
A-30 manufacturing?

-John

On Dec 27, 2:17 pm, Andy wrote:
For the bench test the encoder was
cold soaked overnight in the freezer at approx +5 deg F. It was then
powered and the current draw and altitude code were monitored.

Time zero current 0.37 A. No altitude report.
4 minutes 0.40 A. No altitude report.
10 minutes 0.35 A. No altitude report.
11 minutes 0.30 A. 22,100 ft.

After about 13 minutes the current stabilized between 0.24A and 0.26A
and the reported altitude continued to reduce. (Current includes
monitor LED current so varies with altitude)

[ In a later post on Dec 27, 4:25 pm, Andy wrote: ]

BTW additional current measurements with monitor LEDs out of circuit
showed post warm up current of 0.15A with strobe open and 0.20 with
strobe enabled. Bench power supply set to 12v.

On Dec 28, 6:03*am, Andy wrote:
On Dec 27, 6:48*pm, JS wrote:

Good documentation. I assume the altimeter was reading normally, so no
icebergs in the lines.
* Andy , do any other aircraft on your field have an ACK on board? If
so, try hooking your encoder to another transponder (one connector and
you won't even need to move the hose for this test). It will probably
do the same thing.
* The tech who certified my transponder installation referred to the
A-30 as a "disposable" encoder. They're cheap, and easy enough to
change. You could spend more on repairing the old one.
Jim

The results provided are for a bench test of the encoder with the
static port vented to ambient pressure. *There was no aircraft,
altimeter, or transponder involved. *I designed the test box so it
could be inserted both "in- line", and to interface with the encoder
alone. *My first test was on the aircraft as I wanted to see how the
installed system behaved. *Once I knew it behaved the same with or
without the transponder in circuit I knew the problem was the encoder,
not the wiring or the transponder. *I then moved to the comfort of my
living room instead of working on the aircraft floor.

After the first test results were posted I did additional testing with
a fridge cold soak (less severe than freezer) and also a room
temperature soak. *For the fridge test, soak at +37 deg F, it took 40
minutes to stabilizes at correct altitude. For the room temperature
soak (67 deg F) it still took over 30 minutes from power on to
stabilize at the correct altitude. *Only an engineer would understand
the quest for data

I can replace the A-30 for $150 + shipping but I think it can be
repaired for under half that. *Maybe I'll do both.

Andy

Andy,

Where do you get ACK A-30 for $150+ shipping.?

Richard
www.craggyaero.com

On Dec 28, 7:32*am, Richard wrote:

Where do you get ACK A-30 for $150+ shipping.?

I bet it starts with "e."

Thanks, Bob K.