SSA petition to allow transponder to be turned off

Experts, correct me where I'm wrong.

1. An operating transponder, not being interrogated, is simply a receiver
with a low current drain.
2. When interrogated by a ground radar or TCAS, a transponder transmits a
short "squitter" at 175 or 250 watts which is a BIG current drain.
3. Flying in an area with many interrogations per minute is likely to be a
congested area where the transponder is needed and a wise pilot would keep
it on despite the current draw.
4. Flying away from a congested area toward a remote area with few
interrogations, the transponder automatically cuts back on its current draw
by operating less and less as a transmitter and more as a receiver.

So, where is the need to turn it off? Doesn't the transponder effectively
manage its own current draw to match the level of congestion? Move away
from congested areas and the current draw is minimal. I haven't read of a
case where the transponder is sucking batteries flat and if that happened,
how much additional battery capacity is needed to keep it running for the
whole flight? (I can remember pilots carrying car starting batteries to run
a vacuum tube radio. No imaginable suite of avionics would draw that much
current today.)

So, aside from the cost of a transponder installation, what is the concern?

Bill Daniels

p.s. Having suffered a couple of alternator failures at night in hard IFR
with single engine airplanes, I became very interested in the current draw
of each bit of avionics. My calculations showed is that if the aircraft
battery is in good condition and fully charged at the point where the
alternator failed, and the pilot swiftly switches off the alternator field
with the split master, the battery will run a full IFR panel longer than the
fuel will last. I tested that calculation and it proved true with good
safety margins.

Bill Daniels wrote:
Experts, correct me where I'm wrong.

1. An operating transponder, not being interrogated, is simply a receiver
with a low current drain.

Not so low: my Becker (175 W version), with the ACK 30 encoder, draws
about 400 ma. In very cold weather (winter wave flying), this rises
50-100 ma because of encoder heating.

2. When interrogated by a ground radar or TCAS, a transponder transmits a
short "squitter" at 175 or 250 watts which is a BIG current drain.

True, though internal to the instrument; the battery and wiring won't
see a spike.

3. Flying in an area with many interrogations per minute is likely to be a
congested area where the transponder is needed and a wise pilot would keep
it on despite the current draw.

I agree, but the increase is only 130 ma at 1200 interogations/sec! The
increases I've seen in Southern California are more like 30 ma, and just
a few milliamps when there are only 3 or 4 radars hitting it.

4. Flying away from a congested area toward a remote area with few
interrogations, the transponder automatically cuts back on its current draw
by operating less and less as a transmitter and more as a receiver.

So, where is the need to turn it off? Doesn't the transponder effectively
manage its own current draw to match the level of congestion? Move away
from congested areas and the current draw is minimal.

It's still 400 ma with the encoder, a significant but not overwhelming
amount for the typical 7 amphour battery. 400 ma may be what all the
other instruments are pulling, so it cuts the battery life in half.
Still, a 7 AH battery should easily last 7+ hours.

I haven't read of a
case where the transponder is sucking batteries flat and if that happened,
how much additional battery capacity is needed to keep it running for the
whole flight? (I can remember pilots carrying car starting batteries to run
a vacuum tube radio. No imaginable suite of avionics would draw that much
current today.)

So, aside from the cost of a transponder installation, what is the concern?

Having a transponder on for the entire flight likely means the pilot has
to charge it (or put in a different one) everyday, rather than every
other day. The battery may need replacing more often, say every 3 years
instead of 4 or 5. Gliders with batteries smaller than 7 AH probably
could get by with, say, a 4 AH battery by using the transponder for only
a couple of hours instead of full time.

I don't think these concerns are important if you think you need a
transponder, and I suspect it's the ~$2000 cost that stops most people,
plus the $50-$70 every two years to have it tested.

--
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Eric Greenwell
Washington State
USA

Now this is a little silly, but are there any manufacturers that
make modern Mode "A" transponders (no altitude encoding)?
I've seen a portable one (battery operated) that looked pretty
old.

I think this is TSO-C74b, ref 91.215(a). I just wonder how much
of the extra price and inspection is just the mode "C" part of modern
transponders.

I wonder how many more owners would install a mode "A" if it only
drew 50mA max, was the size of a cell phone, required no recurring
inspection, and cost "about $100" (quote from Rain Man).

Better than nothing. And allows you into A,B,C by deviation
request. And gives at least lateral avoidance to airliners.

Maybe a market here to compete with the over-capability of the
mode "C". Better is the enemy of good, especially when talking
about optional equipment.

Bill Daniels wrote:

Experts, correct me where I'm wrong.

I'm not an expert, but I'll answer anyway, this is RAS, after all.

1. An operating transponder, not being interrogated, is simply a receiver
with a low current drain.
2. When interrogated by a ground radar or TCAS, a transponder transmits a
short "squitter" at 175 or 250 watts which is a BIG current drain.
3. Flying in an area with many interrogations per minute is likely to be a
congested area where the transponder is needed and a wise pilot would keep
it on despite the current draw.
4. Flying away from a congested area toward a remote area with few
interrogations, the transponder automatically cuts back on its current draw
by operating less and less as a transmitter and more as a receiver.

So, where is the need to turn it off? Doesn't the transponder effectively
manage its own current draw to match the level of congestion? Move away
from congested areas and the current draw is minimal.

You are assuming that areas of low traffic are also areas with few
interrogations, which is not necessarily true. In my case, the primary
concern is the area within 30 miles or so of Reno, where there is high
likelihood of encountering airliners in the 10 to 18K foot range.
Outside of the immediate area of Reno, my transponder still gets
interrogated, due to enroute and military radar.

I haven't read of a case where the transponder is sucking batteries flat
and if that happened, how much additional battery capacity is needed to
keep it running for the whole flight?

Our Duo came standard with dual 7ah batteries. A combination of an
LNAV, GPS-NAV, radio, and transponder sucks a single 7ah battery dry in
a little more than 4 hours. We were replacing the batteries roughly
once per year, since some of the partners (not me, of course) would
forget to either switch batteries or turn off the transponder before
draining the first battery completely. We switched to using dual 12ah
batteries, and the problem appears to have gone away.

Marc