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