wrote:
wrote:
: You must be one of the few that has ever had a tube crap out. You
: will surely run out of gas before you run out of tubes.

: Having it on also allows all the toy collision avoidance things to see
: you.
: Like TCADS.

: Small price to pay for collision avoidance. Please have it on. It's
: not just the rule, it's a great idea!!

It's not that the tube craps out in-flight... it's just that repeatedly running the transponder when you do not
"need" it (read: in B.F.E., VFR, local-flights) means that you shorten its useful life when you *DO* need it (flight
following, Class-[ABC], IFR). Any time I actually *GO* anywhere where collision avoidance might be an additional
feature, chances are it's a cross-country flight and I'll have it on for that reason.

As I mentioned, I have personally gone through two tube-based transponders in this manner. My mechanic has
also gone through two that lost transmit power due to a dying cavity tube, as has another pilot friend of mine.
Granted, I'm on the low end of the avionics totem pole (ebay specials mostly)... the fact of the matter is that
replacing the cavity tube has been made cost-prohibitive by the part costing $500 or more.

-Cory


The cavities are pretty robust, and it takes lots of hours to wear them
out. Perhaps you are buying used transponders that are already close to
end of life of the cavity? If you are concerned about cavity life that
much, then why not bite the bullet and get a modern solid-state slide-in
replacement? A new one costs no more than 3 or 4 used ones, comes with
a warranty, and will last you a good long time. Plus some of them come
with added features like altitude read-out, timers, single push VFR and
such that you'll find handy.

Leaving the transponder off makes you blind to TCAS, TCAD, and all of
the low cost passive anti-collision boxes like the Monroy (granted, they
won't see you if you are not in radar coverage anyway unless an active
TCAS interrogates you).

Also, the tube isn't dissipating any power unless it is replying to an
interrogation, so it doesn't take any life off the tube by keeping the
transponder on when it isn't getting interrogated. If you are in an
area where it is getting interrogated, then you are invisible to
whatever radar or TCAS is interrogating you if you have it off. Please
turn it on unless you are on the ground.