Loss of Mode C and Transponder Reply

[mikem]

Lost Mode C and Transponder reply while in Class B airspace the other
day. After landing, was looking at all of the simple obvious things,
like transponder not seated in tray etc. Little voice told me
to look at the antenna, too.

Seems the whip broke at the base and just fell off. It is mounted on the
bottom of the boot cowl just aft of the firewall on the PA22/20, and
probably gets subjected to some buffeting from the propellor pressure
pulses.

Mounting a new one further back from the propellor might be better, but
that would put it closer to the landing gear legs. Alternatively, it
would have to go all the way back to mid fuselage where there is a
16x16" metal access plate for the elavator trim jackscrew. The strobe is
mounted on this plate. Otherwise the fuselage is covered with fabric.

Any ideas?

MikeM
Pacer '00Z

[G.R. Patterson III]

mikem wrote:
>
> Any ideas?

I would probably put the new one in exactly the same place. They aren't real
expensive, and I think that's the best place for it on that plane (mine is in
the same place on my Maule).

How long did it last? Maybe the best bet is to replace it periodically before
it fails, like some people replace dry vacuum pumps.

George Patterson
A diplomat is a person who can tell you to go to hell in such a way that
you look forward to the trip.

[[email protected]]

mikem > wrote:
> Lost Mode C and Transponder reply while in Class B airspace the other
> day. After landing, was looking at all of the simple obvious things,
> like transponder not seated in tray etc. Little voice told me
> to look at the antenna, too.

> Seems the whip broke at the base and just fell off. It is mounted on the
> bottom of the boot cowl just aft of the firewall on the PA22/20, and
> probably gets subjected to some buffeting from the propellor pressure
> pulses.

> Mounting a new one further back from the propellor might be better, but
> that would put it closer to the landing gear legs. Alternatively, it
> would have to go all the way back to mid fuselage where there is a
> 16x16" metal access plate for the elavator trim jackscrew. The strobe is
> mounted on this plate. Otherwise the fuselage is covered with fabric.

> Any ideas?

> MikeM
> Pacer '00Z

Go to an avionics shop that knows how to properly install a metal backing
plate in a fabric aircraft.

Ask how much to replace the existing antenna and how much to move it back.

--
Jim Pennino

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[mikem]

wrote:

> Go to an avionics shop that knows how to properly install a metal backing
> plate in a fabric aircraft.

My AI and I installed a backing plate under the TXP antenna about a year
ago. Prior to this, it was mounted only through the (very thin) aluminum
boot cowl, and was very flexible. I'm guessing that adding the backing
plate stiffened the mount to where the whip now hits some vibration
modes excited by the propellor pulses...

> Ask how much to replace the existing antenna and how much to move it back.

No expertise with fabric aircraft at any local avionics shop. The
replacement antenna TED 12-10 can be had from Spruce for $20, Chief for
$23, and eBay for $15+

[[email protected]]

mikem > wrote:


> wrote:

> > Go to an avionics shop that knows how to properly install a metal backing
> > plate in a fabric aircraft.

> My AI and I installed a backing plate under the TXP antenna about a year
> ago. Prior to this, it was mounted only through the (very thin) aluminum
> boot cowl, and was very flexible. I'm guessing that adding the backing
> plate stiffened the mount to where the whip now hits some vibration
> modes excited by the propellor pulses...

Could well be. Better to fatigue off a cheap antenna than a chunk of the
cowl though.

> > Ask how much to replace the existing antenna and how much to move it back.

> No expertise with fabric aircraft at any local avionics shop. The
> replacement antenna TED 12-10 can be had from Spruce for $20, Chief for
> $23, and eBay for $15+

That's not really surprising. Back in the early 70's when I worked in an
avionics shop and fabric aircraft were a lot more common, we only did one
or two a year if you didn't count the Maules which had metal in the
right place already.

Anyway, I think you answered your own question; $20 now and then is a lot
cheaper than finding someone with the expertise to mount it on the belly.

You could mount it inside the fabric, but that would probably be even
more expensive and problematic.

--
Jim Pennino

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