Transponder requirement confusion

[[email protected]]

Quick (hopefully) question to a confusing part of the FARs. With my new (to
me) solid-state transponder, I'm now willing to power it up for even local VFR
flights. Before, I would only turn on my KT-76 if I needed to (VFR advisories, IFR,
Class B/C operation) since I knew the cavity tube had a finite number of hours before
it died. Now that I read through the transponder requirement carefully, it appears
that that's not allowed. Google groups appear to contain previous usenet arguments
regarding this, but the answer's still not quite clear. In particular:

FAR 91.215(5)(c)
(c) Transponder-on operation. While in the airspace as specified in paragraph (b) of
this section or in all controlled airspace, each person operating an aircraft equipped
with an operable ATC transponder maintained in accordance with ??91.413 of this part
shall operate the transponder, including Mode C equipment if installed, and shall
reply on the appropriate code or as assigned by ATC.

"Controlled" airspace also includes Class E... so if you have one (and it's
within the 2-year test), you *have* to turn it on? ... but you aren't *required* to
have one?

If so, that's a very unclear requirement from most colloquial references on
equipment requirements...

Curious...
-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[john smith]

In article >,
wrote:

> Quick (hopefully) question to a confusing part of the FARs. With my new (to
> me) solid-state transponder, I'm now willing to power it up for even local
> VFR
> flights. Before, I would only turn on my KT-76 if I needed to (VFR
> advisories, IFR,
> Class B/C operation) since I knew the cavity tube had a finite number of
> hours before
> it died. Now that I read through the transponder requirement carefully, it
> appears
> that that's not allowed. Google groups appear to contain previous usenet
> arguments
> regarding this, but the answer's still not quite clear. In particular:
>
> FAR 91.215(5)(c)
> (c) Transponder-on operation. While in the airspace as specified in paragraph
> (b) of
> this section or in all controlled airspace, each person operating an aircraft
> equipped
> with an operable ATC transponder maintained in accordance with ??91.413 of
> this part
> shall operate the transponder, including Mode C equipment if installed, and
> shall
> reply on the appropriate code or as assigned by ATC.
>
> "Controlled" airspace also includes Class E... so if you have one (and it's
> within the 2-year test), you *have* to turn it on? ... but you aren't
> *required* to
> have one?
>
> If so, that's a very unclear requirement from most colloquial references on
> equipment requirements...

The really dumb part of that requirement is that terminal radar doesn't
see below a given altitude at a given distance. Our local site (900 MSL)
cannot see aircraft with transponders on below (5000 MSL) at airports 40
south of the radar site. It gets really exciting when they are
controlling you into that airport on an instrument approach.

[Bob Noel]

In article
>,
john smith > wrote:

> The really dumb part of that requirement is that terminal radar doesn't
> see below a given altitude at a given distance.

but a TCAS II equipped aircraft can see you.

--
Bob Noel
Looking for a sig the
lawyers will hate

[Vaughn Simon]

"Bob Noel" > wrote in message
...
> but a TCAS II equipped aircraft can see you.

...which gives you plenty of reason to leave the thing on. In fact, it is
reason enough that significant numbers of glider people are installing some of
the new low-drain units, even though there is no FAA requirement.

Vaughn

[[email protected]]

: ...which gives you plenty of reason to leave the thing on. In fact, it is
: reason enough that significant numbers of glider people are installing some of
: the new low-drain units, even though there is no FAA requirement.

My big beef was just the finite lifespan of the old cavity tube units. As long as it's solid
state like my new unit, there's really no reason NOT to turn it on. Maybe I was just ultra-sensitive
to the cavity thing since I had to replace two of them with low power.

The other beef is how vague the FAR is.

-Cory


--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[Ron Natalie]

wrote:

> The other beef is how vague the FAR is.
>
What's vague. If you're in controlled airspace and you've got
a transponder, it's supposed to be on.

[Greg B[_1_]]

> wrote in message
...
> "Controlled" airspace also includes Class E... so if you have one (and
> it's
> within the 2-year test), you *have* to turn it on? ... but you aren't
> *required* to
> have one?
>

Yes, I don't have a transponder so I don't have to turn it on. I have gotten
requests to turn on my transponder which I reply 'Slant X-Ray'...

[john smith]

In article >,
Bob Noel > wrote:

> In article
> >,
> john smith > wrote:
>
> > The really dumb part of that requirement is that terminal radar doesn't
> > see below a given altitude at a given distance.
>
> but a TCAS II equipped aircraft can see you.

Only if you have a Mode-S transponder.

[[email protected]]

john smith wrote:

> > but a TCAS II equipped aircraft can see you.
>
> Only if you have a Mode-S transponder.

Nope, all they need is mode C.

[[email protected]]

wrote:
> john smith wrote:
>
> > > but a TCAS II equipped aircraft can see you.
> >
> > Only if you have a Mode-S transponder.
>
> Nope, all they need is mode C.

Actually, they don't even need mode C, now that I think about it.
You get different levels of service depending on what the target has.

If they've got Mode A you'll get an callout.
If they've got Mode C then you'll get an RA (climb / descend)
If they've got Mode S and they've also got TCAS, the two TCAS's
will boogie with each other to coordinate the RA.

So really mode S by itself does nothing for TCAS II unless you've
also got a TCAS as well.

[[email protected]]

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!!

Bill Hale


wrote:
> Quick (hopefully) question to a confusing part of the FARs. With my new (to
> me) solid-state transponder, I'm now willing to power it up for even local VFR
> flights. Before, I would only turn on my KT-76 if I needed to (VFR advisories, IFR,
> Class B/C operation) since I knew the cavity tube had a finite number of hours before
> it died. Now that I read through the transponder requirement carefully, it appears
> that that's not allowed. Google groups appear to contain previous usenet arguments
> regarding this, but the answer's still not quite clear. In particular:
>
> FAR 91.215(5)(c)
> (c) Transponder-on operation. While in the airspace as specified in paragraph (b) of
> this section or in all controlled airspace, each person operating an aircraft equipped
> with an operable ATC transponder maintained in accordance with ??91.413 of this part
> shall operate the transponder, including Mode C equipment if installed, and shall
> reply on the appropriate code or as assigned by ATC.
>
> "Controlled" airspace also includes Class E... so if you have one (and it's
> within the 2-year test), you *have* to turn it on? ... but you aren't *required* to
> have one?
>
> If so, that's a very unclear requirement from most colloquial references on
> equipment requirements...
>
> Curious...
> -Cory
>
> --
>
> ************************************************** ***********************
> * Cory Papenfuss, Ph.D., PPSEL-IA *
> * Electrical Engineering *
> * Virginia Polytechnic Institute and State University *
> ************************************************** ***********************

[Newps]

john smith wrote:


>>
>>but a TCAS II equipped aircraft can see you.
>
>
> Only if you have a Mode-S transponder.


?????

[[email protected]]

> 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

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[darthpup]

In the current atmosphere of fear in the US regarding terrorism I do
not think it would be wise to fly VFR anywhere without Mode C in
operation, regulations or not withstanding.
Being right and being safe are two different things.

[Steven P. McNicoll[_1_]]

> wrote in message
...
>
> 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).
>

What's "B.F.E."?

[john smith]

In article om>,
wrote:

> wrote:
> > john smith wrote:
> >
> > > > but a TCAS II equipped aircraft can see you.
> > >
> > > Only if you have a Mode-S transponder.
> >
> > Nope, all they need is mode C.
>
> Actually, they don't even need mode C, now that I think about it.
> You get different levels of service depending on what the target has.
>
> If they've got Mode A you'll get an callout.
> If they've got Mode C then you'll get an RA (climb / descend)
> If they've got Mode S and they've also got TCAS, the two TCAS's
> will boogie with each other to coordinate the RA.
>
> So really mode S by itself does nothing for TCAS II unless you've
> also got a TCAS as well.

My understanding is that the Mode-S provides the communication
information. Mode C may contain altitude, but there is no distance or
direction information.

[Ray Andraka]

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.

[RST Engineering]

Bum F@#k Egypt. i.e. out in the tules

Jim


"Steven P. McNicoll" > wrote in message
k.net...

>
> What's "B.F.E."?
>

[Newps]

john smith wrote:

>
>
> My understanding is that the Mode-S provides the communication
> information. Mode C may contain altitude, but there is no distance or
> direction information.

Mode S doesn't provide any distance or direction info to another TCAS.
Each TCAS figures that stuff out on its own.

[Ray Andraka]

john smith wrote:

> In article om>,
> wrote:
>
>
wrote:
>>
>>>john smith wrote:
>>>
>>>
>>>>>but a TCAS II equipped aircraft can see you.
>>>>
>>>>Only if you have a Mode-S transponder.
>>>
>>>Nope, all they need is mode C.
>>
>>Actually, they don't even need mode C, now that I think about it.
>>You get different levels of service depending on what the target has.
>>
>>If they've got Mode A you'll get an callout.
>>If they've got Mode C then you'll get an RA (climb / descend)
>>If they've got Mode S and they've also got TCAS, the two TCAS's
>>will boogie with each other to coordinate the RA.
>>
>>So really mode S by itself does nothing for TCAS II unless you've
>>also got a TCAS as well.
>
>
> My understanding is that the Mode-S provides the communication
> information. Mode C may contain altitude, but there is no distance or
> direction information.


That's correct. Mode C only transmits either the squawk code or the
encoded altitude, depending on the specific interrogation. However,
TCAS is an active system that sends an interrogation and listens for the
reply. It uses the time delay between the interrogation it sends and
the reply it recieves back to accurately measure range. Many use an
antenna array and beamforming techniques to determine azimuth as well.
Therefore, it can determine both distance and direction, and of course
it gets the altitude directly from the reply.

The passive TCAD boxes do not transmit interrogations. Instead they
listen for replies to other interrogations and use some signal
processing to glean rough range information from the dispersion and
amplitude of the received pulse. Considering the lack of a hard
measurement, they do a pretty decent job at estimating range. Strike
finders work under a similar principle. In the case of a transponder
reply, the job is easier because the transmit pulse shape and amplitude
are pretty narrowly defined for an in-spec transponder. Most of the
TCAD boxes do not provide azimuth info. The ones that do also use a
phase array of antennas and signal processing to do beamforming to
resolve the azimuth.

[Bob Noel]

In article <qAPEg.4109$SZ3.3107@dukeread04>, Ray Andraka >
wrote:

> That's correct. Mode C only transmits either the squawk code or the
> encoded altitude, depending on the specific interrogation.

Mode 3A provides the code, 3C provides the altitude.

>,
> TCAS is an active system that sends an interrogation and listens for the
> reply. It uses the time delay between the interrogation it sends and
> the reply it recieves back to accurately measure range. Many use an
> antenna array and beamforming techniques to determine azimuth as well.

TCAS II systems MUST use at least one directional antenna (the top one).
Normally both the top and bottom ones are directional.

--
Bob Noel
Looking for a sig the
lawyers will hate

[Bob Noel]

In article
>,
john smith > wrote:

> My understanding is that the Mode-S provides the communication
> information.

Mode S provides a datalink so that cooperating TCAS II-equipped
aircraft can coordinate RAs. When a TCAS II-equipped aircraft
encounters the older Mode 3A/C transponder, the TCAS-II will
still provide the flying pilot an RA if necessary.

--
Bob Noel
Looking for a sig the
lawyers will hate

[Ron Natalie]

john smith wrote:

> My understanding is that the Mode-S provides the communication
> information. Mode C may contain altitude, but there is no distance or
> direction information.

TCAS doesn't rely on the units communicating position information.
It does everything via the normal radar behavior. You might
be confusing it with ADS-B where units broadcast their position.

[Ron Natalie]

wrote:

>
> 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.
>

I had the same KT78 in the plane for years and always turned it on.
It's still working (and soon should be in a friends RV). I only
pulled it because I did install a mode S transponder.

[Ray Andraka]

Bob Noel wrote:

>
> Mode 3A provides the code, 3C provides the altitude.
>

Absolutely right. There are two different interrogations differentiated
by the spacing of pulses in the interrogation. The transponder replies
with the 12 bit squawk code when it receives a mode 3/A interrogation,
or the 12 bit altitude code from the altitude encoder for a mode C
interrogation. If you have mode C turned off on the panel or if there
is no encoder attached, the reply to a mode C interrogation has
all 12 data bits set to 0. The transpoder always replys with the squawk
code to A mode 3/A interrogation.

>
> TCAS II systems MUST use at least one directional antenna (the top one).
> Normally both the top and bottom ones are directional.
>
Right. TCAS II requires the azimuth. This is typically done with a
small array of antennas and the reciever processes the differences in
signal phase between antennas to determine the direction of arrival. The
advantage of a phased array is that there are no moving parts needed to
steer the antenna's look direction. I don't believe TCAS (not TCAS II)
was required to resolve azimuth.