why would you want two transponders?

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cirrusmx

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i was looking at a seneca for sale in controller.com and it was listed as having twp transponders g327's. why would someone install a stand by transponder. is there a reg for a particular type of operation requiring 2 transponders?
 
if you fly in airspace which requires a transponder it might be nice to have a back-up...
 
Clark1961 said:
if you fly in airspace which requires a transponder it might be nice to have a back-up...
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. If the 327 is the cheapest thing in the panel then it won't cost much (relatively speaking) to have a back up for when #1 decides to quit
 
If the plane operates near the Washington,DC area then it's not a surprise they would want two. losing a transponder is very bad here.
 
Especially if the plane is used for charter or other revenue-generating activity where losing the transponder means losing the revenue stream. Dual transponders get more and more common as you go up the ladder from light singles where they are most uncommon to commercial transports in 121 operations where they are de rigeur.
 
This was more important with cavity tube containing transponders that like to drift off frequency ever so often.
Suggests that this was a 135 Seneca.
 
BigBadLou said:
Follow-up question: in a dual-transponder configuration, does each one have its own encoder (blind or not)?
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Depends how it's installed. No reason you couldn't feed both from the same encoder via a splitter, but I guess if you want complete redundancy, you'd probably want two encoders. Not sure what the general tread is in reality.
 
Ron Levy said:
but I guess if you want complete redundancy, you'd probably want two encoders.
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That was absolutely my thinking, Ron.

Two transponders fed from one source, you can see how it would be pointless in case the encoder fails.

I wonder whether there is any reg for dual transponder installation and encoder pairing.
Disclaimer: I am just a VFR guy so I don't really fall into the crowd that normally operates such aircraft and I am too lazy to look it up. :)
 
Looking over the shoulder of the guys at Cape Air last year, the second transponder serves as an altitude bug.

At least, below 7,778' MSL. :D
 
Ron Levy said:
Especially if the plane is used for charter or other revenue-generating activity where losing the transponder means losing the revenue stream.
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Ding ding. What costs more? Buying a redundant transponder, or cancelling scheduled charter flights while still paying the crew their salary?:yes:
 
The installation I have seen had a selector switch and two antennae. One was a mode-C one a mode-S box.
 
jtheune said:
If the plane operates near the Washington,DC area then it's not a surprise they would want two. losing a transponder is very bad here.
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If you are based within the Flight Restricted Zone, no transponder means you can't come home.
 
redundancy in critical instruments/parts is understandable. but if you are in imc in bravo within those no fly zones like they have in dc with icing shooting an approach to minimums with one engine inop and a pax dying and your transponder craps out and you dont have the back up i am pretty sure that can be explained after landing and shouldnt affect the safety of the flight,.after all theres no reg that says that you need two?. now, if you are in the ground and your transponder craps out the you cant launch unless you are planning to stayout of the transponder required airspace. so i am guessing the redundancy is moreso to be able to launch rather than for inflight breakups.
 
BigBadLou said:
That was absolutely my thinking, Ron.

Two transponders fed from one source, you can see how it would be pointless in case the encoder fails.
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Well, maybe. ATC gets a lot less upset about losing Mode C (altitude data) but having Mode A (4-digit code) still operational than about having no transponder at all. In fact, when you're having Mode C problems, they may tell you to turn Mode C off (i.e., ON rather than ALT) and continue with it that way. You don't often get told to turn your transponder completely off and continue that way.

I wonder whether there is any reg for dual transponder installation and encoder pairing.
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None of which I am aware.
 
It's so when ATC says you're at the wrong altitude, you can say, "let me try the #2 transponder.":yesnod:
 
cirrusmx said:
redundancy in critical instruments/parts is understandable. but if you are in imc in bravo within those no fly zones like they have in dc with icing shooting an approach to minimums with one engine inop and a pax dying and your transponder craps out and you dont have the back up i am pretty sure that can be explained after landing and shouldnt affect the safety of the flight,.after all theres no reg that says that you need two?.
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Correct, but there are places where you must have one, and if that one fails in flight, you ain't goin' there (or going back there). Big issue around where I live near DC.

now, if you are in the ground and your transponder craps out the you cant launch unless you are planning to stayout of the transponder required airspace. so i am guessing the redundancy is moreso to be able to launch rather than for inflight breakups.
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In the FRZ/SFRA, you'd be wrong. No transponder right after takeoff? You either turn around and land right back where you took off and stay there until it's fixed, or you depart the airspace ASAP and don't come back until it's fixed. Lose it on the way to FRZ/SFRA before entry? You divert somewhere outside that airspace and get it fixed before you can enter. That can put major crimp in your operation, especially if you're doing it for money.
 
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Ron Levy said:
Well, maybe. ATC gets a lot less upset about losing Mode C (altitude data) but having Mode A (4-digit code) still operational than about having no transponder at all. In fact, when you're having Mode C problems, they may tell you to turn Mode C off (i.e., ON rather than ALT) and continue with it that way. You don't often get told to turn your transponder completely off and continue that way.

None of which I am aware.
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I had this happen in Memphis Class B. Was going into KOLV and my Mode C was reporting 13,500'. They directed me to report altitude and turn off Alt reporting. Needed a new encoder and certification.



Jim R
Collierville, TN

N7155H--1946 Piper J-3 Cub
N3368K--1946 Globe GC-1B Swift
N4WJ--1994 Van's RV-4
 
Ron Levy said:
Correct, but there are places where you must have one, and if that one fails in flight, you ain't goin' there (or going back there). Big issue around where I live near DC.
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I've had ATC get picky with me above 10,000' (over the mountains) with a failed transponder.
 
N3368K said:
I had this happen in Memphis Class B. Was going into KOLV and my Mode C was reporting 13,500'. They directed me to report altitude and turn off Alt reporting. Needed a new encoder and certification.



Jim R
Collierville, TN

N7155H--1946 Piper J-3 Cub
N3368K--1946 Globe GC-1B Swift
N4WJ--1994 Van's RV-4
Click to expand...

I had it happen down low...same result, but shuttung off the heater solved it....or at least helped my avionics guy troubleshoot it.:goofy:
 
cirrusmx said:
if you are in the ground and your transponder craps out the you cant launch unless you are planning to stayout of the transponder required airspace. so i am guessing the redundancy is moreso to be able to launch rather than for inflight breakups.
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Unless you have a test rig, there is no way to know whether a transponder is working properly until after launch, when ATC either sees you and your Mode C or does not. If you have just taken off from within the FRZ and you don't have a good signal, you will most likely be ordered to exit the area, not to return until the transponder is fixed.
 
MauleSkinner said:
It's so when ATC says you're at the wrong altitude, you can say, "let me try the #2 transponder.":yesnod:
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While pushing the nose over to lose the extra 400 feet before the other transponder has a chance to cycle. :D
 
I had mine break in mid-flight and was returning to the LA Area where there's a Mode C veil 30nm around. My airport is inside that. I asked permission over the freq enroute and was clear to continue. They didn't even ask me to stay clear of B or C airspace, but I told them I was going to anyway. No problem.
 
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