Transponder check required?

[SpamPilot]

My GNX 375 with GAE 12 altitude encoder passed its IFR and VFR certs (91.411 + 91.413) just a few weeks ago.  On a subsequent flight, in remote areas, ATC reported they were not receiving my Mode C.  At these times, the interrogation indicator on the GNX 375, which should be going off at least once every 12 seconds when ground radar paints and interrogates the airplane, was inactive.  My A&P checked it out and found a poorly made crimp on the BNC connector that connected the coax from the antenna to the pigtail at the back of the GNX 375 mounting tray.  The connector pulled off easily by hand.  He replaced the entire length of 40-year-old RG58 coax with new M17/128-RG400 coax and connectors.

Do I need to re-do the recent IFR/VFR certs? (which cost me $1200, BTW; also BTW, I'm inside a Mode C veil, and under Class Bravo)

I think not.  My A&P is uncertain.  I welcome opposing or supporting viewpoints, especially if anyone has direct experience.

14 CFR 91.413 covers transponder tests.  It requires transponders to be inspected and tested every 24 months and found to comply with Part 43 Appendix F.  Appendix F lists all the various tests that a transponder must pass.  It does not require the transponder to be in the aircraft for such testing.  You don't even need to connect it to an actual antenna - it's just a test of the transponder itself, not the coax that connects to the antenna.  You can bench test a transponder and be in compliance with 91.413.  More to the point, this clause doesn't require Appendix F testing to be redone after maintenance; it's only elapsed time that compels new testing.

14 CFR 91.413 and 91.411 also require an integrated system test per Part 43 Appendix E paragraph (c) when you perform maintenance "where data correspondence error could be introduced".  "Correspondence" is defined in AC 43.6D.  It is "the maximum absolute difference between altimeter display and encoder output for a constant encoder output".  This is about making sure that the static system, altimeter, altitude encoder, connection from the encoder to the transponder, and the transponder configuration/interpretation of the data coming from the encoder are all working properly so the altitude reported over Mode C is within 125 feet of the panel altimeter reading.  If, for example, you disconnect the altitude encoder from the transponder, or install a new one, you'd have to do the Appendix E paragraph (c) tests.  If you opened the static system, other than by using the static drain port, then not only would you have to do Appendix E paragraph (c), you would also have to do Appendix E paragraph (a) per 91.411.

So...

1)   My 24-month test was done just a few weeks ago, so it's still valid.  The 24-month transponder test regulation doesn't require you to re-test when you perform maintenance on the system.

2) You only need to re-test when you perform maintenance "where data correspondence error could be introduced".  Replacing the coax between the transponder and the antenna is maintenance that is entirely downstream of the altimeter, altitude encoder, and transponder.  It cannot introduce data correspondence error, because it doesn't change any of the subsystems or interfaces that are involved in encoding altitude correctly.

3) We didn't touch the static system, altimeter, or altitude encoder.  The connection between the altitude encoder and the GNX 375 was not affected.  We never removed the transponder from its mounting tray, and we didn't change its configuration relative to the encoder (or any other configuration item).

4) We did a function check on the ground and confirmed that the new coax to the antenna connection is working.  The GNX 375 indicates that it is receiving and responding to Mode C interrogations.  It's also getting ADS-B In, which uses the same antenna, and displaying traffic.

I conclude that I don't have to redo the IFR cert.

Thoughts?  Comments?

 

Edited April 21, 2021 by SpamPilot

[carusoam]

Kind of like opening up the static system... after it has been tested...

There are rules to follow... that are a challenge to remember in such fine detail...

And the rules can be ancient, and not be a good fit for modern technology...

Wait for an IR pilot who is also really familiar with avionics or a mechanic...

There are a few around here...

Best regards,

-a-

[N201MKTurbo]

Wow, I can get 5 certs for what you paid. My guy would just stop by and check that the power freq and codes are working. Probably wouldn’t charge me anything for that.

[N201MKTurbo]

When you remove the transponder, you break the connection between the encoder and the transponder. That could affect the data connection. Just being the devils advocate. 
 

But then again, if it is working ok, nobody cares how it got that way. If ATC gives you grief, just say “I just had that fixed, I’ll have to call the shop again”

[N201MKTurbo]

I see you didn’t take it out of the tray. You are probably ok. 

[Shadrach]

2 hours ago, SpamPilot said:

My GNX 375 with GAE 12 altitude encoder passed its IFR and VFR certs (91.411 + 91.413) just a few weeks ago.  On a subsequent flight, in remote areas, ATC reported they were not receiving my Mode C.  At these times, the interrogation indicator on the GNX 375, which should be going off at least once every 12 seconds when ground radar paints and interrogates the airplane, was inactive.  My A&P checked it out and found a poorly made crimp on the BNC connector that connected the coax from the antenna to the pigtail at the back of the GNX 375 mounting tray.  The connector pulled off easily by hand.  He replaced the entire length of 40-year-old RG58 coax with new M17/128-RG400 coax and connectors.

Do I need to re-do the recent IFR/VFR certs? (which cost me $1200, BTW; also BTW, I'm inside a Mode C veil, and under Class Bravo)

I think not.  My A&P is uncertain.  I welcome opposing or supporting viewpoints, especially if anyone has direct experience.

14 CFR 91.413 covers transponder tests.  It requires transponders to be inspected and tested every 24 months and found to comply with Part 43 Appendix F.  Appendix F lists all the various tests that a transponder must pass.  It does not require the transponder to be in the aircraft for such testing.  You don't even need to connect it to an actual antenna - it's just a test of the transponder itself, not the coax that connects to the antenna.  You can bench test a transponder and be in compliance with 91.413.  More to the point, this clause doesn't require Appendix F testing to be redone after maintenance; it's only elapsed time that compels new testing.

14 CFR 91.413 and 91.411 also require an integrated system test per Part 43 Appendix E paragraph (c) when you perform maintenance "where data correspondence error could be introduced".  "Correspondence" is defined in AC 43.6D.  It is "the maximum absolute difference between altimeter display and encoder output for a constant encoder output".  This is about making sure that the static system, altimeter, altitude encoder, connection from the encoder to the transponder, and the transponder configuration/interpretation of the data coming from the encoder are all working properly so the altitude reported over Mode C is within 125 feet of the panel altimeter reading.  If, for example, you disconnect the altitude encoder from the transponder, or install a new one, you'd have to do the Appendix E paragraph (c) tests.  If you opened the static system, other than by using the static drain port, then not only would you have to do Appendix E paragraph (c), you would also have to do Appendix E paragraph (a) per 91.411.

So...

1)   My 24-month test was done just a few weeks ago, so it's still valid.  The 24-month transponder test regulation doesn't require you to re-test when you perform maintenance on the system.

2) You only need to re-test when you perform maintenance "where data correspondence error could be introduced".  Replacing the coax between the transponder and the antenna is maintenance that is entirely downstream of the altimeter, altitude encoder, and transponder.  It cannot introduce data correspondence error, because it doesn't change any of the subsystems or interfaces that are involved in encoding altitude correctly.

3) We didn't touch the static system, altimeter, or altitude encoder.  The connection between the altitude encoder and the GNX 375 was not affected.  We never removed the transponder from its mounting tray, and we didn't change its configuration relative to the encoder (or any other configuration item).

4) We did a function check on the ground and confirmed that the new coax to the antenna connection is working.  The GNX 375 indicates that it is receiving and responding to Mode C interrogations.  It's also getting ADS-B In, which uses the same antenna, and displaying traffic.

I conclude that I don't have to redo the IFR cert.

Thoughts?  Comments?

 

Where are you located? IFR certification is $400 in these parts an I’m less than 50 miles from Washington DC.

[Hank]

53 minutes ago, Shadrach said:

Where are you located? IFR certification is $400 in these parts an I’m less than 50 miles from Washington DC.

Expensive in The Big City! But I think the OP was taken for a ride . . . .

[GeeBee]

Why don't you fly it, do a PAPR request. If the PAPR shows good, you're good to go.

 

https://adsbperformance.faa.gov/PAPRRequest.aspx

 

 

[DMM]

See attached AC43-6C.  I glanced thru the document.  A functional check may be the only thing required to be legal.  Take a look at the decision flow chart.   What GeeBee says makes sense.  

ac 43-6c.pdf

[N231BN]

Which Garmin dealer installed a GNX-375 with old RG-58??

A Garmin GDL-50 or 52 can verify your system using the ADS-B Compliance function in Garmin Pilot. As stated above, a function test is all that is required.

 

[Marauder]

Which Garmin dealer installed a GNX-375 with old RG-58??

All of them

From my experience, both Garmin shops reused the RG-58 that was in my plane for the GTN 650 and the GNC 255B. The exception being new RG-400 that was used for the GPS antenna on my GTN. When I had issues with the new radios I ran all new RG-400 and had the shop make up new connectors.

The only time I saw all new RG-400 run was when I had the Lynx installed.

I suspect reusing coax, even with new radios, is pretty prevalent.


Sent from my iPad using Tapatalk Pro

[JimB]

No a re-cerfication is not required. The transponder is what is certified. In fact to accomplish the 24 month check you can send the transponder out or you can bypass the antennae connection. And yes, I do this for a living. 

Edited April 21, 2021 by JimB

[EricJ]

I'd +1 that a coax re-crimp is not justification for a recert.   I think your research was good.

 

[RobertGary1]

My Garmin installer replaced all the coax. 

[RobertGary1]

9 hours ago, Shadrach said:

Where are you located? IFR certification is $400 in these parts an I’m less than 50 miles from Washington DC.

At least here is based on equipment. $400 may get a minimum ifr platform but many of us now technically have 3 altitude indicators and 2 air data computers. They add to the cost. 

[SpamPilot]

Answers to some questions:

- $1200 for the cert was on the West Coast, using a well-known avionics maintenance chain that typically deals in corporate jets.  They diagnosed but did not fix a small static leak (see my post on the static drain leak), which added to the cost.  They did get me in very quickly.  My preferred shop on the field couldn't get me in for at least a month.

- RG-58 has about 50% more loss per unit length at transponder and GPS frequencies than RG-400, but that's not enough to make a noticeable performance difference over the short runs you get in a Mooney (often less than 10 feet).  Improperly crimped connectors are a different story, as would be corroded connectors.

New question:

Why does ATC care so much about Mode C anymore?  I naiively thought ADS-B was supposed to provide ATC with the same (and better) information as Mode C.  My ADS-B out was still working well, even when ATC couldn't get Mode C.  My FlightAware track is pretty clean.

[A64Pilot]

I can’t answer why we need Mode-C, it would seem that ADSB would supersede it.

‘However I’d bet lunch money that while the FAA required all of us to buy ADSB, they themselves haven’t fully integrated it, meaning I’d bet that a lot of their equipment is using Mode C and not ADSB yet.

‘That’s just my guess

Edited April 21, 2021 by A64Pilot

[cferr59]

16 minutes ago, A64Pilot said:

I can’t answer why we need Mode-C, it would seem that ADSB would supersede it.

‘However I’d bet lunch money that while the FAA required all of us to buy ADSB, they themselves haven’t fully integrated it, meaning I’d bet that a lot of their equipment is using Mode C and not ADSB yet.

‘That’s just my guess

Last time I asked ATC to confirm they were receiving my ADS-B, they did not know and didn't know how to figure it out.

[Rick Junkin]

15 minutes ago, A64Pilot said:

I can’t answer why we need Mode-C, it would seem that ADSB would supersede it.

‘However I’d bet lunch money that while the FAA required all of us to buy ADSB, they themselves haven’t fully integrated it, meaning I’d bet that a lot of their equipment is using Mode C and not ADSB yet.

‘That’s just my guess

@A64Pilot Curiosity is killing me - why is there a " ' " at the beginning of almost every line of your posts?

[bradp]

Only the new generation of radar terminals can use and display ads-b.  Not all ATC facilities have the new equipment. That’s how my local ATC explained it to me when they had a demo unit in their meeting room but hadn’t had the installation yet. 

[Matt M]

2 hours ago, SpamPilot said:

Answers to some questions:

- $1200 for the cert was on the West Coast, using a well-known avionics maintenance chain that typically deals in corporate jets.  They diagnosed but did not fix a small static leak (see my post on the static drain leak), which added to the cost.  They did get me in very quickly.  My preferred shop on the field couldn't get me in for at least a month.

- RG-58 has about 50% more loss per unit length at transponder and GPS frequencies than RG-400, but that's not enough to make a noticeable performance difference over the short runs you get in a Mooney (often less than 10 feet).  Improperly crimped connectors are a different story, as would be corroded connectors.

New question:

Why does ATC care so much about Mode C anymore?  I naiively thought ADS-B was supposed to provide ATC with the same (and better) information as Mode C.  My ADS-B out was still working well, even when ATC couldn't get Mode C.  My FlightAware track is pretty clean.

I believe all ATC facilities have ADS-B capabilities now. Your GNX-375 is a 1090ES transponder. ATC radar interrogate transponders for both altitude and your 3A code. The transponder sends these signals back to the radar. The ES portion of your transponder (ADS-B) sends an unsolicited message independent of the radar interrogation. This signal is received by ground stations, not the radar.  The ground stations then send your position back to the radar facility where it is fused on the controllers display. If you had a bad connection on your coax the signal could have been weak and the radar would not not see your transponder reply The ADS-B signal may have been received by a ground station. The controller may have seen your ADS-B on the display , but it wasn’t fused with your radar track because the radar did not get your transponder reply.

FlightAware uses their own ADS-B receivers independent of the ground stations the FAA uses. So if you saw yourself on FlightAware that doesn’t mean the FAA saw it. 

At lastly you still need Mode-C for TCAS to operate. The TCAS on bigger aircraft still rely on Mode C. The new TCAS standard is coming which will incorporate ADS-B data.

[EricJ]

2 hours ago, Junkman said:

@A64Pilot Curiosity is killing me - why is there a " ' " at the beginning of almost every line of your posts?

The government adds those after processing his outbox.

[RobertGary1]

3 hours ago, A64Pilot said:

I can’t answer why we need Mode-C, it would seem that ADSB would supersede it.

‘However I’d bet lunch money that while the FAA required all of us to buy ADSB, they themselves haven’t fully integrated it, meaning I’d bet that a lot of their equipment is using Mode C and not ADSB yet.

‘That’s just my guess

For sure. When I’ve asked atc to check my ads-b I always get a “hold on I need to check another system”.