Nice having Mode C back

[Anthony]

For a month or so, the various APP's have not been receiving my transponders Mode C. They were getting the transponder squawk, but not the altitude. I did some troubleshooting, but eventually just took it to the avioincs shop where they replaced the unit that transmits Mode C, not the entire transponder thankfully.

Earlier this week I transitted Philly Class B to head to the Jersey Shore for a few days. The Philly controllers were curteous, and professional as usual. I was able to go direct at 5500 ft. It was nice to not hear, Grummanxxx we are not receiving Mode C. However, they have let me in with permission without Mode C previously. In fact many years ago, when I had total transponder failure, New York Approach let me in, AND out of their Class B when I was dropping off a passenger at Teteboro, and then returning home. I was relieved, but a bit surprised.

Anyway, had a good few days at the shore and nice flights there and back, although I did hit some rain yesterday evening. What's the experiences with entry/transit of Class B's without Mode C or without transponder altogether? Is it common for ATC to allow this?

 

[flyingron]

I find that description hard to believe. The thing that transmits the mode C is the same circuit that transmits the mode A. The only difference is the spacing of the first pulses in the reply chain (P3 comes 21us after P1 rather than 8).

But obviously they fixed it and something got lost in the translations.

I have several times operated in and out of Dulles without *ANY* transponder, even after 9/11. Just takes some amount of coordination. Before 9/11, we'd just call up on CD or approach (depending if we were on the ground) and it would be approved. Now I call up Dulles Tower and PXT both to coordinate.

One time back in the day, I had to ferry a plane with inop transponder from IAD up to HGR where the radio shop was. The neat thing was Dulles's primary radar was also out. That meant, absent a transponder, I was completely visible to ATC. But our standard "Runway Heading 1500' until clear of the class B" clearance worked fine.

I've also come in to Dulles with no XPNDR and no ability to TRANSMIT. They identified us via a couple of turns on our primary return.

 

[Anthony]

I find that description hard to believe. The thing that transmits the mode C is the same circuit that transmits the mode A. The only difference is the spacing of the first pulses in the reply chain (P3 comes 21us after P1 rather than 8).

But obviously they fixed it and something got lost in the translations.

Quite possibly. I thought they said the Mode C unit was seperate, but maybe not. I have to call them for another reason, so I'll ask again.

I have several times operated in and out of Dulles without *ANY* transponder, even after 9/11. Just takes some amount of coordination. Before 9/11, we'd just call up on CD or approach (depending if we were on the ground) and it would be approved. Now I call up Dulles Tower and PXT both to coordinate.

One time back in the day, I had to ferry a plane with inop transponder from IAD up to HGR where the radio shop was. The neat thing was Dulles's primary radar was also out. That meant, absent a transponder, I was completely visible to ATC. But our standard "Runway Heading 1500' until clear of the class B" clearance worked fine.

I've also come in to Dulles with no XPNDR and no ability to TRANSMIT. They identified us via a couple of turns on our primary return.

Yes, NYC APP said they did have me on primary, but I thought it was nice of them to work me in the way they did, especially in very busy airspace like that. I have always found the NYC controllers really good to work with and vey accomodating. For the most part, same with Philly, but there have been a time or two when I guess they've just been too busy, but that is very, very rare.

 

[JGoodish]

I find that description hard to believe. The thing that transmits the mode C is the same circuit that transmits the mode A. The only difference is the spacing of the first pulses in the reply chain (P3 comes 21us after P1 rather than 8).

I'm going to guess that they replaced the encoder or corrected a wiring issue, as I would otherwise tend to agree with you.


JKG

 

[Anthony]

I'm going to guess that they replaced the encoder or corrected a wiring issue, as I would otherwise tend to agree with you.


JKG

I believe it was the encoder they replaced. Is this sometime seperate from the unit? I'll have to look at the logbook entry when I get home, as I don't remember what it said.

 

[denverpilot]

Yes. The altitude encoder on many is a separate small box, stuffed up behind the panel and connected to the static system with a tube, and also to the transponder via what looks like a serial cable, but is actually carrying altitude information in Binary Coded Decimal (BCD) format, usually on a DB-15 connector.

 

[Anthony]

Yes. The altitude encoder on many is a separate small box, stuffed up behind the panel and connected to the static system with a tube, and also to the transponder via what looks like a serial cable, but is actually carrying altitude information in Binary Coded Decimal (BCD) format, usually on a DB-15 connector.

Thanks Nate. That about sums it up.

Anyway, it works great now. I guess when these things go bad they just go bad.

 

[flyingron]

Yes. The altitude encoder on many is a separate small box, stuffed up behind the panel and connected to the static system with a tube, and also to the transponder via what looks like a serial cable, but is actually carrying altitude information in Binary Coded Decimal (BCD) format, usually on a DB-15 connector.

The parallel wiring is almost always Gillham-Gray coded altitude. There's not enough pins on a DB-15 to do BCD. Some transponders put out RS-232 or RS-485 serial. The format varies. Sometimes it's just the 12 bits of the Gray code packed across of a four transmitted bytes, sometimes it IS BCD.

The encoder stuck behind the panel (or in my case it's in the tail cone just behind the baggage compartment) is called a blind encoder because it only has the electrical output. Some are actually built into an altimeter that's got a dial like a regular altimeter (and hence are called encoding altimeters).

The appearance of the thing varies from looking literally like an altimeter with no face (cylindrical object) to a rather nondescript box with as Denver says a static port on one side and a DB connector on the other.

 

[denverpilot]

The parallel wiring is almost always Gillham-Gray coded altitude. There's not enough pins on a DB-15 to do BCD. Some transponders put out RS-232 or RS-485 serial. The format varies. Sometimes it's just the 12 bits of the Gray code packed across of a four transmitted bytes, sometimes it IS BCD.

Bah... Yeah. Not BCD. The one in our airplane is the Gray code shoved across as the 12 bits, if I remember correctly.

Too many "standards" floating around in my head.

Kinda amazing though that this simple stuff pulls $90/hr and the shops are all still using tube gear to test even the modern avionics.

I'm amazed the dude didn't walk out to the ramp with a car battery and some alligator clips to "test" that crappy RG-8U coax hooked to the transponder.

And them fancy serial ports... Who needs 'em! Kids! Get off my lawn!

 

[Red Air Rambo]

Mode C and traffic...I just got mine back yesterday...same issue...dead encoder.