View Full Version : Old transponder TSOs

23rd Mar 2007, 18:01
Anyone got a copy of the now obsolete TSO-C74, TSO-C74a or TSO-C74b?

In particular, does TSO-C74b require the transponder to reply to Mode C interrogation with framing pulses, even though no information pulses are sent, and what were the corresponding requirements in TSO-C74 and TSO-C74a?

For reference, the relevant bits in TSO-C74c are

2.4 Interrogation. The equipment must accept and reply to interrogations on at least Modes 3/A and C

2.14 Pressure-Altitude Transmission. ... The transponder must be provided with a means to remove the information pulses from the Mode C reply when requested by Air Traffic Control. The transponder must continue transmitting the framing pulses on Mode C when the information pulses have been removed or are not provided.


23rd Mar 2007, 20:55
Here's maybe more than you wanted to know:

I had his Altitude Nag in my Bellanca Cruisair back in '90-92, and it was a great product; still is.

He wrote a white paper back then about the Iran Air A300 that was shot down by the USNavy in what, 1987? He postulated that it was the fault of the Navy radar confusing the 6400 ft. altitude reply with an IFF code that was sometimes used by the Iran AF. You know, it's only the spacing of 21.3 microseconds vs 8 microseconds that distinguishes an altitude interrogation from a code interrogation.

I'm fairly new to Pprune; has that accident been discussed?


24th Mar 2007, 14:49
I contacted Keith Peshak and received these links: ( (

He briefly discusses the Vincennes shootdown of Iran Air A-300, but doesn't go into the lengthy explanation that was in the white paper.

I've worked with and on a lot of old transponders, Narco, King, etc., up to Arinc style, and I never saw one that didn't reply to Mode C interrogations with framing pulses.

Contact me offline if you like.

24th Mar 2007, 17:54
Thanks Graybeard

The question arose in another forum as to whether there were transponders around that wouldn't reply to a Mode C interrogation (in that case from TCAS) with at least framing pulses. I've managed to establish that:

* Since 1975 all operating transponders have to conform to TSO-C74b or TSO-C74c

* TSO-C74c definitely requires reply to a Mode C interrogation with framing pulses even in the absence of altitude information

I'd like to be able to say that means that all legal transponders reply to a Mode C interrogation with framing pulses (and thus show on TCAS). However, TSO-C74b is now "historical" and the FARs are phrased as follows:

"For operations not conducted under part 121, 127 or 135 of this chapter, ATC transponder equipment installed must meet the performance and environmental requirements of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with altitude reporting capability) as appropriate..."

That left a slight doubt in my mind as regards the response of a TSO-C74b transponder to a Mode C interrogation. My questions about TSO-C74 and TSO-C74 were merely curiosity as to the history of development.

25th Mar 2007, 03:44
I think you've answered it. A transponder not reporting altitude has to respond to Mode C interrogations with framing pulses, as it has no other reply mechanism than framing pulses.

Peshak is a good read, if you can follow him.