TCAS and Mode-S

[Nigd3]

This is not really relevant to the bigger aircraft types, especially European or US operators.

For those aircraft not mandated to carry Mode-S transponders but have them installed, either because modern Mode C XPDRs are more difficult to purchase, or as provision for if/when they do become mandated, ensure that the XPDR has a 24 bit address strapped/configured.

I have seen aircraft with Mode S XPDR and no 24 bit address codes, as Mode-S is not required where they operate but the XPDR functions without any problem with ATC. However a TCAS unit will reject an "all call" from a XPDR with a 24 bit address of all 1's or all 0's (no address code) and will not place the XPDR on its register for tracking

In effect, the aircraft without a 24 bit address code on its XPDR is invisible to a TCAS and will not be displayed/alerted on suitably equipped aircraft.

Anyone else had experience of this?

[Graybeard]

That's true, and I don't remember the design logic of that. I've seen the situation where the strapping plug was wired, but the Common was open, and the 737's own TCAS would not function. The Fail Warn logic in the TCAS was no help, either.

Early on, after Mode S and before TCAS, Boeing was delivering new planes with the strapping open, but they soon fixed that.

Somewhere in the nethers of my computer is a Basic program to convert N numbers to hex, octal and binary codes for strapping. Officially, you need to get the strapping code for your tail number from your nation's certification authority.

GB

[CJ Driver]

I think you will find that this is fairly old news. Most modern Mode S transponders will not allow you to "set" an all zeroes or all ones address - it is prohibited in the TSO standard. Only older types which used actual wire links to set the address were susceptible to having the plug fall out and effectively no address at all - and then depending on whether the logic was active high or low you could get either all zeroes or all ones.

[Nigd3]

Old news, maybe. Still happening or installed, definately

The old style wire strapped XPDRs are still available, also offering EHS options, although I agree a maintenance mode/programmable dongle that flags an error should ensure this doesn't happen with a more modern unit.

I dont think an all zero address can be unflagged if the XPDR is installed together with a TCAS, as the TCAS checks the XPDR, however if the older style XPDR is installed alone, it can be missed.

[Graybeard]

I thought not. It finally came to me upon awakening this morning. The name before Mode S was Discreet Address Beacon System, pre-1986. Its intent was to allow ATC radar to individually interrogate transponders as needed. However, TCAS came in before more than just a few ATC facilities were equipped.

TCAS needs unique targets for tracking, rather than just blips, so it can make full use of the target aircraft's info, and can increase the discreet interrogation rate in keeping with the level of threat.

A mode C reply or All Call is of more limited use in collision avoidance, in high traffic areas, meaning possible target merging and signal saturation.

GB