TCAS did not display a/c

[New F/O]

To all you techies out there,

There I was at FL170 in my office when a small jet went from right to left about 600 - 800 ft above us and about a mile or so away and nothing on the TCAS to say it was comming :ooh:

Once it was to the left side of our a/c it then showed up as a blue dimond only and then later (5 - 7sec) showed the level and trend to be 500ft above.

Any idea why it was not detected?
It passed 90 deg from right to left.
I'm reasonabley technical so please fire away, I just wondered why this happened.
Our TCAS showed other traffic a few minutes ago, so our system TCAS was OK.
At the time there were no other a/c displayed in our 12nm range window.

Please feel free to ask me more questions in case I have not given all the info here.

Cheers,

New F/O

[wrenchbender]

Perhaps the other ship had the equipment problem?

[SR20flyDoc]

The small jet could have had a Garmin 330 transponder .

This mode S transponder (/current software version) has a problem with TCAS interrogations, but not with SSR.

There is an SB out for this problem, but I think this might answer to the question without opening the box (TCAS)


Jaap

("old" Cirrus SR20 owner (2003) without PFD but with MFD and all other bells and w's)

[New F/O]

Thanks for the info.

If the SSR response is OK on the other a/c then surely my equipment would be able to detect the reply and show direction with a blue dimond with no level or trend to the right of me, there was nothing until the other a/c was ahead of us.

OK about the software, it was a Learjet type of a/c (I'm not much of an anorak so can't tell you the type even though it was close), could his transponder be intermitant or switched off at the time ?

Cheers:ok: