Tcas

[TheDrop]

Here is what could have happened:

As stated above, it is true, that normally, if you get something on TCAS, it should be on ATCs radar as well. I can think of two instances where it is not so.

1. The aircrafts transponder is partly broken and so weak that the signal cannot reach the ATC radar 100 miles away, but can reach the other aircraft a few miles ahead.

More probable in this case though:

2. A low altitude aircraft has an altitude encoding problem (this is very common - have you never been told in low altitude something like "your mode charley indicates FL145, please switch to mode alpha"). If there is a low altitude aircraft 5 miles away from you, and his transponder by mistake indicates same FL as you are flying in, TCAS will always see his direction in azimuth, which is independent of altitude, and his mode charley will be used to tell altitude.

There you go, an aircraft way below you triggers your TCAS, and ATC doesn't see him if he is below radar range. Two minutes after he is "gone", because his altitude encoding indicates something else, that is away from the TCAS range.

Also, some bored person might find a transponder with altitude encoder and set it to a certain flight level, just to see if he can trigger some other aircrafts TCAS. Why ? There are idiots "down there" who gives false clearences for whatever reason !

Therefore, any such ghost TCAS instance should be reported with accurate position, FL, time etc, and if it is the same place, I would send someone out to search for a ground based TCAS transponder !!!