TCAS Antenna and Discrete Problems

Battling a problem with a TCAS system that was going inoperative almost every flight.

Two issues:

1. A intermittent upper antenna co-ax lead.


2. A discrete wiring failure.



For the antenna issue we got lucky and found the problem right at the rack connector. By wiggling the test lead inside the suspect socket, we could see the wire length drop from 30+ feet (distance to antenna) to about 10 feet (distance of test lead).





We jumped to the back of the rack and disconnected from there just to make sure our Site Hawk readings were correct.



On this TCAS antenna installation, the co-ax leads have three sections. A short one from the rack plug to a disconnect on the rear of the electronics shelf (about 1-1/2 foot). A long run up the sidewall. And near the antenna itself there is another short run.




All four co-ax's must be replaced as a set for each particular wire run. They are impedance matched with each other.


As far as the discrete issue. We verified that the three other discrete grounds were there for TA Display #2 and both RA #1 Display and RA #2 Display. I believe this was a missed wire during install. The fault most likely wasn't causing the system to fail. It still required a ground wire be installed on the pin (the RMP 7E represents Right Middle Plug, Pin E7).

On the discrete problem, it wasn’t quite known that this was a wiring problem. This aircraft didn’t have stand alone IVSI-TA/RA indicators. You would be led to think maybe a Symbol Generator or similar.

Since the discrete wiring was so easily read on the front of the unit, a quick glance revealed the problem without much digging into other user systems.

There was no pin in E7, so this was modification screw up.