ATA 34-45, Directional TCAS Antenna

Inbound M.E.L. on the TCAS system. A system test from the front of the processor showed an upper antenna fault.



The Fault Isolation Manual 34-45 Fault Tree 803 asks to verify antenna integrity with a meter, but it does not give any resistance values.

Upper/Lower Antenna Connections on both Processor and Rack.

Plug "A" (rack upper right) is for the top antenna elements. "B" for lower antenna.

These numbers are for reference only:
A1 : 5.75K Ohms B1 : 5.68K Ohms
A2 : 15.05K Ohms B2 : 15.19K Ohms
A3 : 31.3K Ohms B3 : 33.1K Ohms
A4 : 85K Ohms B4 : 83.7K Ohms



On this particular aircraft, the upper antenna was recently replaced after a brief history of failures. Two processors had been installed, but the failure repeated after only a couple of days of the processor(s) being replaced.

I suspect (without seeing the removed antenna) that one of the four elements had shorted out in the antenna itself. This short was taking out (frying) the transceiver in each of the two previous processors. After a third processor was installed, the system has been operating normally.

A previous discussion of a directional antenna fault on a MD-11 can be found here .

767-300 SSM 34-45-01

I always see R&R of the easiest LRUs over and over. After two, that's it time to troubleshoot. See this a lot. Thanks Mark.