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"Fail" indication in Radar Altimeter...


gulti
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We are using Combined Altitude Radar Altimeter (CARA), model: AN/APN-232(V). After switching it on 'FAIL' indication shows on ground in both the indicators (Pilot side & Nav side) for about 30-45 sec. After that period it become normal.

In air, above 1000 ft both the indicators fluctuate with all indications (RT FAIL). But, at the same time TCAS system operation remain normal.

For this prob, we have done the fol troubleshooting:

a. CARA R/T, Height Indicators, Antennas are changed.

b. Continuity of wiring was measured.

But still we are facing the same problem.:confused:

It will be appreciated if anybody is having any other solution.

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The CARA should be good for 50,000 ft with a signal validity supplied to the TCAS. If the TCAS is not failing I would think the CARA T/R is working and sending valid signals but the indicators are playing up, possible signal loss! Where is the CARA transmitter/receiver located on your aircraft?

When you switch on does the needle drive to the mask area above the 5000' mark? It takes about 32 seconds for the needle to read zero but there should be no fails RT or LO in this time. the FMCW needs to time in!

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Check the coax connections for your Antennas. I would start at the RT.

If you have a TDR (Time Domain Reflectometer) that graphs "shoot" and graph the Coax cables to the antennas on a good aircraft and then shoot and compare your "bad" aircraft.

You will be able to see the length of the cable as well as any "changes" in the resistance in the cable (think signal loss) along the cable. The antennas may be "open" or "shorted" on the graph (can't remember). I have not "shot" a cable in over 15 years but it works!

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The ETCAS, GCAS/GPWS & Digital Autopilot are fed by the Analog altitude signal. The indicators get the Aux Analog and Digital Altitude signals. If there are no GCAS or ETCAS problems, I would check the Aux and Digital lines to ground. Maybe one of the lines is shorting to the shield. These wires are also spliced. Whether it will cause an R/T fail indication on the indicator, I have no idea.

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  • 4 weeks later...

We had very simular issues with the CARA fail indications above 1,000' and some times above 10,000', After much troubleshooting of the RT, indicators and cable (actually replaceed the cable) replacing the antenna cured the problem. I am not a comm/nav guy, but I was involve in all the troubleshooting and removing the floor panels. Just my 2 cents worth.

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