Nuisance RA Flag during flight 73NG

[theflyingbus]

Returning home last night we had an intermittent "RA" Flag on the FO's EHSI, which is apparently known on this aircraft and I have seen on others. It would come on for anything between 2 and 30 seconds with no regular patterns. It then stayed on constantly for 27 minutes before TOD. Once descent had commenced it flickered on/off for a couple more times (34000ft+) then remained off.
What made it even more fun was that we would be doing an autoland at our destination due FG! We had planned for all outcomes that we could think of at all stages of the approach and suffice to say the old girl worked fine.
The tech guys at base have in the past done all sorts of BITE tests etc and cant find anything. I was just curious to see if anyone knew any more...?

Safe flying

(At one point I was looking down at AMS, with a training Capt next to me, in a 738, with RA issues!! Has a final report been published on the Turkish accident yet?)

[Graybeard]

Typically that's caused by corrosion between an antenna and the structure. It needs a really good radio frequency bond there. The fix is to drop the antennas, inspect and replace any corrosion damaged antennas.

Less common is a leaky connection between the unit and an antenna cable connector in the mating connector.

RF leakage is noticed first at altitude, because the transceiver is operating at max gain, and any leakage will show up there first. Once there is ground return greater than the leakage signal, the gain is reduced and the altitude is accurate. Hence, it's not by itself a safety issue.

Depending on the leakage, you may see a flag, or as in the case of AMS, a negative altitude with no flag in view.
Aircraft systems are built to accommodate failures such as yours, but less likely to accommodate erroneous inputs with no warning flag.

In both your case and THY, additional line maintenance technician training and Maintenance Control training is sorely needed. Heavy check procedures may have been skipped, too.

GB

[BOAC]

I would have thought that if you keep on snagging it the CatIII status will have to be downgraded and then you should find the comany will 'magic' the downtime to fix it.:)

[theflyingbus]

Thanks

Back in Wednesday. Will chat with them again

[matkat]

Sounds like an intermittant failure of the compass coupler.