Phantom aircraft triggering a TCAS RA

[Alphafloor75]

Good evening, a few days ago we were flying level in the hold, we saw an aircraft on TCAS that was +200 ft, following us for 1 complete lap, ATC was advised, they had nothing on their screen apart from us. VMC conditions, 8000 ft, we could not see anything. Then RA "Descend ! ", we comply, and then "Level Off !". There was no "clear of conflict" annunciation. We concluded it must have been a glitch from our TCAS system, did you ever experience this malfunction ? Both GPS were giving reliable information, both transponders as well, B 737 NG.

[GlobalNav]

There was a Eurocontrol advisory in 2014 regarding more than a dozen reported false RA's associated with hybrid TCAS installed on Airbus single aisle models. I believe the TCAS were all vers 7.1.

There may be more conclusive explanations of the events by now, but it was suspected by the TCAS manufacturer that when the hybrid unit, under certain relative position geometries, transitioned from ADS-B to TCAS ranging, the result created a range rate that tripped the RA.

This excerpt from the advisory may help:

"The common factor in these events is that the RA receiving aircraft is fitted with TCAS II version 7.1 with hybrid surveillance function. The RAs are triggered when two aircraft two aircraft are crossing at the same level or are in vertical convergence but where conditions for RA generation are not met and ATC standard horizontal separation is assured. Typically, the unexpected RA is generated on the ‘front’ aircraft only against an aircraft that is crossing between 5 to 7 NM behind it. Only A319 and A320 aircraft were receiving false RAs but the anomaly may affect other aircraft types as well.
In all reported cases, there has been no erosion of horizontal separation or possible risk of collision between the two aircraft at the time when the ‘front’ aircraft in the encounter received an RA; hence, from a pilot and controller perspective the RAs have been unexpected. Nevertheless, the pilots have, quite properly, followed the RA instructions. Whilst this is correct, from a traffic management viewpoint the manoeuvres disrupt the flow and increases cockpit and ATC workload; they may also precipitate follow-on conflicts."

[Alphafloor75]

Thank you, I had found this report but in my case it was different, there was no other aircraft around us, this was confirmed by ATC. It was as if it had been "created" by our own TCAS...

[4Screwaircrew]

Some years ago I had a series of these on a 737-300, I was advised by engineering that a cable was not fastened correctly and that the unit was generating RAs on itself.