IN HOLDING RECEIVED TCASII RA TO DSND.

Narrative

WHILE HOLDING AT SMITY INTL; WE RECEIVED A TA 'TFC; TFC' FOLLOWED A FEW SECONDS LATER BY AN RA 'DSND; NOW; DSND.' WE WERE LEVEL AT FL200. WE WERE WATCHING THE TCASII SCREEN AS WE SAW A TARGET IMMEDIATELY ABOVE US DSNDING OUT OF FL215. OUR CHOICES WERE TO 1) IGNORE THE RA; OR 2) DSND AS COMMANDED FOLLOWING THE IVSI. THERE WAS ANOTHER (3RD) AIRPLANE AT FL190. BECAUSE I WAS FEARFUL THE AIRPLANE ABOVE WAS GOING TO DSND THROUGH FL210 AND DOWN ON TOP OF US AND BECAUSE I DETERMINED THE TFC AT FL190 WAS SPACED ADEQUATELY IN HORIZ DISTANCE; I TOLD THE FO TO BEGIN A DSCNT. WE IMMEDIATELY INFORMED ZDV. AT FL195 THE RA SILENCED AND ZDV RECLRED US TO FL200. BY WATCHING THE TCASII SCREEN WE DETERMINED THAT THE AIRPLANE ABOVE WAS LEVEL AT FL210 AND WE CLBED BACK TO FL200. THE SECOND AIRPLANE (ONE OF OUR MLG'S) SAID HIS TCASII WAS 'ACTING UP' ALSO. WE DID NOT WRITE UP THE TCASII SYS BECAUSE WE BELIEVED IT WORKED AS DESIGNED: EITHER THE RATE OF DSCNT OF THE AIRPLANE ABOVE CAUSED THE ALARM OR PERHAPS THE PROX OF OUR ACFT. THE TCASII SYS ALSO WORKED WELL BEFORE AND AFTER THIS INCIDENT.