TCASII CAUSES LGT TO DSND 1000 FT WITHOUT CLRNC.

Narrative

CTR CLRED AN ACFT AT FL310 TO FL290. THIS ACFT WAS DIRECTLY ABOVE OURS. WE WERE AT FL280. THE DSCNT OF THE OTHER ACFT CAUSED A TCASII RA TO COMMAND DSCNT OF 2000 FPM. SINCE WE WERE IMC AT THE TIME; WE STARTED AN UNCOMMANDED DSCNT TO FL270 AND ADVISED ZDC OF OUR ACTION. ONCE WE WERE AT FL270 ZDC WANTED TO CLB US BACK UP TO FL280 BUT THE TCASII RA HADN'T RESET WHICH PROHIBITED US TO CLB BACK UP. CTR NEEDS TO KNOW THAT IF THERE ARE 2 ACFT WITH LIMITED HORIZ SEPARATION AND ONE IS ASKED TO CLB OR DSND; DEPEND ON THE RATE USED MAY CAUSE A TCASII TA/RA. ALL THE TCASII KNOWS IS THAT IF AN ACFT IS DSNDING TO WITHIN 1000 FT OF ANOTHER PLANE AT A RATE OF 1500-2000 FPM; THERE IS GOING TO BE A CONFLICT. IF THIS WOULD HAVE HAPPENED IN VMC CONDITIONS WE WOULDN'T HAVE HAD TO DSND. CTR SHOULD ALSO ADVISE US IF THEY ARE DSNDING AN ACFT TO WITHIN 1000 FT THAT IS DIRECTLY ABOVE US. OVERALL I LIKE THE TCASII SYS BUT IN HIGH FLOW/CONGESTED AREAS IT CAN CAUSE SOME PROBS FOR CTLRS ESPECIALLY IN CASES LIKE OURS. I'M GLAD THERE WASN'T ANY OPPOSITE TFC AT FL270 TODAY.