ACR Y ACFT EQUIP PROB AUTOPLT DSCNT THROUGH ASSIGNED ALT HAD LTSS FROM ACR X TCASII RA DSND. EVASIVE ACTION TAKEN.

Narrative

ACR X; OVER 'MOL' VORTAC TRAVELING SW AT .76 MACH; FL280 IN DAYLIGHT IMC CONDITIONS. X RECEIVED A TCASII RA ALERT AT CRUISE IN RESPONSE TO AN ACR Y ACFT DSNDING RAPIDLY THROUGH FL290. NO PRIOR ATC ADVISORY WAS ISSUED. TCASII COMMANDS WERE MANUALLY FOLLOWED (GREEN ARC) AFTER OBSERVING RA TARGET AT 2 O'CLOCK AND 2.5 MI ON TA/VSI INST AND VERBAL 'DSND' COMMANDS. ZDC ADVISED US OF TRAFFIC BELOW AT 8 O'CLOCK FL270. DSCNT WAS HALTED AT FL277 WITH AT TA ALERT CONFIRMING OUR LOWER TFC (MONITOR VERT SPD; GREEN ARC AT ZERO VVI). COMMANDS WERE FOLLOWED AS DIRECTED BY OPERATING PROC AND ACFT WAS LEVELED OFF AT FL280 AFTER TCASII COMMANDS CEASED. ACR Y STATED HIS ACFT HAD INADVERTENTLY DSNDED THROUGH FL290 ISOLATING OUR IFR SEPARATION. OUR RESPONSE WAS TAKEN BECAUSE OF AN IMMEDIATE AND SIGNIFICANT HAZARD BROUGHT TO OUR ATTN BY THE TCASII SYS. OBSERVATIONS: WORKLOADS PERMITTING; ATC MUST ISSUE ADVISORIES WHEN AN ACFT DSNDS RAPIDLY THROUGH A CRUISE OR LEVEL OFF ALT ESPECIALLY WITH 1000 FT SEPARATION OVER A VORTAC OR INTXN OF MULTIPLE RTES. SUPPLEMENTAL INFO FROM ACN 267820: CRUISE AT FL290; AUTO PLT ON. ACFT SUDDENLY STARTED CLBING. DISENGAGED AND APPLIED FORWARD PRESSURE. BOTH PITCH TRIM SWITCHES HAD DISCONNECTED; RE-ENGAGED TRIM SWITCHES AND ACFT REVERSED DIRECTION TO A DSCNT. TOP OF CLB WAS 29250 FT; STOPPED DSCNT AT FL288. DURING DSCNT; RECEIVED RA TO 'MONITOR VERT SPD.' PROBABLY CAUSED ACR X DEVS IN OUR VICINITY.