ACR X DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACR Y. SYS ERROR.

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ACR X CHK ON OUR FREQ AND WE REDUCED HIS AIRSPD TO 250 KTS FOR SPACING TO DCA. I OBSERVED ACR X LEVEL FL270. I IMMEDIATELY QUIZZED HIM OF HIS ASSIGNED ALT. HE SAID FL240. I ISSUED IMMEDIATE CLRNC TO FL270 FOR ACR Y AT FL260. HE THEN RPTED LEVEL AT FL270. MUST LISTEN MUCH CLOSER TO PLTS ALT WHEN CHKING ONTO FREQ AND REPEAT ALL ALT IN CLRNCS. SUPPLEMENTAL INFO FROM ACN 279387: ACR X CHKED ON DSNDING TO FL240. I THOUGHT HE SAID FL270. X LEFT FL270 (26800 FT) AND WE ASKED WHAT HIS ALT WAS. HE SAID DSNDING TO FL240. WE STOPPED HIM AND TOLD HIM TO CLB BACK TO FL270 IMMEDIATELY. HE HAD ACR Y AT FL260. HE SAW TFC AND RPTED LEVEL AT FL270. NEED TO LISTEN MORE ATTENTIVELY. SUPPLEMENTAL INFO FROM ACN 280549: ACR X WAS CLRED TO CROSS 85 MI S OF RIC AT FL270. HE READ BACK FL240 AND I DIDN'T CATCH THE WRONG ALT. ACR Y WAS WBOUND UNDER X AT FL260. ACR X DSNDED INTO ACR Y PROTECTED AIRSPACE. AFTER CONFERRING WITH THE NEXT CTLR THAT WAS NOW TALKING TO X; I ISSUED AN IMMEDIATE DSCNT TO FL250 FOR Y WHEN IT WAS CLR THE OTHER CTLR TOLD X TO CLB UP TO FL270. I GAVE A GOOD CLRNC. X READ IT BACK WRONG; AND I MISSED IT; SO DID THE NEXT CTLR WHEN X CHKED ON HIS FREQ. SUPPLEMENTAL INFO FROM ACN 279679: OVER FAYETTEVILLE; NC; ZDC ASKED IF WE HAD READ BACK A CLRNC TO CROSS 85 S OF RICHMOND AT FL240. WE HAD NOT RECEIVED THAT CLRNC; SO I QUESTIONED IF IT WAS FOR US AND THAT WE WOULD COMPLY. CTR ACKNOWLEDGED THAT I ACCEPTED THAT CLRNC. THE DSCNT FROM FL330 WAS STARTED SO WE WOULD REACH FL240 85 NM S OF RIC. CTR HAD US SLOW TO 250 KTS DURING THE DSCNT. WE CHANGED FREQS TO 122.75 AND CHKED IN THAT WE WERE DSNDING TO FL240. CTR THEN CALLED TFC TO US AT 2 O'CLOCK AND 20 MI AT FL260. THE TFC SHOWED UP ON THE TCASII AND I MADE A COMMENT THAT IT DIDN'T LOOK RIGHT. THE CAPT SLOWED THE DSCNT AND CTR THEN CALLED AND ASKED IF WE WERE LEVEL AT FL270. (WE WERE DSNDING THROUGH ABOUT FL265). I TOLD HIM WE WERE DSNDING TO FL240. CTR TOLD US TO CLB IMMEDIATELY TO FL270; AND TURN 40 DEGS L. TCASII COMMANDED A DSCNT OF APPROX 2000 FPM. THE CAPT STARTED A L TURN; BUT THEN WE GOT A VISUAL ON THE OTHER ACFT AND TURNED BACK TO THE R TO AVOID THEM. SUPPLEMENTAL INFO FROM ACN 279975: ORF- ATL. IN CRUISE FLT FL260 ATC ADVISED WE HAD TFC AT 11:00 O'CLOCK 25 MI LEVELING AT FL270. WHILE SHOWING TCASII TO JUMPSEAT PAX; WE OBSERVED TFC CONTINUE THROUGH FL270 ON DOWN TOWARDS 260; TCASII GAVE AN ADVISORY TO CLB. (THE CAPT TOOK THE ACFT AND STARTED A CLB. DURING THIS I OBSERVED THE TCASII ALT TO SHOW FL262 FOR THE TFC. SOON AFTER THE CLB WAS INITIATED CTR TELLS US TO DSND IMMEDIATELY TO FL250. THE CAPT USES IDLE THRUST; FULL SPD BRAKES AND R BANK TO GET THE NOSE DOWN AND ACFT TO FL250. DURING THE DSCNT I OBSERVED THE TFC'S ALT ON TCASII TO RETURN FROM FL262 BACK TO FL270.