Narrative:

Cga en route to cleveland, was proceeding direct chardon (cxr) at FL310 from over syracuse (syr). Traffic volume was light to moderate with approximately 10 aircraft on frequency. All equipment was normal. Frequency clarity was good. Cga checked on the frequency approximately 30 mi northeast of jamestown (jhw) level at FL310. An air carrier en route over jhw from the west was direct jhw-J82 alb to bos at FL290. Air carrier was switched to the next frequency approximately 20 mi west of jhw. Cga at the time just crossed my sector boundary 8 east of jhw. My clearance was 'cga descend now to FL280, correction maintain FL310, expect descent clearance in 10 miles maintain 310'. Descent clearance initially issued to FL280 wasn't for the cga but for an small transport eastbound at FL290 which would have delayed cga on the descent. I thought better of it because of the air carrier and amended cga's altitude to FL310 in the same transmission. Air carrier never got a readback from cga. Approximately 42 seconds after clearance to cga was issued, I observed his mode C at 30.7. I immediately instructed cga to maintain FL310, issued air carrier as traffic which was at his 1 O'clock and 8 mi, told him to expedite climb, report reaching, and issued a left turn. Cga said 'roger we're going back up to FL310'. Two radar hits were within 5 mi. The closest being 1200' and 3.5 mi. Make sure you get a readback from the pilot. Some pilots don't give readbacks. Force them to give you one. Pilots aren't required to give you a readback but ATC is required to get one. Cga might have read back a partial clearance while I was amending it but there isn't any way to tell on the tapes.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACFT DESCENDED BELOW ASSIGNED ALT RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative: CGA ENRTE TO CLEVELAND, WAS PROCEEDING DIRECT CHARDON (CXR) AT FL310 FROM OVER SYRACUSE (SYR). TFC VOLUME WAS LIGHT TO MODERATE WITH APPROX 10 ACFT ON FREQ. ALL EQUIPMENT WAS NORMAL. FREQ CLARITY WAS GOOD. CGA CHECKED ON THE FREQ APPROX 30 MI NE OF JAMESTOWN (JHW) LEVEL AT FL310. AN ACR ENRTE OVER JHW FROM THE W WAS DIRECT JHW-J82 ALB TO BOS AT FL290. ACR WAS SWITCHED TO THE NEXT FREQ APPROX 20 MI W OF JHW. CGA AT THE TIME JUST CROSSED MY SECTOR BOUNDARY 8 E OF JHW. MY CLRNC WAS 'CGA DSND NOW TO FL280, CORRECTION MAINTAIN FL310, EXPECT DSCNT CLRNC IN 10 MILES MAINTAIN 310'. DSCNT CLRNC INITIALLY ISSUED TO FL280 WASN'T FOR THE CGA BUT FOR AN SMT EBND AT FL290 WHICH WOULD HAVE DELAYED CGA ON THE DSCNT. I THOUGHT BETTER OF IT BECAUSE OF THE ACR AND AMENDED CGA'S ALT TO FL310 IN THE SAME XMISSION. ACR NEVER GOT A READBACK FROM CGA. APPROX 42 SECONDS AFTER CLRNC TO CGA WAS ISSUED, I OBSERVED HIS MODE C AT 30.7. I IMMEDIATELY INSTRUCTED CGA TO MAINTAIN FL310, ISSUED ACR AS TFC WHICH WAS AT HIS 1 O'CLOCK AND 8 MI, TOLD HIM TO EXPEDITE CLIMB, REPORT REACHING, AND ISSUED A LEFT TURN. CGA SAID 'ROGER WE'RE GOING BACK UP TO FL310'. TWO RADAR HITS WERE WITHIN 5 MI. THE CLOSEST BEING 1200' AND 3.5 MI. MAKE SURE YOU GET A READBACK FROM THE PLT. SOME PLTS DON'T GIVE READBACKS. FORCE THEM TO GIVE YOU ONE. PLTS AREN'T REQUIRED TO GIVE YOU A READBACK BUT ATC IS REQUIRED TO GET ONE. CGA MIGHT HAVE READ BACK A PARTIAL CLRNC WHILE I WAS AMENDING IT BUT THERE ISN'T ANY WAY TO TELL ON THE TAPES.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.