ZBW controller misses incorrect readback from DC9 flight crew which conflicts with carj.

Attributes
ACN	451988
Time
Date	199910
Day	Sat
Local Time Of Day	0601 To 1200
Place
State Reference	NY
Altitude	msl bound lower : 24600 msl bound upper : 25000
Environment
Flight Conditions	IMC
Weather Elements	Rain Turbulence
Light	Dawn
Aircraft 1
Controlling Facilities	artcc : zbw.artcc
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : enroute altitude change
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zbw.artcc
Operator	common carrier : air carrier
Make Model Name	Challenger CL604
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	pilot : student
Experience	controller radar : 10 flight time total : 8
ASRS Report	451988
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe inflight encounter : turbulence non adherence : clearance non adherence : published procedure non adherence : required legal separation
Independent Detector	atc equipment : conflict alert other controllera
Resolutory Action	controller : issued new clearance controller : issued advisory flight crew : returned to assigned altitude
Consequence	faa : investigated
Miss Distance	horizontal : 15000 vertical : 600
Supplementary
Problem Areas	Weather Flight Crew Human Performance ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Pilot Deviation Operational Error

Narrative:

The operational error was the result of a pilot reading back a wrong altitude assignment and my failure to catch the readback error. The DC9 (aircraft X) was at FL280 complaining of light to moderate chop. I told him that I had received complaints at FL260 and FL280 and that FL240 was smooth. He requested FL240. I told him it was unavailable and to descend to FL260. He went to FL260 and was still getting chop so I told him to descend to FL250 and he read back FL240, which I missed. I called out traffic to him at 2 O'clock level at FL240 and he said 'roger,' and continued to descend to FL240. I noticed his mode C reading FL247 as he passed the carj (aircraft Y) and asked him his altitude. He said FL246 descending and I assigned him FL250 again. The WX was a factor in the error because of the chop. I had 4 aircraft level at FL240 because it was smooth. The pilot of the DC9 expected to get FL240 because of the smooth air and I'm sure that was what was on his mind. I still should have caught the readback error, but what I don't understand is why didn't the pilot question me when I told him about crossing traffic at FL240?

Original NASA ASRS Text

Title: ZBW CTLR MISSES INCORRECT READBACK FROM DC9 FLC WHICH CONFLICTS WITH CARJ.

Narrative: THE OPERROR WAS THE RESULT OF A PLT READING BACK A WRONG ALT ASSIGNMENT AND MY FAILURE TO CATCH THE READBACK ERROR. THE DC9 (ACFT X) WAS AT FL280 COMPLAINING OF LIGHT TO MODERATE CHOP. I TOLD HIM THAT I HAD RECEIVED COMPLAINTS AT FL260 AND FL280 AND THAT FL240 WAS SMOOTH. HE REQUESTED FL240. I TOLD HIM IT WAS UNAVAILABLE AND TO DSND TO FL260. HE WENT TO FL260 AND WAS STILL GETTING CHOP SO I TOLD HIM TO DSND TO FL250 AND HE READ BACK FL240, WHICH I MISSED. I CALLED OUT TFC TO HIM AT 2 O'CLOCK LEVEL AT FL240 AND HE SAID 'ROGER,' AND CONTINUED TO DSND TO FL240. I NOTICED HIS MODE C READING FL247 AS HE PASSED THE CARJ (ACFT Y) AND ASKED HIM HIS ALT. HE SAID FL246 DSNDING AND I ASSIGNED HIM FL250 AGAIN. THE WX WAS A FACTOR IN THE ERROR BECAUSE OF THE CHOP. I HAD 4 ACFT LEVEL AT FL240 BECAUSE IT WAS SMOOTH. THE PLT OF THE DC9 EXPECTED TO GET FL240 BECAUSE OF THE SMOOTH AIR AND I'M SURE THAT WAS WHAT WAS ON HIS MIND. I STILL SHOULD HAVE CAUGHT THE READBACK ERROR, BUT WHAT I DON'T UNDERSTAND IS WHY DIDN'T THE PLT QUESTION ME WHEN I TOLD HIM ABOUT XING TFC AT FL240?

Data retrieved from NASA's ASRS site as of July 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.