Air carrier took evasive action to miss another aircraft.

Attributes
ACN	121940
Time
Date	198909
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : gdm
State Reference	MA
Altitude	msl bound lower : 9800 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Large Transport
Flight Phase	cruise other
Route In Use	arrival other enroute airway : bos
Flight Plan	IFR
Aircraft 2
Operator	other
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Flight Plan	VFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 10350 flight time type : 5000
ASRS Report	212940
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	conflict : nmac other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 0 vertical : 200
Supplementary
Primary Problem	Flight Crew Human Performance

Narrative:

We were flying from den to bos and were descending on the gdm 2 arrival. The center had cleared us to cross bronc at 11000' at 250 KTS; however, the first officer thought he said 10000'. I leveled at 11000' and we tried to get clarification from ATC,but we couldn't because of all the talking on the frequency. We were nearing bronc, so I eased down about 250' while waiting for confirmation. The controller said that we had been cleared to 11000', but we could descend to 10000' and he would coordinate it with approach control--so we descended on down to 10000'. We were then handed over to approach control. At this pint we were flying level at 10000' when I saw a plane at our 2 O'clock position. I said, 'there's a plane; watch it.', then 'what's he doing?' I then noticed that he was getting larger, but not moving in the window. I suddenly had the distinct impressions that we were on a collision course, so I pushed forward on the control column and we descended about 250'. The first officer saw the plane very clearly, and he saw an FAA logo on its tail. He estimated that it was 50-100' above us and passed immediately overhead. The second officer also saw it very clearly, and estimated that it was about 50' or so above us. I estimated that he had passed about 100-200' above us.

Original NASA ASRS Text

Title: ACR TOOK EVASIVE ACTION TO MISS ANOTHER ACFT.

Narrative: WE WERE FLYING FROM DEN TO BOS AND WERE DSNDING ON THE GDM 2 ARR. THE CENTER HAD CLRED US TO CROSS BRONC AT 11000' AT 250 KTS; HOWEVER, THE F/O THOUGHT HE SAID 10000'. I LEVELED AT 11000' AND WE TRIED TO GET CLARIFICATION FROM ATC,BUT WE COULDN'T BECAUSE OF ALL THE TALKING ON THE FREQ. WE WERE NEARING BRONC, SO I EASED DOWN ABOUT 250' WHILE WAITING FOR CONFIRMATION. THE CTLR SAID THAT WE HAD BEEN CLRED TO 11000', BUT WE COULD DSND TO 10000' AND HE WOULD COORDINATE IT WITH APCH CTL--SO WE DSNDED ON DOWN TO 10000'. WE WERE THEN HANDED OVER TO APCH CTL. AT THIS PINT WE WERE FLYING LEVEL AT 10000' WHEN I SAW A PLANE AT OUR 2 O'CLOCK POS. I SAID, 'THERE'S A PLANE; WATCH IT.', THEN 'WHAT'S HE DOING?' I THEN NOTICED THAT HE WAS GETTING LARGER, BUT NOT MOVING IN THE WINDOW. I SUDDENLY HAD THE DISTINCT IMPRESSIONS THAT WE WERE ON A COLLISION COURSE, SO I PUSHED FORWARD ON THE CONTROL COLUMN AND WE DSNDED ABOUT 250'. THE F/O SAW THE PLANE VERY CLEARLY, AND HE SAW AN FAA LOGO ON ITS TAIL. HE ESTIMATED THAT IT WAS 50-100' ABOVE US AND PASSED IMMEDIATELY OVERHEAD. THE S/O ALSO SAW IT VERY CLEARLY, AND ESTIMATED THAT IT WAS ABOUT 50' OR SO ABOVE US. I ESTIMATED THAT HE HAD PASSED ABOUT 100-200' ABOVE US.

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.