Air carrier saw traffic, thought at same altitude, descended without clearance and then realized traffic was below him. Air carrier climbed back to altitude, but had caused less than standard separation.

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Attributes
ACN	175238
Time
Date	199104
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : amg
State Reference	GA
Altitude	msl bound lower : 27800 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zjx
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	climbout : intermediate altitude cruise other descent other
Route In Use	enroute airway : zjx
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 8000 flight time type : 500
ASRS Report	175238
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance non adherence : required legal separation
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Miss Distance	horizontal : 12000 vertical : 800
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Was working way around trw's. In level flight at FL280 with 20 mi visibility, picked up visibility on air carrier Y at about 5 mi. He looked to be close to my altitude. Started a descent and right turn. At 27800' I realized he was below me, climbed back to 28000'. It took place so fast did not have time to call ATC. Air carrier Y passed 2-3 mi behind. ATC was working with a lot of aircraft deviating around WX. If ATC would have called the traffic, no action would have been necessary.

Original NASA ASRS Text

Title: ACR SAW TRAFFIC, THOUGHT AT SAME ALT, DESCENDED WITHOUT CLRNC AND THEN REALIZED TRAFFIC WAS BELOW HIM. ACR CLIMBED BACK TO ALT, BUT HAD CAUSED LESS THAN STANDARD SEPARATION.

Narrative: WAS WORKING WAY AROUND TRW'S. IN LEVEL FLT AT FL280 WITH 20 MI VISIBILITY, PICKED UP VIS ON ACR Y AT ABOUT 5 MI. HE LOOKED TO BE CLOSE TO MY ALT. STARTED A DSNT AND RIGHT TURN. AT 27800' I REALIZED HE WAS BELOW ME, CLBED BACK TO 28000'. IT TOOK PLACE SO FAST DID NOT HAVE TIME TO CALL ATC. ACR Y PASSED 2-3 MI BEHIND. ATC WAS WORKING WITH A LOT OF ACFT DEVIATING AROUND WX. IF ATC WOULD HAVE CALLED THE TFC, NO ACTION WOULD HAVE BEEN NECESSARY.

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.