Near midair collision between an medium large transport climbing out on a SID and a single engine cessna descending. ATC provided TA to alert flight crew who sighted the other aircraft. When reporter's aircraft took evasive action by descending, the other aircraft was also observed descending with them.

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Attributes
ACN	372268
Time
Date	199706
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sfo
State Reference	CA
Altitude	msl bound lower : 3500 msl bound upper : 4000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sfo
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	climbout : intermediate altitude cruise other
Route In Use	departure other departure sid : sid
Flight Plan	IFR
Aircraft 2
Make Model Name	Cessna Single Piston Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Flight Phase	descent other
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 180 flight time total : 8000 flight time type : 2000
ASRS Report	372268
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : nmac other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 500 vertical : 100
Supplementary
Primary Problem	Flight Crew Human Performance

Narrative:

Bay departure (135.1) leveled us off at 4000 ft for traffic. We were on a heading of 180 degrees and subsequently cleared porte. VFR traffic reported approximately 2 O'clock, 4500 ft. We had them in sight (southeast cessna) and I think they had called us in sight. We were converging and got a descend RA. Followed RA as per company procedure but cessna decided to descend also, creating a near miss situation. We still might have gotten a descend RA due to proximity of aircraft but it would not have been a near miss if the cessna hadn't descended because we were already lower than them.

Original NASA ASRS Text

Title: NMAC BTWN AN MLG CLBING OUT ON A SID AND A SINGLE ENG CESSNA DSNDING. ATC PROVIDED TA TO ALERT FLC WHO SIGHTED THE OTHER ACFT. WHEN RPTR'S ACFT TOOK EVASIVE ACTION BY DSNDING, THE OTHER ACFT WAS ALSO OBSERVED DSNDING WITH THEM.

Narrative: BAY DEP (135.1) LEVELED US OFF AT 4000 FT FOR TFC. WE WERE ON A HDG OF 180 DEGS AND SUBSEQUENTLY CLRED PORTE. VFR TFC RPTED APPROX 2 O'CLOCK, 4500 FT. WE HAD THEM IN SIGHT (SE CESSNA) AND I THINK THEY HAD CALLED US IN SIGHT. WE WERE CONVERGING AND GOT A DSND RA. FOLLOWED RA AS PER COMPANY PROC BUT CESSNA DECIDED TO DSND ALSO, CREATING A NEAR MISS SIT. WE STILL MIGHT HAVE GOTTEN A DSND RA DUE TO PROX OF ACFT BUT IT WOULD NOT HAVE BEEN A NEAR MISS IF THE CESSNA HADN'T DSNDED BECAUSE WE WERE ALREADY LOWER THAN THEM.

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.