A B737-700 captain reported an near midair collision while on approach to sfo.

Attributes
ACN	513160
Time
Date	200106
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : sfo.vor
State Reference	CA
Altitude	msl single value : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : a80.tracon
Operator	common carrier : air carrier
Make Model Name	B737-700
Flight Phase	descent : intermediate altitude
Route In Use	arrival star : golde gate 4
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : o90.tracon
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise : level
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 13000 flight time type : 1200
ASRS Report	513160
Events
Anomaly	conflict : nmac non adherence : published procedure
Resolutory Action	flight crew : took evasive action
Miss Distance	horizontal : 90 vertical : 400
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

On golden gate 4 arrival at sfo, level at 11000 ft, over north bay, heading 100 degrees magnetic. Bay approach issued TA of light aircraft at our 12 - 12:30 O'clock position, 5 mi, 10500 ft MSL. It popped up on TCASII at approximately 4 mi and 400 ft below our altitude. (He was supposed to be at 10500 ft according to bay approach.) TCASII alert almost immediately turned into a TA alert and just a few seconds later (2-4 seconds) became an RA alert for a climb. First officer (PF) took immediate action to comply with the RA. I notified approach control of climb from 11000 ft for the RA and informed him of our close proximity to this traffic. Controller apologized. First officer corrected back to 11000 ft when 'clear of conflict' was annunciated on TCASII.

Original NASA ASRS Text

Title: A B737-700 CAPT RPTED AN NMAC WHILE ON APCH TO SFO.

Narrative: ON GOLDEN GATE 4 ARR AT SFO, LEVEL AT 11000 FT, OVER NORTH BAY, HDG 100 DEGS MAGNETIC. BAY APCH ISSUED TA OF LIGHT ACFT AT OUR 12 - 12:30 O'CLOCK POS, 5 MI, 10500 FT MSL. IT POPPED UP ON TCASII AT APPROX 4 MI AND 400 FT BELOW OUR ALT. (HE WAS SUPPOSED TO BE AT 10500 FT ACCORDING TO BAY APCH.) TCASII ALERT ALMOST IMMEDIATELY TURNED INTO A TA ALERT AND JUST A FEW SECONDS LATER (2-4 SECONDS) BECAME AN RA ALERT FOR A CLB. FO (PF) TOOK IMMEDIATE ACTION TO COMPLY WITH THE RA. I NOTIFIED APCH CTL OF CLB FROM 11000 FT FOR THE RA AND INFORMED HIM OF OUR CLOSE PROX TO THIS TFC. CTLR APOLOGIZED. FO CORRECTED BACK TO 11000 FT WHEN 'CLR OF CONFLICT' WAS ANNUNCIATED ON TCASII.

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.