Air carrier turned base leg inside traffic he was supposed to follow and required resequencing.

Attributes
ACN	124996
Time
Date	198910
Day	Wed
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 : oak
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport
Navigation In Use	Other
Flight Phase	descent : approach descent other
Route In Use	approach : visual
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 4 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent : approach descent other
Route In Use	approach : visual
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : cfi pilot : atp pilot : commercial
Experience	flight time last 90 days : 150 flight time total : 6000 flight time type : 800
ASRS Report	124996
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	non adherence : clearance other anomaly other other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 10000 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

As we were passing abeam sfo the controller asked us to report the airport in sight. We reported it in sight at this time. He cleared us for the visibility, to cross the 'bridge' (OM) at or above 4000'. I informed him that we were starting our turn at this time. We were just inside bridge. We lowered the gear and began a quick descent. As we were on base the controller advised us to use caution for wake following the widebody transport. I immediately questioned him on this, since we had only a widebody transport in sight to our left, behind us for 28R. He then said he didn't remember clearing us for the visibility and instructed us to turn left and maintain 4000'. This turn took us first directly toward the widebody transport, then diverging. I was positive that we were cleared for the visibility and informed him of this, then asked for a phone #. The only thing I can think of that led to this was the fact that the controller had told us few mins before clearing us for the visibility to expect to follow a widebody transport for 28R. When he cleared us for the visibility, we had the widebody transport in sight about 3-5 mi behind us and expedite our turn and descent in order to allow proper spacing in front of him. Looking back, I believe he wanted us to cross bridge outbnd at 4000' and then follow the widebody transport.

Original NASA ASRS Text

Title: ACR TURNED BASE LEG INSIDE TRAFFIC HE WAS SUPPOSED TO FOLLOW AND REQUIRED RESEQUENCING.

Narrative: AS WE WERE PASSING ABEAM SFO THE CTLR ASKED US TO RPT THE ARPT IN SIGHT. WE RPTED IT IN SIGHT AT THIS TIME. HE CLRED US FOR THE VIS, TO CROSS THE 'BRIDGE' (OM) AT OR ABOVE 4000'. I INFORMED HIM THAT WE WERE STARTING OUR TURN AT THIS TIME. WE WERE JUST INSIDE BRIDGE. WE LOWERED THE GEAR AND BEGAN A QUICK DSCNT. AS WE WERE ON BASE THE CTLR ADVISED US TO USE CAUTION FOR WAKE FOLLOWING THE WDB. I IMMEDIATELY QUESTIONED HIM ON THIS, SINCE WE HAD ONLY A WDB IN SIGHT TO OUR LEFT, BEHIND US FOR 28R. HE THEN SAID HE DIDN'T REMEMBER CLRING US FOR THE VIS AND INSTRUCTED US TO TURN LEFT AND MAINTAIN 4000'. THIS TURN TOOK US FIRST DIRECTLY TOWARD THE WDB, THEN DIVERGING. I WAS POSITIVE THAT WE WERE CLRED FOR THE VIS AND INFORMED HIM OF THIS, THEN ASKED FOR A PHONE #. THE ONLY THING I CAN THINK OF THAT LED TO THIS WAS THE FACT THAT THE CTLR HAD TOLD US FEW MINS BEFORE CLRING US FOR THE VIS TO EXPECT TO FOLLOW A WDB FOR 28R. WHEN HE CLRED US FOR THE VIS, WE HAD THE WDB IN SIGHT ABOUT 3-5 MI BEHIND US AND EXPEDITE OUR TURN AND DSCNT IN ORDER TO ALLOW PROPER SPACING IN FRONT OF HIM. LOOKING BACK, I BELIEVE HE WANTED US TO CROSS BRIDGE OUTBND AT 4000' AND THEN FOLLOW THE WDB.

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.