Narrative:

Copilot flying, IFR lax to ont. While on V21 (sli-pdz) approaching pdz at 9000', controller asked if we wanted a visual approach. I replied that we did. He said (to the best of my recollection), 'left heading 040 for visual approach, report airport in sight.' as we were nearly abeam the airport at this time, the first officer asked me to alert the cabin team of landing (this is a new procedure on short legs to compensate for their no longer having the no smoke sign as an alert). I also tried to call our company to get a gate assignment (as required by company policy). As I switched back to approach, copilot called for flaps 1. I noticed we were descending, looked at the altitude select window and saw that the first officer had set 4500'. I thought all was well until controller said to climb immediately to 8000'. I saw traffic at 12 O'clock about 2 mi and said we could maintain visual sep. Controller said to maintain visual sep and 7000'. He said we should still be at 9000' assigned. The copilot had thought we were cleared for a visual approach and I had accepted his selection of 4500' in the altitude window as a clearance west/O confirming it with him. I should have confirmed reason for descent with first officer. Being so close to the airport it seemed to be a logical thing to be doing, so I accepted it. There should be no procedure which takes 1 pilot on a 2-M crew off of ATC frequency after leaving high level cruise altitude, as it leaves no back up. While controller acted properly and deviation was crew error, a confirmation to 'maintain 9000'' after 'heading 040 degrees for visual approach, report airport in sight,' would have cleared the confusion that the first officer had.

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Original NASA ASRS Text

Title: MLG ON VECTOR FOR VISUAL APCH MADE UNAUTH DESCENT AND CREATED A LESS THAN STANDARD SEPARATION OCCURRENCE WITH ANOTHER ACFT. APCH CTLR SAW POTENTIAL CONFLICT AND APPLIED VISUAL SEPARATION AFTER MLG REQUESTED IT.

Narrative: COPLT FLYING, IFR LAX TO ONT. WHILE ON V21 (SLI-PDZ) APCHING PDZ AT 9000', CTLR ASKED IF WE WANTED A VISUAL APCH. I REPLIED THAT WE DID. HE SAID (TO THE BEST OF MY RECOLLECTION), 'LEFT HDG 040 FOR VISUAL APCH, RPT ARPT IN SIGHT.' AS WE WERE NEARLY ABEAM THE ARPT AT THIS TIME, THE F/O ASKED ME TO ALERT THE CABIN TEAM OF LNDG (THIS IS A NEW PROC ON SHORT LEGS TO COMPENSATE FOR THEIR NO LONGER HAVING THE NO SMOKE SIGN AS AN ALERT). I ALSO TRIED TO CALL OUR COMPANY TO GET A GATE ASSIGNMENT (AS REQUIRED BY COMPANY POLICY). AS I SWITCHED BACK TO APCH, COPLT CALLED FOR FLAPS 1. I NOTICED WE WERE DSNDING, LOOKED AT THE ALT SELECT WINDOW AND SAW THAT THE F/O HAD SET 4500'. I THOUGHT ALL WAS WELL UNTIL CTLR SAID TO CLB IMMEDIATELY TO 8000'. I SAW TFC AT 12 O'CLOCK ABOUT 2 MI AND SAID WE COULD MAINTAIN VISUAL SEP. CTLR SAID TO MAINTAIN VISUAL SEP AND 7000'. HE SAID WE SHOULD STILL BE AT 9000' ASSIGNED. THE COPLT HAD THOUGHT WE WERE CLRED FOR A VISUAL APCH AND I HAD ACCEPTED HIS SELECTION OF 4500' IN THE ALT WINDOW AS A CLRNC W/O CONFIRMING IT WITH HIM. I SHOULD HAVE CONFIRMED REASON FOR DSCNT WITH F/O. BEING SO CLOSE TO THE ARPT IT SEEMED TO BE A LOGICAL THING TO BE DOING, SO I ACCEPTED IT. THERE SHOULD BE NO PROC WHICH TAKES 1 PLT ON A 2-M CREW OFF OF ATC FREQ AFTER LEAVING HIGH LEVEL CRUISE ALT, AS IT LEAVES NO BACK UP. WHILE CTLR ACTED PROPERLY AND DEVIATION WAS CREW ERROR, A CONFIRMATION TO 'MAINTAIN 9000'' AFTER 'HDG 040 DEGS FOR VISUAL APCH, RPT ARPT IN SIGHT,' WOULD HAVE CLRED THE CONFUSION THAT THE F/O HAD.

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.