Narrative:

While giving OJT at the AR2 position at bay TRACON, a mooney and a cessna were handed off to our sector at the panoche arrival into oak airport's north field. Both aircraft were instructed to intercept the runway 27R localizer. The cessna was ahead of the mooney. The plan was to let the mooney out-run the cessna and use altitude separation until the mooney was far enough ahead. Traffic was issued to each aircraft by the developmental and the mooney stated he had the traffic in sight. Visual separation was not used and attention was directed elsewhere. The developmental then noticed the mooney left his assigned altitude and instructed the cessna to turn north to avoid a collision. I inquired as to why the mooney left his altitude and he stated...I didn't see the cessna and was on the localizer so I thought it was ok to descend. After pointing out his mistake, the mooney pilot realized his error and was very apologetic. Supplemental information from acn 521282: while in contact with oakland approach and level at 6000 ft MSL, we were given a vector to follow, until intercepting the localizer for runway 27R at oak. A few mins later upon intercepting the localizer and tracking inbound the GS for ILS runway 27R started to come down on us and I could not recall whether we were instructed to intercept the GS also. The radio frequency was very busy, we were in VMC conditions and I had the 2 VFR airplanes that were ahead and under me in sight. I started my descent, following the ILS GS while still trying to get on the radio. We were at 5200 ft MSL when the controller informed us that we should not have descended until cleared to do so. In retrospect, I should have been more alert when copying down his instructions and maintained altitude until cleared to descend.

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Original NASA ASRS Text

Title: M20M DSNDS FROM ASSIGNED ALT WHILE ON APCH TO OAK ON 090 FREQ AND CONFLICTS WITH IFR C172.

Narrative: WHILE GIVING OJT AT THE AR2 POS AT BAY TRACON, A MOONEY AND A CESSNA WERE HANDED OFF TO OUR SECTOR AT THE PANOCHE ARR INTO OAK ARPT'S N FIELD. BOTH ACFT WERE INSTRUCTED TO INTERCEPT THE RWY 27R LOC. THE CESSNA WAS AHEAD OF THE MOONEY. THE PLAN WAS TO LET THE MOONEY OUT-RUN THE CESSNA AND USE ALT SEPARATION UNTIL THE MOONEY WAS FAR ENOUGH AHEAD. TFC WAS ISSUED TO EACH ACFT BY THE DEVELOPMENTAL AND THE MOONEY STATED HE HAD THE TFC IN SIGHT. VISUAL SEPARATION WAS NOT USED AND ATTN WAS DIRECTED ELSEWHERE. THE DEVELOPMENTAL THEN NOTICED THE MOONEY LEFT HIS ASSIGNED ALT AND INSTRUCTED THE CESSNA TO TURN N TO AVOID A COLLISION. I INQUIRED AS TO WHY THE MOONEY LEFT HIS ALT AND HE STATED...I DIDN'T SEE THE CESSNA AND WAS ON THE LOC SO I THOUGHT IT WAS OK TO DSND. AFTER POINTING OUT HIS MISTAKE, THE MOONEY PLT REALIZED HIS ERROR AND WAS VERY APOLOGETIC. SUPPLEMENTAL INFO FROM ACN 521282: WHILE IN CONTACT WITH OAKLAND APCH AND LEVEL AT 6000 FT MSL, WE WERE GIVEN A VECTOR TO FOLLOW, UNTIL INTERCEPTING THE LOC FOR RWY 27R AT OAK. A FEW MINS LATER UPON INTERCEPTING THE LOC AND TRACKING INBOUND THE GS FOR ILS RWY 27R STARTED TO COME DOWN ON US AND I COULD NOT RECALL WHETHER WE WERE INSTRUCTED TO INTERCEPT THE GS ALSO. THE RADIO FREQ WAS VERY BUSY, WE WERE IN VMC CONDITIONS AND I HAD THE 2 VFR AIRPLANES THAT WERE AHEAD AND UNDER ME IN SIGHT. I STARTED MY DSCNT, FOLLOWING THE ILS GS WHILE STILL TRYING TO GET ON THE RADIO. WE WERE AT 5200 FT MSL WHEN THE CTLR INFORMED US THAT WE SHOULD NOT HAVE DSNDED UNTIL CLRED TO DO SO. IN RETROSPECT, I SHOULD HAVE BEEN MORE ALERT WHEN COPYING DOWN HIS INSTRUCTIONS AND MAINTAINED ALT UNTIL CLRED TO DSND.

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.