A B777 WAS DISPATCHED IN NON COMPLIANCE WITH THE L AIR CONDITIONING PACK DOOR UNLATCHED WHICH DEPARTED THE ACFT REQUIRING A RETURN TO THE FIELD.

Narrative

I; ABC ZXZX ON MAR/WED/00; DISPATCHED ACFT XYZ. ON PUSHBACK TO TXWY THE CREW AND PAX RPTED A GRINDING NOISE IN MID-CABIN AREA; ROWS 12-18. THE PUSHBACK AMT (ACFT MAINT TECHNICIAN) WXVV TOLD THE CREW TO RETURN TO THE GATE SO WE COULD INVESTIGATE THE NOISE. AT THE GATE WE SPOKE TO THE CREW. AMT'S WXVV; XWXW AND MYSELF. I WENT TO THE COCKPIT AND TALKED WITH THE CREW AND TOLD THEM I SUSPECTED A LOOSE CARGO CONTAINER TO BE THE SOURCE OF THE NOISE. I THEN CALLED THE SMC (STATION MAINT CTLR) TO HAVE RAMP OPEN THE FORWARD PIT; AND LEFT THE COCKPIT AND ALL 3 OF US MET IN THE FORWARD PIT; WE FOUND THE CONTAINERS SECURED AND THEN REMOVED THE AFT AIR MIX MANIFOLD COVER TO GAIN ACCESS TO THE RECIRCULATION FANS. AT THIS TIME I WENT OUTSIDE AND THOUGHT WE MIGHT HAVE A PACK PROB; OR SOMETHING LOOSE IN THE L AIR CONDITIONING BAY. AS I STARTED TO UNLATCH THE L AIR CONDITIONING DOOR FORWARD AND AFT LATCHES I GOT CALLED TO GO AND SHUT THE APU BLEED OFF; BECAUSE AMT'S WXVV AND XWXW HEARD A NOISE COMING FROM THE AIR MIX MANIFOLD. AS I LEFT TO GO TO THE COCKPIT WITH THE AIR CONDITIONING BAY DOOR HALFWAY UNLATCHED TO SHUT THE BLEEDS OFF; I RETURNED TO THE FORWARD PIT TO SEE WHAT THEY FOUND. THEY FOUND A 2 INCH BY 2 FT LONG BRACKET THAT WAS IN THE MIX MANIFOLD. AT THIS TIME WE DIDN'T KNOW WHAT WE HAD; SO I WENT TO THE IPC (ILLUSTRATED PARTS CATALOG) TO FIND OUT AND WE CALLED THE ENGINEERS FOR ASSISTANCE. THEY CALLED THE LINE MAINT CTLR AND HAD US DEFER THE BRACKET WHICH IS A SCREEN STIFFENER; TO BE REPLACED AT DENVER. SO WE CLOSED UP AND DISPATCHED THE ACFT. I DIDN'T REALIZE THAT I DID NOT RELATCH THE AIR CONDITIONING DOOR PANEL UNTIL THE CREW CALLED AND SAID THEY HEARD AN EXPLOSION AND RETURNED TO ZZZ.