PAX NOTES A LOOSE COWLING AFTER DEPARTING LAX. FLT RETURN LAND.

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I AM EMPLOYED AS A FO WITH A FAR PART 135 CARRIER. OUR CREW ARRIVED LATE INTO LAX AND HAD 20 MINS TO SWAP INTO A SECOND ACFT IN ORDER TO MAKE OUR SCHEDULED DEP. THE PIC ARRIVED AT THE ACFT FIRST; WAS BRIEFED ON 3 OPEN MAINT ITEMS AND DEPARTED TO SPEAK WITH DISPATCH REGARDING A LATE DEP. I ARRIVED AT THE ACFT AND COMPLETED AN EXTERIOR SAFETY INSPECTION AT THE SAME TIME MAINT PERSONNEL BEGAN WORK ON THE OUTSTANDING ITEMS. I THEN COMPLETED A COCKPIT SAFETY INSPECTION. WHILE THERE; I ASSISTED MAINT PERSONNEL BY UNFEATHERING AND THEN REFEATHERING THE #1 PROP WHICH THEY WERE WORKING ON. AFTERWARDS; I ENSURED THAT THIS ITEM; AND THE SECOND ITEM (AN EXTERIOR LIGHT) WERE COMPLETED AND SECURED. AT THIS POINT; THE PIC RETURNED AND WAS BRIEFED; ALONG WITH MYSELF; THAT THE FIRST 2 ITEMS WERE COMPLETED AND SIGNED OFF; AND THE THIRD ITEM WAS TO BE DEFERRED AS PER THE MEL. AT NO TIME WAS THE CREW INFORMED THAT A COWLING HAD BEEN OPENED IN ORDER TO LOOK AT THE THIRD MAINT ITEM; AND THEN BEEN CLOSED. HOWEVER; ONLY 1 LATCH WAS SECURED IN THE EVENT THAT THE PIC CHOSE NOT TO DEFER THE ITEM FORCING THEM TO GO BACK INTO THE NACELLE. CONSEQUENTLY; THE CREW ACCEPTED THE ACFT AND MAINT DEPARTED. AT THIS POINT WE WERE ABOUT 20 MINS BEHIND SCHEDULE. TAXI AND TKOF WERE NORMAL. CLBING THROUGH 2000 FT MSL ON THE DEP; THE FLT ATTENDANT CALLED FORWARD TO INFORM THE PIC THAT A PAX HAD NOTICED THE #2 NACELLE OUTER COWLING WAS FULLY OPEN; FLYING PARALLEL TO THE WIND SO THAT IT WAS NOT MOVING OR FLAPPING ABOUT. THE PIC INFORMED ATC OF OUR SIT AND REQUESTED VECTORS BACK FOR A LNDG AT LAX. HE DETERMINED THAT IT WOULD NOT BE NECESSARY TO DECLARE AN EMER. WE RECEIVED VECTORS FOR A VISUAL APCH AND MADE AN UNEVENTFUL LNDG. PERHAPS THE GREATEST CONTRIBUTING FACTOR WAS A FAILURE IN COM BTWN MAINT AND THE CREW AS TO WHAT EXACTLY WAS DONE WITH THE ACFT. THIS WAS EXACERBATED BY THE SHORT TURN TIME; ACFT SWAP; AND THE PENDING LATE DEP. IN MY MIND; IT IS INCONCEIVABLE THAT THE COWLING COULD BE CLOSED BUT ONLY PARTIALLY LATCHED. FROM MY PREVIOUS EXPERIENCE; WHEN MAINT IS COMPLETED; A SECOND QUALITY ASSURANCE CHKER INSPECTS AND THEN SIGNS OFF THE WORK. THE CREW IS THEN COMPLETELY DEBRIEFED ON ALL ACTION ACCOMPLISHED. THIS DOES NOT APPEAR TO HAVE HAPPENED IN THIS INSTANCE. PERHAPS; TOO; THE CREWS EAGERNESS TO COMPLETE THEIR TRIP WAS A CONTRIBUTING FACTOR. INSTEAD OF DELVING INTO THE MATTER FURTHER; WE ACCEPTED AT THEIR WORD THAT THE THIRD ITEM WAS TO BE DEFERRED. ALTHOUGH WE WERE UNDER THE IMPRESSION THAT NO WORK HAD BEEN DONE ON THIS ITEM; SOME FURTHER DIGGING COULD HAVE UNCOVERED THAT THE COWLING HAD BEEN OPENED AT SOME POINT IN ORDER FOR MAINT TO DETERMINE THAT NOTHING COULD BE DONE ON THE THIRD ITEM AT THAT PARTICULAR TIME. PRESENTLY; COMPANY POLICY IS BEING REVIEWED TO PREVENT THIS FROM OCCURRING AGAIN. SUPPLEMENTAL INFO FROM ACN 284988: LATER WE DISCOVERED THAT THE MECH HAD OPENED THE #2 COWLING AFTER THE FO HAD DONE HIS WALK AROUND. HE HAD APPARENTLY SHUT THE COWLING BUT ONLY LATCHED 1 OUT OF 6 LATCHES. NEITHER THE FO NOR I HAD SEEN THE COWLINGS OPEN. NOR DID WE HAVE ANY REASON TO THINK THAT THEY HAD BEEN OPEN. THEY INFORMED US THAT EVERYTHING WAS DONE AND SIGNED OFF. IT WOULD BE NICE TO BE ABLE TO TRUST EVERYONE AT ALL TIMES. BUT I FORGET THAT YOU CAN'T.