A B777-200 WAS DISPATCHED IN NON COMPLIANCE WITH THE L REVERSER LOCK PIN INSTALLED FOR MAINT AND NOT REMOVED.

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ON THE EVENING OF NOV/XA/01; ACR X ACFT XXXX FLT XX ARRIVED AT ZZZ WITH A STATUS MESSAGE ON EICAS INDICATING A L ENG BLEED AIR PROB WHICH NEEDED TO BE CORRECTED PRIOR TO DEP THAT NIGHT I'M NOT QUITE SURE ALL THAT TRANSPIRED DURING THIS EVENT; BUT A DECISION WAS MADE FOR REMOVAL AND REPLACEMENT OF THE FAN AIR MODULATING VALVE (FAMV) ON THE LEFT ENG. DURING THE NIGHT OF NOV/XA/01 AT XA45 AT THE START OF MY SHIFT; I WAS ASSIGNED TO RELIEVE THE PRIOR SHIFT WHOSE ENDING TIME WAS XB00; AND WHOM WAS IN COURSE OF RECTIFYING THE PROB MENTIONED ABOVE. UNFORTUNATELY; PUTTING ME IN A SIT AFTER GATHERING MY TOOLS AND REACHING PLANE SIDE LEFT LITTLE OR NO TIME FOR AN ADEQUATE TURNOVER OF THE WORK IN PROGRESS BTWN THE ONGOING AND OFF GOING SHIFTS. AFTER THE REMOVAL AND REPLACEMENT VALVE WAS ACCOMPLISHED; THE MAINT MANUAL REQUIRED FOR AN ENG AIR OPS TEST. WHEREBY DUE TO THE TIME CONSTRAINTS NEEDED TO PERFORM AN OPS TEST OF THE BLEED SYS AND TO REDUCE ANY FURTHER DELAY WHICH WAS OCCURRING A DECISION WAS MADE TO RENDER THE NEW VALVE WHICH HAD BEEN INSTALLED INOP IN OPEN POS FOR FLT PER THE MEL FOR DISPATCH. I AND MY PARTNER THEN PROCEEDED TO PERFORM THE STEPS NEEDED PER THE MEL TO RENDER THE VALVE INOP FLT. ONE OF THE REQUIREMENTS WHICH NEEDED TO BE DONE PRIOR OPENING THE THRUST REVERSER TO GAIN ACCESS TO THE VALVE LOCATED ON THE INSIDE R THRUST REVERSER HALF; WAS TO DEACTIVATE THE THRUST REVERSER AT THE ISOLATION SHUTOFF VALVE LOCATED THROUGH AN ACCESS PANEL ON THE OUTBOARD SIDE OF THE L ENG PYLON 15 FT AGL. WE INSTALLED THE BYPASS PIN WHICH HAD A RED STREAMER ATTACHED TO IT THROUGH THE VALVE AND HUNG THE STREAMER OUT THE ACCESS PANEL. ONCE WE HAD OPEN COWLS AND GAINED ACCESS TO FAN AIR MODULATING VALVE; OUR FOCUS AND ATTN WAS DIRECTED TO PROPERLY SECURING THE VALVE IN OPEN POS FOR FLT PER THE MEL. ONCE I FELT SECURE THAT I AND MY PARTNER HAD PROPERLY SECURED THE VALVE IN THE OPEN POS; WE THEN SHIFTED OUR FOCUS AND ATTN ON PROPERLY LATCHING AND SECURING THE COWLS WHICH WE HAD OPEN TO GAIN ACCESS TO THE FAN AIR MODULATING VALVE. THE ACFT WAS RELEASED AND DEPARTED ZZZ TO ZZZ1 BTWN XC30 AND XD00. AT THE ENDING OF MY SHIFT; ABOUT XI55; MY PARTNER WAS FORTUNATE ENOUGH TO RECALL TO ME WE MAY HAVE INADVERTENTLY; AND BY NO DELIBERATE ACTION BY ME OR HIM; MISTAKENLY FORGOT TO REMOVE THE BYPASS PIN IN THE ISOLATION SHUTOFF VALVE FROM THE L ENG REVERSER SYS BEFORE WE DISPATCHED THE ACFT. I HAD COME TO THE HARSH REALIZATION I HAD INADVERTENTLY AND NOT DELIBERATELY DISPATCHED THE ACFT ILLEGALLY FOR FLT PER MEL. I IMMEDIATELY INFORMED MY SUPVR OF THE INFRACTION WHICH HAD OCCURRED WHEN I BECAME AWARE MYSELF. BY THIS TIME THE ACFT HAD ARRIVED AT ZZZ. WE THEN CHKED IN THE COMPUTER FOR ANY DISCREPANCIES RELATING TO THIS UNEVENTFUL SIT. WE WERE ABLE TO SEE THAT THE L THRUST REVERSER FAILED TO DEPLOY UPON LNDG AND THE ACCESS PANEL HAD BEEN DAMAGED ALONG WITH THE FACT THE BYPASS PIN WAS STILL INSTALLED IN ISOLATION SHUTOFF VALVE WITH THE STREAMER HANGING OUT. TAKE; FOR INSTANCE; I HAD NO CTL OVER LIGHTING CONDITIONS SURROUNDING THE ACFT; AND THAT IT WAS NIGHT WHEN THIS SIT OCCURRED NOR THE UNNEEDED PRESSURE AND STRESS MGMNT AND GND OPS; OH AND NOT TO FORGET THE PLT WHO TOO WAS ANXIOUS FOR A TIMELY DISPATCH. ANOTHER FACTOR WAS THE STREAMER ATTACHED TO THE BYPASS PIN IN THE VALVE WAS NOT VISIBLE FROM THE GND AT EYE LEVEL HT -- YOU NEEDED TO LOOK UP AT LEAST 13 FT TO SEE IT HANGING OUT THE ACCESS PANEL ON THE PYLON.