AN ACR LTT HAD TO RETURN LAND AFTER TKOF WITH A 'SPECIAL PIN' THAT WAS LEFT IN THE NOSE GEAR.

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IT WAS XX30 ON A THU MORNING. OUR ACFT WAS PREFLTED BY THE FO INSIDE THE HANGAR. SINCE THE ACFT HAD TO BE TOWED OUT OF THE HANGAR; THE FO LEFT THE GEAR PINS INSTALLED ACCORDING TO OUR OPS MANUAL PROCS. AS THE FO OBSERVED THE TOWING PROCESS BY THE RAMP AGENTS; HE PULLED 3 GEAR PINS WHEN THE AGENTS WERE FINISHED TOWING THE ACFT (1 NOSE; 1 L MAIN GEAR; AND 1 R MAIN GEAR). THESE 3 GEAR PINS ARE THE REQUIRED PINS THAT OUR FLT STANDARDS MANUAL ADDRESSES FOR PREFLT ACTION. THERE WAS ANOTHER PIN (SPECIAL) THAT OUR COMPANY USES AT OVERNIGHTING STATIONS WHERE OUR ACFT HAVE TO BE TOWED. CREW MEMBERS WERE INFORMED OF THIS SPECIAL PIN; BUT WERE NEVER INSTRUCTED ON THE INSTALLATION OR REMOVAL OF THIS PIN. INSTEAD; ONLY RAMP AGENTS AT SELECTED OUTSTATIONS WERE TAUGHT ON ITS USAGE. RAMP AGENTS WERE TO INSTALL AND REMOVE PIN BEFORE AND AFTER TOWING ACFT. THE FO OBSERVED THE RAMP AGENT REMOVING TOW BAR. ALSO; HE OBSERVED THE AGENT STARTING TO REMOVE THE SPECIAL PIN IN THE NOSEWHEEL. SO; HE REACHED DOWN TO PICK UP HIS 3 REQUIRED GEAR PINS; THEN LOOKED TOWARD THE RAMP AGENT OBSERVING HIM WALKING AWAY FROM THE ACFT. THE FO GLANCED AT THE NOSEWHEEL TO SIGHT THE RED WARNING FLAG THAT HANGS FROM THE SPECIAL PIN. HE DID NOT SEE THE FLAG AND ASSUMED THE PIN HAD BEEN REMOVED FROM THE NOSEWHEEL. THE SAME AGENT THEN MARSHALLED OUR ACFT OUT OF APRON. THE CREW TAXIED THE ACFT TO THE GATE TO LOAD PAX WHILE EXECUTING A SINGLE ENG TURN. (THAT'S WHERE YOU LEAVE 1 ENG RUNNING WHILE LOADING PAX). SHORTLY AFTER DEP; WE NOTICED THE GEAR RETRACTION PROB AND RETURNED FOR LNDG AT CAK.