A DEHAVILLAND DASH 8 WAS RELEASED FOR SVC AFTER HVY MAINT AND A SUPPLEMENTARY TYPE CERTIFICATE MODIFICATION WITH NO FORM 337 SUBMITTED TO THE FAA.

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ACFT HAD A MAJOR ALTERATION INCORPORATED DURING HVY CHK MAINT AND FAA FORM 337 WAS NOT COMPLETED PRIOR TO THE AIRWORTHINESS RELEASE BEING SIGNED IN THE ACFT LOG. IT WAS DISCOVERED SHORTLY AFTER ACFT DEPARTED BY OPERATORS REPRESENTATIVE (SUPVR). THE ACFT WAS BEING REPOSITIONED TO ITS HOME BASE. I IMMEDIATELY COMPLETED AND SIGNED FAA FORM 337 FOR THE ALTERATION (STC) AND SUBMITTED IT TO THE SUPVR FOR INCORPORATION INTO THE ACFT RECORDS AND FAA. CONTRIBUTING FACTORS: THIS WAS THE FIRST ACFT BEING MAINTAINED UNDER A DIFFERENT MAINT PROGRAM. INSPECTION AND OTHER AUTH PERSONNEL WERE VERY UNFAMILIAR WITH PAPERWORK DOCUMENTATION AND RECORDING. LACK OF TRAINING AND FAMILIARITY WITH THEIR MAINT PROCS IS THE MAIN PROB. CURRENTLY UNDER OUR NORMAL MAINT PROGRAM ANY STC; MODIFICATIONS; ETC; WILL INCLUDE INSTRUCTIONS AS A STEP TO FILL OUT FAA FORM 337. I NEVER HAD TO FILL ONE OUT BEFORE. IT IS HANDLED THROUGH SPECIFIC PERSONNEL AND RECORDS. NOT BEING FAMILIAR WITH FAA FORM 337; AND OPERATORS LACK OF GUIDANCE/INSTRUCTION ON WHEN THE FORM IS REQUIRED FOR ANY SPECIFIC TASK HAS LED TO THIS ERROR.