ATX SMT ACFT OPERATED WITHOUT REQUIRED INST EQUIP ON SCHEDULED CARGO OPS SINCE THE COMPANY DID NOT HAVE AN APPROVED MEL FOR THE ACFT.

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ACFT MADE NUMEROUS CARGO ONLY FLTS OPERATING UNDER FAR PART 135 WITH THE PLT'S (L SIDE) DIRECTIONAL GYROSCOPE REMOVED FROM THE ACFT FOR REPAIRS. THE COPLT'S (R SIDE) DIRECTIONAL GYROSCOPE WAS OPERATIVE. AT THE TIME THE PLT'S (L SIDE) DIRECTIONAL GYROSCOPE WAS REMOVED; THE DIRECTOR OF MAINT AND ASSISTANT CHIEF PLT INTERPRETED THAT UNDER FAR PARTS 91.205; 135.159 AND 135.163 IT WOULD BE ALLOWABLE TO OPERATE THE ACFT WITH THE REMAINING COPLT'S (R SIDE) DIRECTIONAL GYROSCOPE AS LONG AS THE ACFT WAS NOT OPERATED IN PAX CARRYING OPS. WHEN I BECAME AWARE OF THE SIT THE ACFT WAS TAKEN OTS UNTIL THE REPAIRED DIRECTIONAL GYROSCOPE WAS INSTALLED IN THE PLT'S (L SIDE) PANEL. I BELIEVE THAT MISINTERPS OF VARIOUS FARS LED TO THE OP OF THE ACFT WHEN IT WAS NOT IN ABSOLUTE CONFORMANCE WITH ITS TYPE CERTIFICATIONS. I BELIEVE THE FAA NEEDS TO PRODUCE BETTER GUIDELINES THAT SPELL OUT IN SIMPLE TERMS THE REQUIREMENTS THAT AN ACFT NEEDS TO MEET TO BE CONSIDERED AIRWORTHY. IN ADDITION A DEFINITION FOR AIRWORTHY NEEDS TO BE LISTED IN PART 1 OF THE FARS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR WAS ADVISED THAT HE SHOULD HAVE AN FAA APPROVED MEL FOR THE ACFT AND IT WILL DESCRIBE THE CONDITIONS AND ACTIONS NECESSARY TO PERMIT OPS WITH INOP EQUIP. HE STATED THAT HE HAD JUST RECEIVED THE APPROVED MEL AND THIS PROB OF NOT KNOWING WHAT TO DO SHOULD NOT HAPPEN AGAIN. HE WAS ALSO ADVISED THAT WITH REGARD TO HIS QUESTION ABOUT INCLUDING THE DEFINITION FOR 'AIRWORTHY' IT WOULD BE TOO DIFFICULT TO DESCRIBE SINCE THERE ARE SO MANY CONSIDERATIONS AND TO MAKE A GENERAL STATEMENT THAT STATED THAT AN ACFT WAS REQUIRED TO BE AIRWORTHY FOR THE TYPE OF OP CONDUCTED; WOULD BE TOO VAGUE.