CAPT OF AN LGT DISCOVERED AFTER THE FACT THAT HE HAD OPERATED THE ACFT CONTRARY TO AN EQUIP MEL'ED ITEM.

Narrative

THIS ACFT WAS OPERATED UNPRESSURIZED WITH CARGO PAYLOAD DUE TO A MISINTERP OF MEL RESTRS AGAINST SUCH OP. CHAIN OF EVENTS LEADING TO FLT: MISINTERP OF MEL RESTRS BY CREW AND DISPATCH. CREW ON DUTY MANY HRS PRIOR TO THIS FLT. A 2 HR REST BREAK IN EARLY EVENING DID NOT ALLOW FOR ACTUAL USEABLE REST. ACFT HAD ARRIVED AT THE STATION UNPRESSURIZED WITH PAYLOAD BEFORE THIS CREW TOOK IT. DISPATCH (FLT FOLLOWING) SHOWED FLT ON PAPERWORK AS BEING LEGAL. MEL STATED CLASS E CARGO COMPARTMENT WITH PAYLOAD NOT LEGAL FOR FLT. DISPATCH INTERPRETED THIS AS 'CATEGORY E HAZMAT PAYLOAD NOT AUTH.' NO ONE IN MY CREW NOR THE DISPATCHER KNEW THE DEFINITION OF A 'CLASS E CARGO COMPARTMENT.' AFTER ARR AT DEST; AFTER I DISCUSSED THE MATTER WITH ANOTHER PLT WHO HAD ENCOUNTERED THIS SAME ISSUE; HE SAID HE BELIEVED THAT THE CLASS E COMPARTMENT WAS; IN OUR CASE; THE MAIN CABIN CARGO COMPARTMENT. AT THIS POINT UPON REREADING THE MEL; I REALIZED THE MISINTERP WHICH HAD OCCURRED BTWN 'CLASS E CARGO COMPARTMENT;' AND 'CATEGORY E EXPLOSIVES;' I TOLD THEM THAT I WOULD NOT TAKE THE ACFT BACK OUT UNTIL WE COULD DEFINE WHAT A 'CLASS E' CARGO COMPARTMENT IS. THEY AGREED; AND TOOK THE ACFT OUT OF SVC FOR PRESSURIZATION SYS REPAIRS. AFTER AN HR OR SO; DISPATCH HAD STILL NOT BEEN ABLE TO FIND THE DEFINITION. OBSERVATIONS: THIS SIT COULD HAVE BEEN PREVENTED IF THE DEFINITION OF THE CARGO COMPARTMENT IN QUESTION HAD BEEN INCLUDED IN THE MEL; EITHER SPELLED OUT IN THE TEXT; IN A FOOTNOTE; OR IN A GLOSSARY OF DEFINITIONS -- NOT BURIED WITHIN VOLUMES OF BOOKS LOCATED ELSEWHERE. WHILE THE WRITERS OF THE MEL'S MAY BE VERY KNOWLEDGEABLE OF SUCH DEFINITIONS AS THESE; THOSE OF US WHO USE THE MEL'S TO DETERMINE LEGALITY AND SAFETY OF FLT MAY NOT BE AWARE OF THEM UNTIL WE ENCOUNTER THEM; WHICH MAY BE VERY INFREQUENTLY. PERSONALLY; I WILL CHK ANY SUCH SIMILAR AMBIGUITIES IN THE MEL MORE CAREFULLY; EVEN AT THE RISK OF DELAYING THE FLT; UNTIL I AM ASSURED OF A PROPER UNDERSTANDING BY ALL CONCERNED.