A B767-300; B757 PLT RPTS ON THE LACK OF THOROUGH DAILY MAINT INSPECTIONS OF THEIR ACFT BY QUALIFIED PERSONNEL IS CAUSING NOT ONLY UNNECESSARY DELAYS; BUT AFFECTING THE SAFETY OF THE AIRLINE.

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DURING PREFLT OF ACFT X; INITIALLY ASSIGNED THIS FLT; FOUND SCREW MISSING AND RPTED SAME TO MAINT. DURING MAINT INSPECTION TRYING TO LOCATE DISCREPANCY; THEY FOUND SOME CRACKS THAT I; AND APPARENTLY PREVIOUS PLTS; DID NOT SPOT. 'R ENG JUST AFT OF IDG AIR/OIL HEAT EXCHANGER OUTLET COWLING HAS 4 RIVETS MISSING AND A 3-INCH CRACK.' THE DAY PRECEDING THIS; WHILE PREFLTING INITIAL ACFT ASSIGNED; I RPTED THIS MAINT DISCREPANCY; WHICH APPARENTLY HAD BEEN HAPPENING PRIOR TO MY FLT AS WELL; DUE TO BLUE STAINS ON UNDERBELLY: 'BLUE LAVATORY FLUID DRIPPING OUT OF AFT BELLY DRAIN HOLE OF B757 ACFT.' BOTH OF THESE ISSUES MAY HAVE; IF NOT SHOULD HAVE; BEEN CAUGHT BY A TRAINED MECH DOING A MORE DETAILED INSPECTION OF THE ACFT. IN BOTH OF THESE CASES; FLTS WERE EXCESSIVELY DELAYED DUE TO ACFT GOING OTS. EITHER OF THESE MAINT DISCREPANCIES COULD HAVE RESULTED IN SERIOUS CONSEQUENCES. I BELIEVE THE LACK OF DAILY MAINT INSPECTIONS OF OUR ACFT BY QUALIFIED PERSONNEL IS CAUSING NOT ONLY UNNECESSARY DELAYS; BUT AFFECTING THE SAFETY OF OUR AIRLINE. FURTHERMORE; RUSHED PREFLTS BY PLTS; DUE TO ROUTINE OPS IS FAILING TO CATCH SOME OF THESE LESS OBVIOUS DISCREPANCIES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED HE FILED THIS RPT BECAUSE OF HIS CONCERNS ABOUT THE MINIMAL MAINT AND LIMITED MAINT WALKAROUNDS WHERE MANY OF THESE DISCREPANCIES COULD HAVE AND MOST LIKELY; WOULD HAVE; BEEN FOUND PRIOR TO FLT CREWS PERFORMING THEIR PREFLT CHKS. RPTR ALSO STATED THE B757 BLUE LAVATORY FLUID DRIPPING OUT OF THE AFT BELLY DRAIN WAS NOT THE DRAIN MAST; BUT THE FUSELAGE DRAIN HOLE. THIS NORMALLY INDICATES EITHER THE LAVATORY TOILET IS LEAKING OR POSSIBLY LINES ARE LEAKING INTO THE AFT CARGO COMPARTMENT AREA. RPTR ALSO NOTED THE B767-300 WAS REMOVED FROM SVC TO FIX THE CRACKED SECTION AROUND THE #2 ENG IDG AIR/OIL HEAT EXCHANGER OUTLET COWLING.