AN SAAB 340B IN DSCNT DECLARED AN EMER DUE TO UNABLE TO REDUCE ENG PWR TO LESS THAN 40%. CAUSED BY DEFERRAL OF AUTOFLT IDLE STOP CTL.

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MY FLT FROM YYY TO ZZZ DEVELOPED A PROB THAT EVENTUALLY REQUIRED ATC PRIORITY HANDLING AND A DECLARED EMER. TECHNICAL ASPECTS: ACFT IS A SAAB 340B EQUIPPED WITH ELECTROMECHANICAL FLT IDLE STOP WITH ASSOCIATED WARNING DEVICES AND MODIFICATIONS. AFTER TKOF; CAUTION AMBER FLT IDLE STOP FAIL LIGHT ILLUMINATED AFTER GEAR RETRACTION. THIS INDICATES AND CAUTIONS THE PLTS TO LIMIT AFT TRAVEL OF THE PWR LEVER WHILE INFLT TO NO FURTHER AFT OF THE PEDESTAL MARKINGS. DURING CLB/LEVEL FLT; NO PROB. DURING DSCNT AND LNDG; BIG PROB. 40% TORQUE WAS LEAST PWR OBSERVED. DSCNT; FINAL APCH AND LNDG REQUIRED MAJOR EFFORT AND COORD OF CREW AND ATC. ACFT DEPARTED YYY AT ALMOST MAX TKOF WT. ACFT WAS DISPATCHED VIA APPROVED MEL PROCS FROM ZZZ TO XXX. BOTH ARE ACR X MAINT BASES. 'ONE TIME REVENUE FLT' PROVISION OF MEL! MY CREW PLUS FAA INSPECTOR OFF ANOTHER CREW ASSIGNED ACFT. MAINT CTL WILL UTILIZE ALL SOURCES (CREW) TO FLY THIS ACFT THAT ARE POSSIBLE. 'IT'S A LEGAL MEL' IS THE MOST COMMON QUOTE! IT MAY BE LEGAL; BUT IS IT/WAS IT SAFE?