ALTDEV ALT UNDERSHOT IN DSCNT.

Narrative

THE EVENT AT ISSUE OCCURRED AT THE END OF A 5 HR FLT FROM THE BAHAMAS DURING A DSCNT FOR LNDG; FROM THE E; INTO OPA LOCKA ARPT (KOPF). AS THE PIC; I HAD BEEN GIVEN CLRNC TO DSND FROM CRUISE ALT DOWN TO 2000 FT MSL. I HAD TRANSITIONED FROM IMC TO VISUAL REF AT APPROX 4500 FT MSL. I WAS THEN 'CLRED FOR THE VISUAL APCH; ENTER L DOWNWIND FOR RWY 9L.' AFTER PASSING BELOW 3000 FT MY ENG STARTED TO RUN ROUGH AND VIBRATED MORE THAN I WOULD HAVE CONSIDERED NORMAL. I STOPPED THE DSCNT AS I ATTEMPTED TO DETERMINE THE CAUSE OF THE ENG RUNNING AS IT WAS. MY ATTN WAS DIVERTED SLIGHTLY TOWARD THE FOLLOWING PRIORITIES: KEEP THE ENG RUNNING. CHK: FUEL SHUTOFF; MAGNETOS; PRIMER; CARB HEAT; ETC. ENG INSTS: CARB TEMP; CYLINDER TEMP; OIL PRESSURE AND TEMP; EXHAUST HAS TEMP (RE-LEAN); MANIFOLD PRESSURE; RPM; ETC. MAINTAIN VISUAL SEPARATION FROM TFC. I WAS NOW CLRLY IN VISUAL CONDITIONS. FIND THE ARPT. I HAD BEEN CLRED TO ENTER L DOWNWIND FOR RWY 9L BUT STILL DID NOT HAVE THE ARPT IN SIGHT. THE ACFT HAD ENTERED A SLOW CLB AND REACHED AN ALT OF 3000 FT MSL. THE CTLR THEN ASKED ME TO CONFIRM MY ALT AND INSTRUCTED ME TO IMMEDIATELY DSND TO 2000 FT. HE THEN GAVE ME VECTORS FOR RWY 9L. I CANCELED IFR ONCE ON FINAL FOR RWY 9L AND LANDED WITHOUT FURTHER INCIDENT. ON THE GND; I 'RAN THE ENG UP' AND FOUND THAT THE VIBRATION DISAPPEARED AT HIGHER PWR SETTINGS. AS I LOOK BACK ON THE INCIDENT; THERE ARE SEVERAL FACTORS THAT MAY HAVE AGGRAVATED THE SIT AND ADDRESSING THEM COULD REDUCE FURTHER PROBS. PLT TRAINING -- PLT TRAINING NEEDS TO CONTINUOUSLY STRESS CTL OF THE AIRPLANE DURING TIMES OF POTENTIAL EMER. PLT FATIGUE -- ALTHOUGH I FELT FINE; IT IS LIKELY THAT I WAS SLIGHTLY FATIGUED DUE TO THE DURATION OF THE FLT AT 'HIGHER-THAN-NORMAL' ALTS. CTLR INSTRUCTIONS -- CTLR INSTRUCTIONS NEED TO BE CLR AND REASONABLE. THE CTLR WAS NOT BUSY; AND IT WOULD HAVE BEEN HELPFUL HAD HE EXPLAINED THE NEED FOR ME TO BE AT 2000 FT (EG; 'NXXXXX DSND TO 2000 FT FOR TFC.'). THIS TYPE OF THING FACILITATES COM BTWN CTLR AND PLT. CTLR DEMEANOR -- THE CTLR WAS HELPFUL AND ACTED IN A PROFESSIONAL MANNER YET IT WAS CLR; BY THE TONE OF HIS VOICE; THAT HE WAS ANNOYED BY THE INCIDENT. THIS TYPE OF BEHAVIOR IS UNDERSTANDABLE BUT DOES NOT HELP THE SIT IF THERE ACTUALLY IS A PROB THAT THE PLT IS DEALING WITH. IN GENERAL; THE MOST IMPORTANT ELEMENT HERE IS THE RELATIONSHIP BTWN THE PLT AND THE CTLR. BOTH ARE REQUIRED TO MAKE THE SYS SAFE. GOOD PLTS DO MAKE MISTAKES AND GOOD CTLRS DO MAKE MISTAKES. WE NEED TO BACK EACH OTHER UP TO KEEP GA SAFE. FOR THIS REASON I THINK CTLRS SHOULD BE REQUIRED TO FLY MORE FREQUENTLY WITH PLTS AND PLTS SHOULD BE REQUIRED TO VISIT ATC FACILITIES. BOTH GROUPS WILL THEN UNDERSTAND BETTER THE CHALLENGES OF THE OTHER.