Narrative:

The event at issue occurred at the end of a 5 hour flight from the bahamas during a descent for landing, from the east, into opa locka airport (kopf). As the PIC, I had been given clearance to descend from cruise altitude down to 2000 ft MSL. I had transitioned from IMC to visual reference at approximately 4500 ft MSL. I was then 'cleared for the visual approach, enter left downwind for runway 9L.' after passing below 3000 ft my engine started to run rough and vibrated more than I would have considered normal. I stopped the descent as I attempted to determine the cause of the engine running as it was. My attention was diverted slightly toward the following priorities: keep the engine running. Check: fuel shutoff, magnetos, primer, carburetor heat, etc. Engine instruments: carburetor temperature, cylinder temperature, oil pressure and temperature, exhaust has temperature (re-lean), manifold pressure, RPM, etc. Maintain visual separation from traffic. I was now clearly in visual conditions. Find the airport. I had been cleared to enter left downwind for runway 9L but still did not have the airport in sight. The aircraft had entered a slow climb and reached an altitude of 3000 ft MSL. The controller then asked me to confirm my altitude and instructed me to immediately descend to 2000 ft. He then gave me vectors for runway 9L. I canceled IFR once on final for runway 9L and landed without further incident. On the ground, I 'ran the engine up' and found that the vibration disappeared at higher power settings. As I look back on the incident, there are several factors that may have aggravated the situation and addressing them could reduce further problems. Pilot training -- pilot training needs to continuously stress control of the airplane during times of potential emergency. Pilot fatigue -- although I felt fine, it is likely that I was slightly fatigued due to the duration of the flight at 'higher-than-normal' altitudes. Controller instructions -- controller instructions need to be clear and reasonable. The controller was not busy, and it would have been helpful had he explained the need for me to be at 2000 ft (eg, 'nxxxxx descend to 2000 ft for traffic.'). This type of thing facilitates communication between controller and pilot. Controller demeanor -- the controller was helpful and acted in a professional manner yet it was clear, by the tone of his voice, that he was annoyed by the incident. This type of behavior is understandable but does not help the situation if there actually is a problem that the pilot is dealing with. In general, the most important element here is the relationship between the pilot and the controller. Both are required to make the system safe. Good pilots do make mistakes and good controllers do make mistakes. We need to back each other up to keep GA safe. For this reason I think controllers should be required to fly more frequently with pilots and pilots should be required to visit ATC facilities. Both groups will then understand better the challenges of the other.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.