Narrative:

I was late switching from auxiliary to main tanks in level flight at 7000 ft; part of a series of step-downs from my cruising altitude of 11000 ft. When the left engine started to run roughly because the auxiliary tank had emptied; I switched to the left main tank and also switched the right engine fuel tank to the right main tank in preparation for landing. The left engine slowed markedly yet smoothly; but did not shut down. Some fuel flow was apparent; but advancing the left mixture; throttle or using the fuel boost pump did not restore normal power. Accordingly; I used right rudder and aileron to compensate. Phx approach noticed my initial loss of altitude and cleared me for altitude at my discretion when I described the engine problem. I then requested a straight-in approach to runway 4R at falcon field; my planned destination. I declined an offer of emergency equipment or the possibility of landing at chandler (chd). Less maneuvering was needed to proceed directly ahead to ffz. I was also considering whether I was better off with an engine running somewhat above idle thrust or shutting it down and feathering it. I chose to let it run unless it slowed further. The RPM was far above what I would expect of a windmilling propeller. My descent and visual approach continued normally until short final when the left engine resumed normal power without any intervention on my part. Quick throttle and rudder adjustments maintained my descent path. I received clearance from ffz tower to land on runway 4R and did so without incident. Both engines continued to idle smoothly; so I taxied the plane back to its hangar. During this time ffz ground requested the registration number and model yr of the aircraft. There was no request for further information. At the time of the initial power loss; I thought that enough air may have entered the left engine's fuel lines to reduce power. Because of the high ambient temperature at 7000 ft was 65-70 degrees F; the fuel may have vaporized and took some time to clear. In estimated order of importance; this sustained power reduction on the left engine would likely not have happened had I 1) been more alert to auxiliary tank fuel levels; or 2) been quicker to switch back to the main tanks; or 3) had the temperature been lower. A subsequent diagnostic flight revealed no apparent problems with the aircraft.

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Original NASA ASRS Text

Title: BE55 PILOT REPORTS POWER LOSS ON LEFT ENGINE AS AUX TANK IS EMPTIED AND FUEL SELECTOR IS SWITCHED TO MAIN. ATTEMPTS TO RESTORE NORMAL FUEL FLOW TO THE LEFT ENGINE ARE UNSUCCESSFUL UNTIL JUST BEFORE LANDING WHEN FULL POWER RETURNS.

Narrative: I WAS LATE SWITCHING FROM AUX TO MAIN TANKS IN LEVEL FLT AT 7000 FT; PART OF A SERIES OF STEP-DOWNS FROM MY CRUISING ALT OF 11000 FT. WHEN THE L ENG STARTED TO RUN ROUGHLY BECAUSE THE AUX TANK HAD EMPTIED; I SWITCHED TO THE L MAIN TANK AND ALSO SWITCHED THE R ENG FUEL TANK TO THE R MAIN TANK IN PREPARATION FOR LNDG. THE L ENG SLOWED MARKEDLY YET SMOOTHLY; BUT DID NOT SHUT DOWN. SOME FUEL FLOW WAS APPARENT; BUT ADVANCING THE L MIXTURE; THROTTLE OR USING THE FUEL BOOST PUMP DID NOT RESTORE NORMAL PWR. ACCORDINGLY; I USED R RUDDER AND AILERON TO COMPENSATE. PHX APCH NOTICED MY INITIAL LOSS OF ALT AND CLRED ME FOR ALT AT MY DISCRETION WHEN I DESCRIBED THE ENG PROB. I THEN REQUESTED A STRAIGHT-IN APCH TO RWY 4R AT FALCON FIELD; MY PLANNED DEST. I DECLINED AN OFFER OF EMER EQUIP OR THE POSSIBILITY OF LNDG AT CHANDLER (CHD). LESS MANEUVERING WAS NEEDED TO PROCEED DIRECTLY AHEAD TO FFZ. I WAS ALSO CONSIDERING WHETHER I WAS BETTER OFF WITH AN ENG RUNNING SOMEWHAT ABOVE IDLE THRUST OR SHUTTING IT DOWN AND FEATHERING IT. I CHOSE TO LET IT RUN UNLESS IT SLOWED FURTHER. THE RPM WAS FAR ABOVE WHAT I WOULD EXPECT OF A WINDMILLING PROP. MY DSCNT AND VISUAL APCH CONTINUED NORMALLY UNTIL SHORT FINAL WHEN THE L ENG RESUMED NORMAL PWR WITHOUT ANY INTERVENTION ON MY PART. QUICK THROTTLE AND RUDDER ADJUSTMENTS MAINTAINED MY DSCNT PATH. I RECEIVED CLRNC FROM FFZ TWR TO LAND ON RWY 4R AND DID SO WITHOUT INCIDENT. BOTH ENGS CONTINUED TO IDLE SMOOTHLY; SO I TAXIED THE PLANE BACK TO ITS HANGAR. DURING THIS TIME FFZ GND REQUESTED THE REGISTRATION NUMBER AND MODEL YR OF THE ACFT. THERE WAS NO REQUEST FOR FURTHER INFO. AT THE TIME OF THE INITIAL PWR LOSS; I THOUGHT THAT ENOUGH AIR MAY HAVE ENTERED THE L ENG'S FUEL LINES TO REDUCE PWR. BECAUSE OF THE HIGH AMBIENT TEMP AT 7000 FT WAS 65-70 DEGS F; THE FUEL MAY HAVE VAPORIZED AND TOOK SOME TIME TO CLEAR. IN ESTIMATED ORDER OF IMPORTANCE; THIS SUSTAINED PWR REDUCTION ON THE L ENG WOULD LIKELY NOT HAVE HAPPENED HAD I 1) BEEN MORE ALERT TO AUX TANK FUEL LEVELS; OR 2) BEEN QUICKER TO SWITCH BACK TO THE MAIN TANKS; OR 3) HAD THE TEMP BEEN LOWER. A SUBSEQUENT DIAGNOSTIC FLT REVEALED NO APPARENT PROBS WITH THE ACFT.

Data retrieved from NASA's ASRS site as of January 2009 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.