Narrative:

Arrived at gate in buf at XA48 with left engine running. Ramp plugged in ground power; but aircraft would not accept. Started APU and connected generator to busses and personally shut down left engine. First officer turned on APU bleed and left pack. We completed parking checklist and secured aircraft for overnight with APU running. We then went to hotel; which took approximately 20 mins. Upon arrival at hotel; I was given the phone where a frantic ramp worker from buf was telling me we had left an engine running. With complete certainty that we had shut the engine down; I still felt I had no choice but to proceed back to airport and address concerns. I did; arriving at aircraft at approximately XB30. There; I found the left engine rotating but not running. I went to cockpit and confirmed start levers were in cutoff and fuel flows were 0. I then turned off APU bleed and pack. Then the ramp agent that had called me and I went outside where the engine was still turning but appeared to be slowing down. He showed me how there was air and heat coming from exhaust of engine prompting him to believe it was running. The engine finally came to a stop apparently as a result of turning off APU bleed. I called maintenance and conferred with 2 technicians whom along with me; had never heard of a CFM56 continuing to motor without being commanded to. There was no indication in cockpit that start valve was opened but this appears to me what was happening. Once APU bleed was closed; rotation ceased.

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Original NASA ASRS Text

Title: B737-500 CAPT RECEIVED CALL TO RETURN TO ARPT. GND CREW BELIEVED ACFT ENG WAS STILL RUNNING. ENG WAS ACTUALLY MOTORING FROM APU BLEED AIR. BLEED VALVE CLOSED AND MOTORING CEASED.

Narrative: ARRIVED AT GATE IN BUF AT XA48 WITH L ENG RUNNING. RAMP PLUGGED IN GND PWR; BUT ACFT WOULD NOT ACCEPT. STARTED APU AND CONNECTED GENERATOR TO BUSSES AND PERSONALLY SHUT DOWN L ENG. FO TURNED ON APU BLEED AND L PACK. WE COMPLETED PARKING CHKLIST AND SECURED ACFT FOR OVERNIGHT WITH APU RUNNING. WE THEN WENT TO HOTEL; WHICH TOOK APPROX 20 MINS. UPON ARR AT HOTEL; I WAS GIVEN THE PHONE WHERE A FRANTIC RAMP WORKER FROM BUF WAS TELLING ME WE HAD LEFT AN ENG RUNNING. WITH COMPLETE CERTAINTY THAT WE HAD SHUT THE ENG DOWN; I STILL FELT I HAD NO CHOICE BUT TO PROCEED BACK TO ARPT AND ADDRESS CONCERNS. I DID; ARRIVING AT ACFT AT APPROX XB30. THERE; I FOUND THE L ENG ROTATING BUT NOT RUNNING. I WENT TO COCKPIT AND CONFIRMED START LEVERS WERE IN CUTOFF AND FUEL FLOWS WERE 0. I THEN TURNED OFF APU BLEED AND PACK. THEN THE RAMP AGENT THAT HAD CALLED ME AND I WENT OUTSIDE WHERE THE ENG WAS STILL TURNING BUT APPEARED TO BE SLOWING DOWN. HE SHOWED ME HOW THERE WAS AIR AND HEAT COMING FROM EXHAUST OF ENG PROMPTING HIM TO BELIEVE IT WAS RUNNING. THE ENG FINALLY CAME TO A STOP APPARENTLY AS A RESULT OF TURNING OFF APU BLEED. I CALLED MAINT AND CONFERRED WITH 2 TECHNICIANS WHOM ALONG WITH ME; HAD NEVER HEARD OF A CFM56 CONTINUING TO MOTOR WITHOUT BEING COMMANDED TO. THERE WAS NO INDICATION IN COCKPIT THAT START VALVE WAS OPENED BUT THIS APPEARS TO ME WHAT WAS HAPPENING. ONCE APU BLEED WAS CLOSED; ROTATION CEASED.

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.