Narrative:

Two weeks prior to incident, aircraft annual and #2 cylinder exhaust and intake valves replaced due to low compression. Aircraft flew six hours since work on cylinder until incident. Pilot and one passenger aboard. VFR flight plan with flight following from ZZZ-ZZZ1. Normal takeoff and cruise. Was on frequency with xyz approach, when engine suddenly started running very rough and vibrating. Altitude 4500 MSL (4200 AGL). Did the initial check of instruments/knobs. Knew exactly where I was on map and turned immediately toward closest airport (ZZZ2/8 mi to the northwest). No change in fuel, engine, or magneto settings changed status. Good engine instrument indication, only vibration and roughness indicated problem (no engine analyzer display, only egt gauge). Told xyz approach of problem and declared an emergency. Told them I was heading to ZZZ2 and asked them to confirm my heading. Began to lose altitude at cruise power, so increased power slightly until airport in sight and ready to descend. Xyz approach handed me off to ZZZ2 approach controller. Announced emergency on unicom frequency and flew straight in to runway. On flare/landing, unexpectedly ballooned up a little and took longer to land and roll to stop. Later on the ground, we discovered that due to cylinder crack, engine did not go to idle when throttle at idle. Apparently we landed with unintended slight power in the flare even though throttle was at idle. Post flight inspection: #2 cylinder was cracked in half. It separated into two pieces at the juncture of the barrel and the cylinder head, all the way around the circumference of the cylinder. The cylinder head was held in place only by the valve cover and push rods. Since the piston continued to move within the barrel sleeve, it was protected from contact with the broken part. Not one drop of oil was lost. Engine = continental io-520k1b. Cylinder is rebuilt/overhauled. Cylinder had 817 hours since installed. Other cylinders from the rebuilt/overhaul company are covered by ad's or mandatory service bulletins for similar problems. This cylinder was not within the time span of cylinders covered by ad's. Lesson learned: I knew where I was and knew immediately where to land. Flight following is handy help. Hadn't thought of inability to go to idle, be prepared to shut down engine. Callback conversation with reporter revealed the following information: the reporter stated the airplane was a bellanca scout with a continental io-520k1b with 6 hours out of annual inspection where #2 cylinder intake and exhaust valves were replaced. The reporter said the valves were replaced due to low compression. The reporter stated on troubleshooting the engine it was discovered the #2 cylinder was cracked just below the cylinder head completely around the cylinder. The reporter said one point on the cylinder looked as if exhaust leaked from a small crack. The reporter stated the cylinder was rebuilt/overhauled by abc and had 814 hours since installed. The reporter said these abc cylinders are subject to airworthiness directives, but this cylinder was not in the correct time span. The reporter stated these cylinders should be covered by the airworthiness directive also.

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Original NASA ASRS Text

Title: A BL17-30A IN CRUISE AT 4500 FT THE ENG BEGAN TO RUN ROUGH. GOOD ENG INDICATIONS. DECLARED AN EMER AND DIVERTED. LATER DETERMINED TO BE A CRACKED CYLINDER.

Narrative: TWO WEEKS PRIOR TO INCIDENT, ACFT ANNUAL AND #2 CYLINDER EXHAUST AND INTAKE VALVES REPLACED DUE TO LOW COMPRESSION. ACFT FLEW SIX HOURS SINCE WORK ON CYLINDER UNTIL INCIDENT. PLT AND ONE PAX ABOARD. VFR FLT PLAN WITH FLT FOLLOWING FROM ZZZ-ZZZ1. NORMAL TKOF AND CRUISE. WAS ON FREQ WITH XYZ APCH, WHEN ENG SUDDENLY STARTED RUNNING VERY ROUGH AND VIBRATING. ALT 4500 MSL (4200 AGL). DID THE INITIAL CHECK OF INSTRUMENTS/KNOBS. KNEW EXACTLY WHERE I WAS ON MAP AND TURNED IMMEDIATELY TOWARD CLOSEST ARPT (ZZZ2/8 MI TO THE NW). NO CHANGE IN FUEL, ENG, OR MAGNETO SETTINGS CHANGED STATUS. GOOD ENG INSTRUMENT INDICATION, ONLY VIBRATION AND ROUGHNESS INDICATED PROB (NO ENG ANALYZER DISPLAY, ONLY EGT GAUGE). TOLD XYZ APCH OF PROB AND DECLARED AN EMER. TOLD THEM I WAS HEADING TO ZZZ2 AND ASKED THEM TO CONFIRM MY HDG. BEGAN TO LOSE ALT AT CRUISE POWER, SO INCREASED POWER SLIGHTLY UNTIL ARPT IN SIGHT AND READY TO DSND. XYZ APCH HANDED ME OFF TO ZZZ2 APCH CTLR. ANNOUNCED EMER ON UNICOM FREQ AND FLEW STRAIGHT IN TO RWY. ON FLARE/LNDG, UNEXPECTEDLY BALLOONED UP A LITTLE AND TOOK LONGER TO LAND AND ROLL TO STOP. LATER ON THE GND, WE DISCOVERED THAT DUE TO CYLINDER CRACK, ENG DID NOT GO TO IDLE WHEN THROTTLE AT IDLE. APPARENTLY WE LANDED WITH UNINTENDED SLIGHT POWER IN THE FLARE EVEN THOUGH THROTTLE WAS AT IDLE. POST FLT INSPECTION: #2 CYLINDER WAS CRACKED IN HALF. IT SEPARATED INTO TWO PIECES AT THE JUNCTURE OF THE BARREL AND THE CYLINDER HEAD, ALL THE WAY AROUND THE CIRCUMFERENCE OF THE CYLINDER. THE CYLINDER HEAD WAS HELD IN PLACE ONLY BY THE VALVE COVER AND PUSH RODS. SINCE THE PISTON CONTINUED TO MOVE WITHIN THE BARREL SLEEVE, IT WAS PROTECTED FROM CONTACT WITH THE BROKEN PART. NOT ONE DROP OF OIL WAS LOST. ENG = CONTINENTAL IO-520K1B. CYLINDER IS REBUILT/OVERHAULED. CYLINDER HAD 817 HOURS SINCE INSTALLED. OTHER CYLINDERS FROM THE REBUILT/OVERHAUL COMPANY ARE COVERED BY AD'S OR MANDATORY SERVICE BULLETINS FOR SIMILAR PROBS. THIS CYLINDER WAS NOT WITHIN THE TIME SPAN OF CYLINDERS COVERED BY AD'S. LESSON LEARNED: I KNEW WHERE I WAS AND KNEW IMMEDIATELY WHERE TO LAND. FLT FOLLOWING IS HANDY HELP. HADN'T THOUGHT OF INABILITY TO GO TO IDLE, BE PREPARED TO SHUT DOWN ENG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS A BELLANCA SCOUT WITH A CONTINENTAL IO-520K1B WITH 6 HOURS OUT OF ANNUAL INSPECTION WHERE #2 CYLINDER INTAKE AND EXHAUST VALVES WERE REPLACED. THE RPTR SAID THE VALVES WERE REPLACED DUE TO LOW COMPRESSION. THE RPTR STATED ON TROUBLESHOOTING THE ENG IT WAS DISCOVERED THE #2 CYLINDER WAS CRACKED JUST BELOW THE CYLINDER HEAD COMPLETELY AROUND THE CYLINDER. THE RPTR SAID ONE POINT ON THE CYLINDER LOOKED AS IF EXHAUST LEAKED FROM A SMALL CRACK. THE RPTR STATED THE CYLINDER WAS REBUILT/OVERHAULED BY ABC AND HAD 814 HOURS SINCE INSTALLED. THE RPTR SAID THESE ABC CYLINDERS ARE SUBJECT TO AIRWORTHINESS DIRECTIVES, BUT THIS CYLINDER WAS NOT IN THE CORRECT TIME SPAN. THE RPTR STATED THESE CYLINDERS SHOULD BE COVERED BY THE AIRWORTHINESS DIRECTIVE ALSO.

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.