NTSB Factual Report

On December 14, 2006, at 0804 central standard time, a Beech B36TC, N30799, experienced a loss of engine power and an in-flight fire during cruise flight, and was subsequently destroyed by fire after a forced landing in a field about 1.5 miles west of the Greater Peoria Regional Airport (PIA), Peoria, Illinois. The private pilot was not injured. The 14 Code of Federal Regulations Part 91 personal flight departed Dubuque Regional Airport (DBQ), Dubuque, Iowa, en route to Centralia Municipal Airport (ENL), Centralia, Illinois. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed.

The pilot reported that the airplane was in cruise flight west-northwest of PIA when the engine started "missing." The pilot put the mixture to full rich, switched fuel tanks, and adjusted the propeller and throttle, but the engine continued to run rough. The pilot reported that he turned to the nearest airport for an emergency landing. He reported that the airplane was about 5 - 6 miles from PIA at 4,500 feet mean sea level (msl) at 100 knots, but he was unable to maintain altitude. The propeller was still turning, but the windshield was 60 percent covered with oil and smoke was coming into the cockpit. The pilot reported that he realized that he could not reach the airport, so he executed a forced landing in a field. About 150 feet above ground level (agl), he lowered the landing gear and turned to the north to align the airplane with the corn rows. The airplane was still on fire when the airplane came to a stop, and the pilot exited the airplane uninjured.

Federal Aviation Administration airworthiness and operations inspectors examined the wreckage. Fire consumed the engine compartment, cockpit, and cabin. The empennage was still intact. The engine was separated from the firewall. A hole was found in the top of the engine crankcase.

The engine was shipped to Teledyne Continental Motors for a teardown inspection. The inspection revealed that there was thermal discoloration throughout much of the engine. The crankcase on the #1, #3, and #5 side of the engine had mechanical damage. The #5 lifter housing was fractured. The #3 cylinder bay was fractured just forward of the lifter bay. The #5 cylinder skirt was missing from the 9 o'clock to 1 o'clock position. The top of the dome combustion area inside cylinder #5 exhibited thermal and mechanical damage. The #5 piston head was disintegrated. The #5 connecting rod was twisted and bent about 30 degrees near the piston pin bushing bore. The #5 intake and exhaust valves were missing. The intake and exhaust valve stems and the exhaust valve guide were removed from the cylinder for further examination at the National Transportation Safety Board (NTSB) Materials Laboratory.

The NTSB Materials Laboratory report stated that both valves were fractured in the transition between the valve head and stem. The fracture surface of the intake valve was relatively rough and on irregular planes consistent with overstress fracture. The fracture surface of the exhaust valve stem was on a flat plane perpendicular to the axis, consistent with a brittle fracture mechanism such as fatigue.

The exhaust valve stem had "SA643767P005 C FAA-PMA" stamped on its side. The hardness was measured at several locations near the outboard end of the exhaust valve and was compared to the engineering drawing. In the tip, the hardness was 40 HRC, which was below the minimum specified hardness of 50 HRC for the tip. In the stem, the hardness was 35.5 HRC, which was within the specified hardness range for the body of the valve.

The diameter of the exhaust valve stem was measured at several locations. Near the tip, the diameter was 0.4336 inch. In the dark oxidized region just inboard of the middle of the length of stem, the diameter was 0.4262 inch. At a location approximately corresponding to the inboard end of the exhaust valve guide, the diameter was 0.4309 to 0.4317 inch. According to the engineering drawing for an SA643837 valve, revision Z, the diameter of the stem of a new exhaust valve is 0.4334 to 0.4341 inch.

The outboard end of the exhaust valve guide was deformed. The opening was reportedly damaged with a mechanical driver during removal from the cylinder. The internal surface of the valve guide appeared relatively rough and had brown deposits or oxides. The internal diameter of the valve guide was measured. Near the inboard end, the inner diameter of the guide was 0.434 to 0.436 inch. After cleaning the inboard end of the guide with a soft-bristle brush, the inner diameter measured 0.440 inch at the inboard end. Near the middle of the valve guide, the inner diameter measured approximately 0.443 to 0.450 inch.

The inboard end of the exhaust valve guide, the measured clearance (difference between the inner diameter of the valve guide and out diameter of the valve stem) was 0.0083 to 0.0091 inch. The exhaust valve guide clearance is approximately 0.0005 to 0.0015 inches smaller when installed in the cylinder due to the press fit. Subtracting the press fit from the measured clearances, the approximate clearance when installed in the cylinder was 0.007 to 0.009 inch. Near the middle of the exhaust valve guide, the clearance was measured as 0.019 inch. According to the 1989 Teledyne Continental Motors (TCM) overhaul manual for the IO-520 engine, the maximum serviceable limit for exhaust valve guide clearance is 0.006 inch.

The aircraft maintenance records included records of oil sample analysis. On September 26, 2003, an oil analysis was performed. The engine had 641 hours time since overhaul (TSO). The lab comments stated, "NOTE HIGH IRON AND BORDERLINE NICKEL. HIGH IRON AND NICKEL ARE TYPICALLY CAUSED BY VALVE GUIDE WEAR. WE WILL MONITOR AT THE NEXT NORMAL INTERVAL."

The next oil analysis was taken on June 23, 2004. The engine had 663 hours TSO. The header on the oil analysis stated, "RESULT!!!! CAUTION!!!!!***CONTACT ENGINE MANUFACTURER'S SERVICE REP******. The lab comments stated, "NOTE EXTREME NICKEL CONCENTRATION. HIGH CONCENTRATIONS OF NICKEL ARE CAUSED BY VALVE GUIDE WEAR. CONTACT THE ENGINE MANUFACTURER'S SERVICE REP. RESAMPLE IN 25 HOURS."

The aircraft owner reported that no maintenance was performed on the engine in regards to the oil analysis. The maintenance records indicated that the engine received seven additional oil analyses at the normal 25-hour intervals. The results indicated that the samples appeared normal and to continue checking the oil samples at the regular 25-hour intervals. Maintenance records indicated that the engine had 936 hours TSO at the last inspection conducted on July 6, 2006.

NTSB Probable Cause

The loss of engine power due to a fatigue fracture of the exhaust valve as a result of maintenance not performed by the aircraft owner, and the unsuitable terrain encountered during the forced landing.