Narrative:

I was flying with flight following from center. ATC was very helpful as I detoured around two thunderstorm cells over ZZZ. Cloud bases ranged from 5000-8000 MSL. Cloud tops were about 14000 MSL. ZZZ was in heavy rain. About ten miles northwest of ZZZ; a vibration started. I added carb heat; no change. I checked both magnetos; suspecting a fouled spark plug. Vibration continued on either or both magnetos. The egt of all four cylinders remained high in all magneto switch positions indicating that the engine had combustion in all four cylinders. I started a precautionary turn toward lower terrain. The vibration suddenly got worse. I informed ATC of the vibration and announced my intention to go to ZZZ. I accidentally said 'ZZZ1' then amended it to ZZZ. ATC was very responsive. Having narrowed down the range of possible problems; none of them good; [advised ATC]. I asked ATC for heading to ZZZ and to tell the tower I was coming. ATC provided winds; and frequencies. I reduced throttle and descended. ATC asked if I was unable to maintain altitude. I replied that I was reducing power and descending to the airport. The vibration got worse in sudden steps. When I reduced RPM; the amplitude of the vibration got worse. I could still see clearly and could see that the cowl was shaking; but did not see any evidence of things banging it from the inside. Oil pressure was normal. Oil temperature was high at 220F (usually 180-190F). I continually considered shutting off the fuel and ignition as I had enough altitude to safely reach the runway as a glider; and have practiced that scenario. I left the engine running at reduced power but was unsure of that decision and ready to change it.a few miles from ZZZ; I switched frequencies to ZZZ tower. ZZZ ATC read out descriptions of the runways. I chose the longer runway xx that was aligned with my arrival path. Reaching the approach end of runway xx that was aligned with my arrival path. Reaching the approach end of runway xx with 2000 MSL I asked the tower for runway elevation and heard 500 feet. That matched my visual estimate of 1500 AGL. I started a 360 turn to the left to descend to the runway. ATC asked me to tell them when the gear was down. I lowered the gear normally and completed the 360 turn to short final at about 500 AGL. I entered the heavy rain just before landing. Landing was uneventful; and I chose to taxi off the runway rather than shut down immediately. Tower gave me progressive taxi instructions; including immediately clearing me onto runway 12-30 when I turned there rather than taxiway B. I shut down mixture; and ignition; then thanked tower and asked him to relay my thanks to center. On the radio I told him no fire and no smoke. Then turned off the master electrical switch and exited the aircraft. ZZZ fire department was right there with a fire engine.in heavy rain; a very quick inspection showed that the propeller assembly was loose from the crankshaft. Several screw heads were gone from the spinner. Subsequent disassembly found four of the six propeller bolts were broken. Two broke at the base of the threads. Two nuts and washers were not recovered and presumable exited through the cowl. Two bolts were broken at about their midpoints. Both ends were held captive. The alignment lugs in the 4 inch prop extension had wobbled in their holes and wallowed out both the back of the propeller and the front face of the aluminum extension. The front face of the prop extension was heavily fretted. The extension was still securely bolted to the crankshaft. The back of the wooden propeller was heavily coated with black aluminum dust. After wiping that off; some burn areas were visible and detectable by smell also. The spinner back plate was heavily eroded where it was in contact with the propeller; and the alignment holes were deformed. Inside the cowl; a 3 inch scat tube feeding cold air to the oil cooler had come loose; explaining the high oil temperature. The engine mount; exhaust; and other components were inspected and showed no visible signs of damage. The observed damage is consistent with a loose propeller that started working with each revolution; generating heat; and fatiguing the (remaining?) bolts until they broke. The loss of adequate preload on the bolts could be due to humidity decrease between the time the prop was last torqued and the incident date; or due to defective or worn bolts. All four of the failed bolts show clean fracture. Some of the bolts were eroded after fracture. The outer surface of the bolts are heavily burnished by working within the prop alignment bushings while loose. I last checked the torque on the prop bolts during the annual conditional inspection approximately one month before the incident. The torques were recorded as 16 ft-lbs on a piece of tape affixed to the prop hub. The bolts were AN6-56; with prevailing torque all-metal locking nuts. The preflight inspection included pushing and pulling on the propeller and tapping the rim of the spinner to listen for any looseness. No anomalies were detected in preflight inspection. In recent flights; I had noticed some minor vibration; and intended to do a dynamic balancing procedure within the next week. In the past; I have been able to get to under 0.02 ips (inches per second) vibration at the front of the engine. I am satisfied with my inflight decisions; though I question whether I should have shut the engine down once I was sure of reaching the runway. A departing propeller or failure of the engine mount; which also carries the nose gear assembly; could have made things much worse. My experiences in gliders and in formation flying with military-style overhead approaches were both useful. ATC at both center and ZZZ tower were superb. They provided everything I needed; when I needed it.in retrospect; I could have checked the torque again as the weather dried this spring. I could have used new bolts instead of re-using bolts. When making repairs; I will use the larger 7/16 inch or ½ inch bolts and a larger 7 inch prop flange and crushplate. I may choose a composite prop that is not sensitive to weather changes.

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Original NASA ASRS Text

Title: Lancair 235 pilot reported a vibration in cruise which increased in a turn. After diverting; post flight disassembly revealed that four of the six propeller bolts were broken and the propeller assembly was loose from the crankshaft.

Narrative: I was flying with flight following from Center. ATC was very helpful as I detoured around two thunderstorm cells over ZZZ. Cloud bases ranged from 5000-8000 MSL. Cloud tops were about 14000 MSL. ZZZ was in heavy rain. About ten miles northwest of ZZZ; a vibration started. I added carb heat; no change. I checked both magnetos; suspecting a fouled spark plug. Vibration continued on either or both magnetos. The EGT of all four cylinders remained high in all magneto switch positions indicating that the engine had combustion in all four cylinders. I started a precautionary turn toward lower terrain. The vibration suddenly got worse. I informed ATC of the vibration and announced my intention to go to ZZZ. I accidentally said 'ZZZ1' then amended it to ZZZ. ATC was very responsive. Having narrowed down the range of possible problems; none of them good; [advised ATC]. I asked ATC for heading to ZZZ and to tell the tower I was coming. ATC provided winds; and frequencies. I reduced throttle and descended. ATC asked if I was unable to maintain altitude. I replied that I was reducing power and descending to the airport. The vibration got worse in sudden steps. When I reduced RPM; the amplitude of the vibration got worse. I could still see clearly and could see that the cowl was shaking; but did not see any evidence of things banging it from the inside. Oil pressure was normal. Oil temperature was high at 220F (usually 180-190F). I continually considered shutting off the fuel and ignition as I had enough altitude to safely reach the runway as a glider; and have practiced that scenario. I left the engine running at reduced power but was unsure of that decision and ready to change it.A few miles from ZZZ; I switched frequencies to ZZZ tower. ZZZ ATC read out descriptions of the runways. I chose the longer RWY XX that was aligned with my arrival path. Reaching the approach end of Runway XX that was aligned with my arrival path. Reaching the approach end of Runway XX with 2000 MSL I asked the tower for runway elevation and heard 500 feet. That matched my visual estimate of 1500 AGL. I started a 360 turn to the left to descend to the runway. ATC asked me to tell them when the gear was down. I lowered the gear normally and completed the 360 turn to short final at about 500 AGL. I entered the heavy rain just before landing. Landing was uneventful; and I chose to taxi off the runway rather than shut down immediately. Tower gave me progressive taxi instructions; including immediately clearing me onto runway 12-30 when I turned there rather than taxiway B. I shut down mixture; and ignition; then thanked Tower and asked him to relay my thanks to Center. On the radio I told him no fire and no smoke. Then turned off the master electrical switch and exited the aircraft. ZZZ fire department was right there with a fire engine.In heavy rain; a very quick inspection showed that the propeller assembly was loose from the crankshaft. Several screw heads were gone from the spinner. Subsequent disassembly found four of the six propeller bolts were broken. Two broke at the base of the threads. Two nuts and washers were not recovered and presumable exited through the cowl. Two bolts were broken at about their midpoints. Both ends were held captive. The alignment lugs in the 4 inch prop extension had wobbled in their holes and wallowed out both the back of the propeller and the front face of the aluminum extension. The front face of the prop extension was heavily fretted. The extension was still securely bolted to the crankshaft. The back of the wooden propeller was heavily coated with black aluminum dust. After wiping that off; some burn areas were visible and detectable by smell also. The spinner back plate was heavily eroded where it was in contact with the propeller; and the alignment holes were deformed. Inside the cowl; a 3 inch SCAT tube feeding cold air to the oil cooler had come loose; explaining the high oil temperature. The engine mount; exhaust; and other components were inspected and showed no visible signs of damage. The observed damage is consistent with a loose propeller that started working with each revolution; generating heat; and fatiguing the (remaining?) bolts until they broke. The loss of adequate preload on the bolts could be due to humidity decrease between the time the prop was last torqued and the incident date; or due to defective or worn bolts. All four of the failed bolts show clean fracture. Some of the bolts were eroded after fracture. The outer surface of the bolts are heavily burnished by working within the prop alignment bushings while loose. I last checked the torque on the prop bolts during the annual conditional inspection approximately one month before the incident. The torques were recorded as 16 ft-lbs on a piece of tape affixed to the prop hub. The bolts were AN6-56; with prevailing torque all-metal locking nuts. The preflight inspection included pushing and pulling on the propeller and tapping the rim of the spinner to listen for any looseness. No anomalies were detected in preflight inspection. In recent flights; I had noticed some minor vibration; and intended to do a dynamic balancing procedure within the next week. In the past; I have been able to get to under 0.02 IPS (Inches per Second) vibration at the front of the engine. I am satisfied with my inflight decisions; though I question whether I should have shut the engine down once I was sure of reaching the runway. A departing propeller or failure of the engine mount; which also carries the nose gear assembly; could have made things much worse. My experiences in gliders and in formation flying with military-style overhead approaches were both useful. ATC at both Center and ZZZ Tower were superb. They provided everything I needed; when I needed it.In retrospect; I could have checked the torque again as the weather dried this spring. I could have used new bolts instead of re-using bolts. When making repairs; I will use the larger 7/16 inch or ½ inch bolts and a larger 7 inch prop flange and crushplate. I may choose a composite prop that is not sensitive to weather changes.

Data retrieved from NASA's ASRS site 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.