Narrative:

Aircraft was being flown in loose formation with another aircraft of the same type. A minor vibration was noticed and the aircraft was turned around to head to home airport. Within 3 mins the vibration had become extreme and the power was reduced to idle. Within 30 seconds after total power reduction the propeller, spinner and propeller hub departed the aircraft. An emergency landing was affected on the freeway below resulting in no damage to persons, property or the aircraft. There was no warning of any problem prior to the vibration. The only action that could have been taken was, I was well rested the night before and had always been somewhat prepared for the possibility of an engine failure especially since it was an automatic conversion for aircraft. The engine was disassembled and returned to new condition 109 hours prior to the failure, including magnaflux of the crankshaft. A new engine will be built including a new crankshaft. According to the local FAA office, handling the investigation it was classed as an incident as there was no damage or injury of any kind.

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Original NASA ASRS Text

Title: PLT FLYING EXPERIMENTAL ACFT LOSES SPINNER, PROPELLER AND HUB AFTER BRIEF VIBRATION. EMERGENCY LNDG.

Narrative: ACFT WAS BEING FLOWN IN LOOSE FORMATION WITH ANOTHER ACFT OF THE SAME TYPE. A MINOR VIBRATION WAS NOTICED AND THE ACFT WAS TURNED AROUND TO HEAD TO HOME ARPT. WITHIN 3 MINS THE VIBRATION HAD BECOME EXTREME AND THE PWR WAS REDUCED TO IDLE. WITHIN 30 SECS AFTER TOTAL PWR REDUCTION THE PROP, SPINNER AND PROP HUB DEPARTED THE ACFT. AN EMER LNDG WAS AFFECTED ON THE FREEWAY BELOW RESULTING IN NO DAMAGE TO PERSONS, PROPERTY OR THE ACFT. THERE WAS NO WARNING OF ANY PROB PRIOR TO THE VIBRATION. THE ONLY ACTION THAT COULD HAVE BEEN TAKEN WAS, I WAS WELL RESTED THE NIGHT BEFORE AND HAD ALWAYS BEEN SOMEWHAT PREPARED FOR THE POSSIBILITY OF AN ENG FAILURE ESPECIALLY SINCE IT WAS AN AUTO CONVERSION FOR ACFT. THE ENG WAS DISASSEMBLED AND RETURNED TO NEW CONDITION 109 HRS PRIOR TO THE FAILURE, INCLUDING MAGNAFLUX OF THE CRANKSHAFT. A NEW ENG WILL BE BUILT INCLUDING A NEW CRANKSHAFT. ACCORDING TO THE LCL FAA OFFICE, HANDLING THE INVESTIGATION IT WAS CLASSED AS AN INCIDENT AS THERE WAS NO DAMAGE OR INJURY OF ANY KIND.

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.