Posted for "Jeffrey Liegner, MD" <liegner@embarqmail.com>:

 
 My Malibu friends remind me that a similar piston engine/prop combo lost it's
prop in Aspen last Winter, and returned to the airfield successfully.  The
NTSB brief report is below.  Two seemingly identical events makes this a
significant potential problem of the crank shaft.
 
 <http://www.ntsb.gov/ntsb/brief.asp?ev_id=20071231X02008&key=1>http://www.ntsb.gov/ntsb/brief.asp?ev_id=20071231X02008&key=1
 
  NTSB Identification: DEN08IA044
 14 CFR Part 91: General Aviation
 Incident occurred Wednesday, December 26, 2007 in Aspen, CO
 Probable Cause Approval Date: 12/24/2008
 Aircraft: PIPER PA-46-310P, registration: N47BC
 Injuries: 4 Uninjured.
 During initial climb at 16,000 feet, the pilot noticed oil droplets forming
on his windscreen. On seeing the oil, he contacted air traffic control,
declared an emergency, and began a standard rate, 180-degree turn, back to the
airport. As he completed the turn, he heard a "boom," felt a shutter, and then
the windscreen was covered with oil. The pilot said he may have noted a pitch
up with the bang, but he immediately got on the best glide speed, 90 knots,
and began retracing his ground track via the global positioning system (GPS)
back to the airport. The airplane landed uneventfully back to the airport. An
examination of the airplane showed minor damage to the front of the cowling,
and the engine showed the crankshaft fractured aft of the flange. The
propeller and spinner, and the forward part of the crankshaft were missing.
Several months after the incident, the separated propeller assembly and
crankshaft were located. Examination of the engine revealed the crankshaft was
fractured through, aft of the propeller flange. The fracture of the crankshaft
was caused by a fatigue crack that stemmed from the aft relief radius of the
propeller flange. The surface of the aft radius contained no evidence of
mechanical damage and the fatigue crack emanated from multiple origins over a
wide area, indicating that a great amount of stress was necessary to initiate
the fatigue crack. The reason for the crack initiation could not be
determined. The fatigue crack origin area was located in the relief radius in
line with the axis of the propeller blades, consistent with forward and aft
bending stresses imposed by the blades during flight. The location indicates a
possibility that the blades were out of balance while rotating or that the
crankshaft was somehow damaged in the past. No evidence of fretting was noted
between the faces of the crankshaft propeller flange and the alternator
pulley. The propeller assembly was attached firmly to the crankshaft.
 The National Transportation Safety Board determines the probable cause(s) of
this incident as follows:
 The failure of the crankshaft due to fatigue. The fatigue crack
 initiation was not determined.


[I happened to have some photos of that Malibu... one is attached here if you're interested.  <Marv>   ]