A PA30B; twin comanche; pilot canceled his IFR clearance and returned to his departure airport after...

Narrative:

After reaching cruise altitude of 6;000 MSL and setting cruise power I heard a bang and the aircraft exhibited a medium to high rate of vibration in the floor. I verified engine operation was good and looked for any missing [aircraft components]. Not being able to find the problem I canceled my IFR flight plan with approach and turned toward a nearby suitable airport. I slowed to 120 mph and begin a 500 foot per minute descent. As the aircraft remained controllable I determined I could be on ground at same time if I continued to back to my departure airport. Upon landing the vibration got worse as aircraft was in flare. On landing rollout I noticed the left spinner was moving about one inch side to side and so I shut down the left engine. The prop stopped with a damaged blade end up. I taxied single engine to the hanger and secured the aircraft.post flight inspection revealed four inches of one of the left prop [blades was] missing. The left side of the aircraft; just forward of the prop center line; had damage with a dent and cracked gel coat to the fiberglass nose at the battery jumper connection. There was no known prior impact to the blade or history of prop damage.

Original NASA ASRS Text

Title: A PA30B; Twin Comanche; pilot canceled his IFR clearance and returned to his departure airport after experiencing significant vibration ultimately found to be the result of the loss of several inches of one left prop blade which had broken off and struck the fuselage.

Narrative: After reaching cruise altitude of 6;000 MSL and setting cruise power I heard a bang and the aircraft exhibited a medium to high rate of vibration in the floor. I verified engine operation was good and looked for any missing [aircraft components]. Not being able to find the problem I canceled my IFR flight plan with Approach and turned toward a nearby suitable airport. I slowed to 120 MPH and begin a 500 foot per minute descent. As the aircraft remained controllable I determined I could be on ground at same time if I continued to back to my departure airport. Upon landing the vibration got worse as aircraft was in flare. On landing rollout I noticed the left spinner was moving about one inch side to side and so I shut down the left engine. The prop stopped with a damaged blade end up. I taxied single engine to the hanger and secured the aircraft.Post flight inspection revealed four inches of one of the left prop [blades was] missing. The left side of the aircraft; just forward of the prop center line; had damage with a dent and cracked gel coat to the fiberglass nose at the battery jumper connection. There was no known prior impact to the blade or history of prop damage.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1137937
Time
Date	201312
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	PA-30 Twin Comanche
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Propeller Blade
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Flight Engineer Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 150 Flight Crew Total 18000 Flight Crew Type 750
Events
Anomaly	Aircraft Equipment Problem Critical

A PA30B; twin comanche; pilot canceled his IFR clearance and returned to his departure airport after experiencing significant vibration ultimately found to be the result of the loss of several inches of one left prop blade which had broken off and struck the fuselage.