On June 8, 2008, at 1315 eastern daylight time, a Piper PA-44-180, N159WM, sustained substantial damage due to a separated propeller blade tip impacting the airplane's nose during cruise near Battle Creek, Michigan. The airplane landed without further incident at the W K Kellogg Airport (BTL), near Battle Creek, Michigan. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 instructional flight was not operating on a flight plan. The certified flight instructor (CFI) and the commercial pilot receiving dual instruction were uninjured. The local flight originated from BTL about 1230. The flight was returning to BTL at the time of the accident. CFI's report, in part, stated: A large BANG was heard and felt 8 to 10 miles south east of the airport. At that point we scanned the gauges and saw no sign of problems. When we looked out the windscreen; however we saw a large depression in the nose section of the aircraft. Initially we thought we had ... a bird strike. N159WM was a 2002, Piper PA-44-180, which was a twin engine, four seat airplane with serial number 4496159. The airplane was powered by two Lycoming engines. Each engine drove its own Hartzell propeller. The propeller blade was a model FJC7666A-2R that was installed in a model HC-C2YR-2CLEUF hub assembly. The serial number of the blade with the tip separation was J73543. The airplane accumulated a total time of 1,997 hours. The outboard section of the remaining propeller blade was cut away and sent to the National Transportation Safety Board's Materials Laboratory. A Senior Materials Engineer examined the propeller section and produced Materials Laboratory Factual Report number 08-097. The report referred to repair data from the propeller manufacturer's website indicating that a 10x magnifying glass should be used to examine repairs to ensure damage, file marks, and any course surface finish are removed prior to returning the propeller back to service. The laboratory report, in part, stated: Fatigue features emanated from a fatigue origin area located at the leading edge near the flat side of the blade ... . ... Angled file marks consistent with a leading edge repair were observed between the leading edge and the flat side of the blade ... .