SEA01LA023

During a descent, and while passing through approximately 8,000 feet mean sea level, a sudden, violent airframe vibration developed in the twin engine Piper PA-30, and the pilot shut down the right engine. He then noticed a portion of the one of the propeller's tips missing. Metallurgical examination of the blade revealed a fatigue fracture initiating from a dent about 0.2 inch from the leading edge of the blade. The dent measured approximately 0.07 inch wide in the chordwise direction and about 0.03 inch deep and was covered with flat black paint. The propeller log indicated repeated historical occasions whereby the propeller had been dressed and painted. FAA Advisory Circular 43.13-1B/2A, 'Aircraft Inspection, Repair & Alterations' states in Section 4. 'Repair of Metal Propellers,' paragraph 8-73(a) 'Flaws in Edges,' that the mechanic should '...Exercise care to remove the deepest point of the injury and also remove any raised metal around the edges of the injury....'

On November 17, 2000, approximately 1630 mountain standard time (mst), a Piper PA-30, N7062Y, registered to a private owner, and being operated/flown by a commercial pilot, sustained substantial damage when approximately six inches of the outboard tip of one of the right engine propeller blades separated in flight during a cruise descent approximately 12 nautical miles east-southeast of Stevensville, Montana. The pilot and four passengers were uninjured. Visual meteorological conditions existed and a VFR flight plan was in effect. The flight, which was personal, originated from Bozeman, Montana, approximately 1545 MST, and was destined for Stevensville, Montana.

The pilot reported that he initiated a descent inbound to Stevensville and left the power unchanged. The engine RPM remained constant and several minutes into the descent, while passing through approximately 8,000 feet mean sea level, a sudden, violent airframe vibration ensued. The pilot, suspecting a problem with the right engine, reduced the power, and then feathered the right propeller. Once the propeller was feathered, the pilot was able to observe a portion of the one of the propeller's tips missing. The aircraft was successfully landed at Stevensville shortly thereafter.

The Hartzell propeller blade (serial number BGDP187) was examined at the Safety Board's Materials Laboratory. The metallurgical report revealed in part, the following:

Visual and magnified examinations of the fracture face found features indicative of a fatigue fracture. The fatigue initiated at a large sharp-bottomed dent on the flat face of the blade. The dent was located about 0.2 inches from the leading edge of the blade. The dent measured approximately 0.07 inch wide in the chordwise direction and about 0.03 inch deep. From the initiation site, the fatigue propagated through about 60% of the blade cross-section. The remaining fracture region was consistent with overstress separation from the fatigue area.

Close inspection found that the initiating dent was covered with flat black paint. Further, black paint covered many other damage locations on the flat face of the blade. Much of the damage appeared to be broad but shallow corrosion pits while others were mechanically induced dents. The leading edge of the blade showed several blending repair locations including one at the fracture location. The repairs left the blade with sharp leading edge radii (refer to attached metallurgical report).

According to the aircraft propeller log the right engine propeller (serial number BGDP187) was overhauled on February 22, 1983. The time on the propeller was not recorded on that date and the log indicated that the total time was unknown. A stamp following this entry indicated that the overhaul interval for this propeller was 1500 hours or 4 calendar years.

The log contained an entry dated November 20, 1989, and at a tach time of 3929.47 hours, which stated, "Dress prop, inspect per 100/annual inspection checklist AD's PCW, see above, and are found to be airworthy."

The next sequential entry dated June 29, 1991, and at a tach time of 3966.03 hours, stated, "Inspection for 100 hr/annual AD 900223 NA for this application (repaired spinner)."

The next sequential entry dated December 17, 1997, and at a tach time of 4184.3 hours, stated, "Filed & painted, Greased & Checked Pressure."

The next sequential entry dated January 6, 1999, and at a tach time of 4253.3 hours, stated, "Cleaned & painted prop. Checked pressure & greased prop."

The last sequential entry dated January 18, 2000, and at a tach time of 4291.2 hours, stated "Filed & painted blades, checked pressure & greased."

FAA Advisory Circular 43.13-1B/2A, "Aircraft Inspection, Repair & Alterations" states in Section 4. "Repair of Metal Propellers," paragraph 8-73(a) "Flaws in Edges," that the mechanic should "...Exercise care to remove the deepest point of the injury and also remove any raised metal around the edges of the injury..." (refer to ATTACHMENT AC-I).