History of Flight
On April 17, 2015, about 1615 Pacific daylight time, a Piper PA-18-150, registration N162SC, made an off-airport landing near La Quinta, California. The commercial pilot, who was also the owner, was operating under 14 CFR Part 91 on a local banner tow flight that had departed Bermuda Dunes (UDD) at 1531. Visual meteorological conditions prevailed, and no flight plan was filed.
The pilot reported that while orbiting over the advertising destination at 1,200 ft above ground level (agl), the airplane began to shake violently and lose altitude. He initiated a right turn away from the banner pattern, reduced power to idle, and the shaking stopped. After making an emergency radio call on the Bermuda Dunes common traffic advisory frequency, he released the banner over an empty golf course fairway at 600 ft agl. Advancing the throttle caused the severe vibration to return, and neither a climb nor level flight could be maintained. The pilot selected the next empty fairway as a landing site, deployed full flaps, and performed a forward slip to touch down. At about 100 to 200 ft agl, he noticed significant elevation changes in the landing area but could not change plans. The airplane touched down on a plateau at 60 knots indicated airspeed (KIAS) but could not stop before encountering a 10-foot drop-off. It became airborne, bounced onto the next plateau (the green), and continued despite full brake application. With a tree line and houses ahead, the pilot attempted a 90° turn; the aircraft contacted the ground sideways, rolled at a 45° angle to the 2-o'clock position, and came to rest inverted. The pilot released his seatbelt and exited.
Damage and Examination
A post-accident examination by the National Transportation Safety Board (NTSB) and the Federal Aviation Administration revealed no anomalies with the airframe or engine that would have precluded normal operation. One magneto passed functional checks. The airplane sustained substantial damage; the pilot sustained minor injuries.
Propeller Blade Analysis
Several inches of one propeller blade tip were missing and not located. The fracture surface was flat and was sent to the NTSB Materials Laboratory. The laboratory reported that the fracture surface was generally flat with a finely textured appearance and a unified crack front consistent with fatigue crack progression. Fatigue cracking had propagated through approximately 83% of the blade cross-section, with overstress separation covering the remaining 17%. The fatigue cracking appeared to initiate from one general area on the pressure side of the blade, which contained an anomaly. This anomaly consisted of deformed material. Parallel lines consistent with hand finishing were on the external surface, overlapping into the deformed area. Thinner circular lines consistent with orbital grinder machining marks overlapped the hand finishing lines. Scanning electron microscope examination revealed feathery features and striations consistent with fatigue cracking emanating from the pressure side in the area of the anomaly. No contamination was observed at the fatigue initiating site.
Propeller Maintenance Records
A review of the propeller logbook showed it was overhauled on June 13, 2011, and installed on the accident airplane on June 19, 2011. An annual inspection on March 1, 2014, recorded 281.9 hours since overhaul. On October 23, 2014, the propeller was removed at 393.6 hours since overhaul and reinstalled on October 30. An entry dated February 23, 2015, for an annual inspection indicated the propeller had been dynamically balanced, inspected, and was in airworthy condition; the time since overhaul was recorded as 380.3 hours, but correlation with the tachometer determined it should have been 480.3 hours. The tachometer read 948.68 hours at the wreckage examination. No logbook entry, including the overhaul, mentioned any work to a deformed area on the propeller.