History of Flight
The flight departed from a private road in Juliaetta about 1720 Pacific standard time on March 11, 2016. The pilot, the sole occupant, was operating under 14 Code of Federal Regulations Part 137 for aerial application. The purpose was to apply 1,600 pounds of dry fertilizer to wheat fields; the pilot had completed about 20 loads earlier in the day, equating to about 4 hours of flight time. With a half-full fuel tank, the airplane climbed to about 500 feet above ground level (agl). After configuring the airplane to the appropriate manifold pressure and turning the carburetor heat on, the pilot maneuvered toward the intended field. In level flight at about 300 feet agl, the engine began to violently shudder and make loud backfiring noises, experiencing a partial loss of power. Unable to maintain altitude, the airplane descended, landed hard, and nosed over, coming to rest inverted.
Aircraft Information
The Grumman G-164A (serial number 1730) was equipped with a Pratt and Whitney R985-AN14B engine (serial number P225620). The operator provided excerpts from the engine logbooks that included the AD lists and the last maintenance performed. The records indicated that the last annual inspection was completed in April 2015 at a tachometer time of 5,402 hours; the tachometer time at the time of the accident was 5,435 hours, or about 35 hours after the maintenance.
In May 1978, the Federal Aviation Administration (FAA) issued Airworthiness Directive (AD) 78-08-07 applicable to Pratt & Whitney R-985 series engines, requiring periodic visual inspections for cracks in the cylinder heads at specified intervals. According to the AD, visual inspections are required at intervals not exceeding 100 or 150 hours of time in service, depending on whether the cylinder heads have been ultrasonically inspected.
The logbook excerpts contained a document listing AD 78-08-07, showing most recent compliances in April 2007 and April 2015 (during which about 1,970 flight hours accrued). The entirety of the AD list only showed AD 78-08-07 as being complied with in May 2001; the only AD showing compliance thereafter was in April 2015 for the cylinder hold-down nuts as per AD 56-06-02.
Additional Information
According to the NTSB aviation accident database, after AD 78-08-07 became effective, there were three accidents involving Pratt and Whitney R-985 engines that had a cylinder failure. A similar query of the FAA's Service Difficulty Report (SDR) database found 34 reports documenting a cylinder crack or separation as the cause of the service difficulty.
A representative at Covington Aircraft Engines, a facility specializing in R-985 engines, stated that cracks in cylinders are often due to two factors: a worn carburetor causing the engine to run too lean (creating hotspots in the cylinder), and thermal fatigue cracks from shock cooling (excessively rapid descent causing high temperature differences within the metal).
Transport Canada Civil Aviation issued Service Difficulty Advisory No. AV-2007-2 regarding R-985 cylinder heads, stating that proper warm-up and cool-down procedures are important to minimize distress and that failure to heat-stretch the cold cylinder before applying high power can lead to fatigue cracks and distress to the cylinder head and other rotating parts.
Tests and Research
Post-accident examination revealed that cylinder no. 2 was cracked around almost the entire circumference of the barrel between cooling fins. The carburetor (part number 391598) was examined at Precision Engines; bench testing revealed no anomalies that would have precluded normal operation. Upon disassembly, investigators found no debris in the main metering nozzle, float needle seat, idle metering tube, and accelerator pump, and no debris was noted in the metering jets.