HISTORY OF FLIGHTOn March 11, 2016, about 1725 Pacific standard time, an Grumman G-164A, N6894Q, experienced a partial loss of power and collided with terrain during an off airport landing in Juliaetta, Idaho. Heimgartner Aviation LLC., was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 137. The commercial pilot, the sole occupant, was not injured; the airplane sustained substantial damage. The local aerial application flight departed from a private road in Juliaetta about 1720. Visual meteorological conditions prevailed and the pilot did not file a visual flight rules (VFR) flight plan. The pilot stated that the purpose of the flight was to apply 1,600 pounds of (dry) fertilizer to wheat fields. He had completed around 20 loads earlier in the day which equated to about 4 hours of flight time. The airplane departed with a half-full fuel tank and 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 the airplane toward the field he intended to spray. While in level flight, about 300 feet agl, the engine began to violently shudder and make loud backfiring noises. The engine experienced a partial loss of power. The airplane continued to descend, unable to maintain level altitude. The airplane landed hard and nosed over, coming to rest inverted. AIRCRAFT INFORMATIONThe airplane, a 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 recorded as being completed in April 2015 at a tachometer time of 5,402 hours; the tachometer time at the time of the accident was 5,435, or about 35 hours after the maintenance. In May 1978 the Federal Aviation Administration (FAA) issued an Airworthiness Directive (AD) 78-08-07 applicable to Pratt & Whitney R-985 series engines. The AD required periodic visual inspections for cracks in the cylinder heads at specified intervals of time in service. According to the AD, visual inspections of the cylinder heads are required at intervals not to exceed 100 or 150 hours of time in service, depending on whether the they have been ultrasonically inspected. The logbook excerpts contained a document listing AD 78-08-07, which showed that the most recent compliances occurred in April 2007 and April 2015 (during which time about 1,970 flight hours accrued). The entirety of the AD list only showed 78-08-07 as being complied with in May 2001. On that list, the only AD that showed compliance thereafter was in April 2015 for the cylinder hold-down nuts as per AD 56-06-02. AIRPORT INFORMATIONThe airplane, a 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 recorded as being completed in April 2015 at a tachometer time of 5,402 hours; the tachometer time at the time of the accident was 5,435, or about 35 hours after the maintenance. In May 1978 the Federal Aviation Administration (FAA) issued an Airworthiness Directive (AD) 78-08-07 applicable to Pratt & Whitney R-985 series engines. The AD required periodic visual inspections for cracks in the cylinder heads at specified intervals of time in service. According to the AD, visual inspections of the cylinder heads are required at intervals not to exceed 100 or 150 hours of time in service, depending on whether the they have been ultrasonically inspected. The logbook excerpts contained a document listing AD 78-08-07, which showed that the most recent compliances occurred in April 2007 and April 2015 (during which time about 1,970 flight hours accrued). The entirety of the AD list only showed 78-08-07 as being complied with in May 2001. On that list, the only AD that showed compliance thereafter was in April 2015 for the cylinder hold-down nuts as per AD 56-06-02. ADDITIONAL INFORMATIONAccording to the NTSB aviation accident database, after AD 78-08-07 became effective there were a total of 3 accidents involving Pratt and Whitney R-985 engines that had a cylinder failure. A similar query was conducted of the FAA's Service Difficulty Report (SDR) database. Among the results, 34 of the reports documented a cylinder crack or separation as the cause of the service difficulty. According to a representative at Covington Aircraft Engines, an aircraft engine maintenance, repair and overhaul facility that specializes in R-985 engines, they seen many cracks in cylinders. He stated that the reasons for the cracks are predominantly twofold: the carburetor is worn, resulting in the engine running too lean (creating hotspots in the cylinder); and thermal fatigue cracks from the pilot shock cooling the engine (an excessively rapid descent going from a high temperature differences within the metal and not the absolute temperature of the metal). The Transport Canada Civil Aviation issued a Service Difficulty Advisory No. AV-2007-2 regarding R-985 cylinder heads. In pertinent part it stated, "It is very important that operators properly warm-up and cool-down the engine before and after flight. This will significantly minimize distress to the engine. It is essential that the cylinder assembly be adequately warmed up in order to "heat stretch" the cold cylinder, especially before applying high power. Failure to do so can lead to fatigue cracks and cause distress to the cylinder head and other rotating parts of the engine. Problems associated with cylinder head separation and cylinder barrel flange cracks can be minimized if attention to cylinder head temperature limitations is closely followed." TESTS AND RESEARCHA post accident examination revealed that cylinder no. 2 was cracked around almost the entire circumference of the barrel in between cooling fins. The carburetor, part number 391598, was examined at Precision Engines. The bench test 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; no debris was noted in the metering jets.