History of Flight
On October 27, 2014, about 0945 Pacific daylight time, an Air Tractor AT-301, registration N8878S, collided with a center pivot irrigation system following a total loss of engine power in Basin City, Washington. The aircraft was operated by B and R Aerial Crop Care, Inc., under 14 CFR Part 137. The commercial pilot, the sole occupant, was not injured; the airplane sustained substantial damage. The flight departed from Connell City Airport, Connell, Washington, about 0925 for a local aerial application flight. Visual meteorological conditions prevailed, and no VFR flight plan was filed.
The pilot stated the purpose was to apply triticale seed to a field about 15 miles from the departure airport. After a reconnaissance pass over the field, the pilot circled back. When the airplane was about 30-50 feet above ground level (agl), the engine experienced a total loss of power. He maneuvered to an adjacent field, and during the landing flare (about 5 feet agl), the engine momentarily restarted for 2-3 seconds, propelling the airplane into the irrigation system.
Aircraft and Engine Information
The airplane was an Air Tractor AT-301, serial number 301-0225, equipped with a Pratt and Whitney R-1340-S3H1-G engine, serial number 32847. Post-accident examination revealed a crack in one cylinder head located in the dome between the forward and aft spark plug holes.
Maintenance and Inspection History
In July 1999, FAA issued Airworthiness Directive (AD) 99-11-02 for Pratt & Whitney R-1340 series engines, requiring periodic visual inspections for cracks in cylinder heads and fluorescent penetration (FP) inspections at overhaul. Pratt & Whitney Service Bulletin (SB) 1787 detailed inspection methods, focusing on cooling fin roots in the dome area between fins 7-8 and 8-9.
For uncowled and unbaffled engines like the Air Tractor AT-301, visual inspections were required every 100 hours. The engine log showed an inspection on April 22, 2014, at tachometer time 1,179.7 hours (692.7 hours since major overhaul). The previous entry on April 19, 2013, showed 999 hours (512 since overhaul). No other AD inspections were noted between these entries. At the accident, tachometer time was 1,404.1 hours, 224.4 hours after the last inspection. The airframe log entry for April 22, 2014, noted the next inspection due in 100 hours. Stamps "FP02 R" and "FP06 R" observed above the intake port indicated FP inspections in 2002 and 2006.
Additional Information
The NTSB aviation accident database recorded 23 accidents involving Pratt and Whitney R-1340 engines with cylinder failure after AD 99-11-02 effective July 1999. Among these, 9 involved circumferential cracks (head to barrel separation), 4 involved dome cracks between spark plug holes (similar to this accident), 3 in the rocker boss area, and 7 unspecified location. None resulted in fatal injury.
For the four dome-crack accidents, inspection intervals varied; none occurred within the required 100-hour interval. The FAA's Service Difficulty Report database documented 22 reports of cylinder cracks, including 7 circumferential, 6 between spark plug holes, and others at different locations.
In February 2009, FAA issued SAIB NE-09-09 alerting to potential cylinder head cracks at the exhaust port on R-1340 engines, recommending that visual inspections per AD 99-11-02 include the exhaust port area and that FP inspections cover the entire cylinder.
Tests and Research
The cracked cylinder was examined at the NTSB Materials Laboratory. The crack was in the dome between the forward and aft spark plug holes, substantial enough to prohibit combustion. Origin areas were between fins 8 and 9, within fin 9, and between fins 11 and 12. The crack extended through the head thickness to the interior surface. Fracture surfaces showed features consistent with fatigue, with multiple origins at the exterior.