DREW, MS, USA
N2368R
Air Tractor 301
The flight was departing from a 3,000 foot runway with a full load of fertilizer. The airplane became airborne, and according to the pilot, the engine lost power. The pilot was not able to maintain altitude, and the airplane impacted the ground about 2,500 feet from the departure end of the runway. The on scene investigation revealed that the No. 4 cylinder exhaust rocker box cover was fractured and missing, and appeared to not have been caused by impact with the ground. The No. 4 cylinder was removed from the engine and sent to the NTSB Materials Laboratory in Washington, D.C., for examination. The laboratory examination revealed that the No. 4 cylinder head had failed from fatigue cracking, probably the result of both mechanical and thermal stresses. It was likely that the fatigue originated first at the corners of the exhaust rocker shaft hole in boss '1', then initiated at localized casting defects in boss '2' as the fatigue in boss '1' grew and the local stresses increased in the remaining structure.
On July 23, 1999, about 1500 central daylight time, an Air Tractor 301, N2368R, registered to Ward Air Service Inc., was substantially damaged during takeoff at Ruleville-Drew Airport, Drew, Mississippi. The commercial-rated pilot reported no injuries. Visual meteorological conditions (VMC) prevailed in the vicinity, and no flight plan had been filed. The local aerial application flight was being conducted in accordance with Title 14 CFR Part 137. The flight was originating at the time. The flight was departing from a 3,000-foot runway with a full load of fertilizer. The airplane became airborne, and according to the pilot, the engine lost power. The pilot was not able to maintain altitude, and the airplane impacted the ground about 2,500 feet from the departure end of the runway. According to the FAA, the on-scene investigation revealed that the No. 4 cylinder exhaust rocker box cover was "...fractured and missing. This appeared to be not caused by the post accident phase or impact with the ground." The No. 4 cylinder was removed from the engine and sent to the NTSB Materials Laboratory in Washington, D.C., for examination. The NTSB Materials Laboratory factual report revealed that the No. 4 cylinder head had failed from fatigue cracking. It was found that the cylinder head was separated through the rocker arm bosses on the exhaust side. The fracture intersected the rocker shaft hole on the boss that had been labeled "1" for identification purposes, and the push rod tube attachment on both bosses "1" and "2". The rocker arm and the mating half of the fracture were not recovered. The fracture through boss "1" was flat, with evidence of casting porosity on the fracture surface. Cracking arrest positions were faintly recognizable on the surface of the fracture in boss "1". According to the NTSB Materials Laboratory factual report, "...the curvature of these arrest markings and the flow patterns on the surface were consistent with fatigue cracking that originated in the corners between the rocker shaft hole and the surface of the casting." The origin areas revealed "heavy oxidation" which prevented identification of the exact origin locations. The fracture surface of boss "2" was stepped, containing several plateaus in the surface. Large ratchet marks (Steps in the surface formed when two fatigue cracks, growing independently on two different planes, join together and proceed as one crack front) joined these plateaus, and many smaller ratchet marks were noted within several of the levels. Multiple crack arrest locations were noted on the surface. Scanning electron microscopy of the surface revealed striations throughout the region. The ratchet marks and striation geometry were "consistent with fatigue from multiple origins...." Many of the origins seemed to originate from two casting discontinuities. These discontinuities did not contain the smooth, nodular surface of the many shrinkage voids that were noted in the material, but appeared relatively flat with a large amount of oxide on the surface. (See the NTSB Materials Laboratory factual report, an attachment to this report.) The reported temperature in the vicinity about the time of the accident was 96 degrees F, and the winds were from 330 degrees, at 3 knots. The calculated density altitude was 2,559 feet. The field elevation was 137 feet.
a loss of engine power due to the failure of cylinder No. 4 boss, which was caused by fatigue cracking, resulting in a forced landing in marshy terrain and subsequent nose over.
Source: NTSB Aviation Accident Database
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