Brazoria, TX, USA
N74420
Grumman AA5
The student pilot and his flight instructor were conducting a local instructional flight when the airplane began to shake violently. A reduction of engine power revealed that the one of the propeller blades had separated about midspan. The flight instructor took control of the airplane and completed an uneventful forced landing in a nearby field. A postaccident examination revealed several fractured engine mounts. A laboratory examination of the propeller revealed that about one-half of one blade had fractured. Further examination of the flat portion of the fracture surface revealed a pattern of crack arrest lines and striations that were consistent with fatigue cracking. The pattern of crack arrest lines indicated that the fatigue cracking emanated from the trailing edge tip of the blade. Damage to the fracture surface at the trailing edge tip of the blade obscured the exact initiation point of the fatigue cracking. The propeller had accumulated about 4,499 hours since its last overhaul, and about 11 hours since it was last examined during an annual inspection.
On July 23, 2018, about 1215 central daylight time, a Grumman AA-5B, N74420, was substantially damaged while maneuvering near Brazoria, Texas. The flight instructor and student pilot were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. According to the student pilot, the airplane began to shake violently while he practiced basic-attitude maneuvers under simulated instrument meteorological conditions. A reduction of engine power revealed that the one of the propeller blades had separated about midspan. The flight instructor took control of the airplane and completed an uneventful forced landing in a nearby field. A postaccident examination revealed several fractured engine mounts. The two-blade propeller, a McCauley Propeller Systems model No. 1A170/FFA7563, serial No. P77410, was examined at the National Transportation Safety Board Materials Laboratory in Washington, D.C. About one-half of one blade had separated and was not located during the investigation. The fracture surface was flat from the trailing edge through roughly two-thirds of the chord length, before angling from the center outwards adjacent to the blade surface. Further examination of the flat portion of the fracture surface revealed a pattern of crack arrest lines and striations that were consistent with fatigue cracking. The pattern of crack arrest lines indicated that the fatigue cracking emanated from the trailing edge tip of the blade. Damage to the fracture surface at the trailing edge tip of the blade obscured the exact initiation point of the fatigue cracking. According to the propeller logbook, the most recent propeller inspection was completed in conjunction with an annual inspection on November 1, 2017, at which time the propeller had accumulated 4,488 hours since its last overhaul. The airplane had been flown about 11 hours since the last annual inspection.
The propeller blade separation due to a fatigue fracture.
Source: NTSB Aviation Accident Database
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