Incident Overview
On July 23, 2018, at approximately 1215 central daylight time, a Grumman AA-5B (registration N74420) sustained substantial damage while maneuvering near Brazoria, Texas. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight with a flight instructor and a student pilot on board; neither occupant was injured.
Sequence of Events
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 one of the propeller blades had separated about midspan. The flight instructor then took control of the airplane and completed an uneventful forced landing in a nearby field.
Postaccident Examination
A postaccident examination of the airplane 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 of the separated blade 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 revealed a pattern of crack arrest lines and striations 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 obscured the exact initiation point of the fatigue cracking.
Maintenance History
According to the propeller logbook, the most recent propeller inspection was completed in conjunction with an annual inspection on November 1, 2017. At that 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.