(Dr. David McDowell, advisor)

"A Comprehensive Approach to On-Condition Evaluation of Rotorcraft Structural Integrity"

Abstract

The Airframe Condition Evaluation (ACE) program is used to conduct an annual structural evaluation of every helicopter in the U.S. Army fleet.  The ACE assesses the condition of the fleet and identifies aircraft requiring depot maintenance.  However, the ACE has been understating the structural degradation occurring in the fleet.  This thesis discusses the author’s efforts to evaluate the performance of the ACE program in assessing the actual condition of the Army helicopter fleet.  A number of significant discoveries and conclusions are presented.  Analysis is presented showing that less than 1% of the causes of major Army helicopter mishaps are attributable to failure of the aircraft structure.  Statistical analysis of ACE data and accident data indicates that ACE is a better predictor of accidents due to material failure than operating hours.  An assessment of Army ACE findings by aircraft type revealing significant deficiencies in the ACE of the AH-64 aircraft is also presented.  An evaluation of ACE corrosion findings is presented revealing that pitting, exfoliation, and corrosion fatigue are not widespread problems on the UH-60 helicopter while general surface corrosion and filiform corrosion are significant problems in some areas of the aircraft.  Finally, a non-linear optimization model is presented which concludes that 8% of the aircraft fleet should be overhauled annually to maintain desired readiness in a cost-effective manner.

Plans to significantly improve the ACE have been developed as a part of this thesis.  These plans have actually been implemented for the UH-60 aircraft and include development of a separate corrosion index for the ACE and improvements to the existing structural index of the ACE.  Provision has also been made for better feedback processes between the ACE program and aircraft depot maintenance activities.  An evaluation of the application of new inspection technologies and on-board usage monitoring has also been completed.  An analysis of the potential for coupling between individual indicators is provided.  Finally, a number of areas are identified for future research and engineering analysis.