N121-043 TITLE: Landing Gear Structural Health Prognostic/Diagnostic System

TECHNOLOGY AREAS: Air Platform, Information Systems, Materials/Processes, Sensors

OBJECTIVE: Develop a low-cost, highly reliable, multifunction prognostic/diagnostic system to assess and monitor the structural health of airframe landing gear systems and improve on methods currently used for fatigue damage tracking of these systems.

DESCRIPTION: Current methods of tracking fatigue damage of airframe landing gear systems and fuselage support structures depend on data collection of aircraft parameters recorded onboard at various sampling rates by structural health monitoring (SHM) devices. These service usage data are processed postflight using applications such as Structural Appraisal of Fatigue Effects (SAFE) to identify flight conditions, recognize landing and ground maneuver events, and determine any overload or exceedance occurrences. The service data are used to determine the airframe's maintenance and inspection requirements and ensure the landing gear systems' flight-worthy condition/flight safety, which can help extend the life of the aircraft and reduce life-cycle costs.

However, the data collected to track fatigue damage has its limitations. Limited sets of parameters recorded and low sampling rates allow for gaps in data quality, requiring conservative assumptions to be made for the characterization of landing, ground, and braking events. Errors in aircraft gross weight and center of gravity (CG) estimations require additional conservatism to be applied to fatigue damage tracking values and fatigue life expended values of landing gear systems and their back-up structure. These assumptions and conservatisms quite often result in too many inspections, premature replacement of parts, and/or an inability to use aircraft to their full capacity (i.e., need to minimize cargo and crew).

New technologies for sensors, wireless communication devices, and miniaturized data acquisition systems have presented a significant opportunity to quickly obtain, via hardware and software modules, a multifunction SHM system that can directly measure landing gear and support structure loads for use in fatigue damage tracking, provide a method for capturing improved estimations of gross weight and CG, and at the same time provide prognostic/diagnostic methods for assessing the condition of landing gear components. Direct load monitoring provides an improved solution to accurately calculate fatigue damage on landing gear components, minimizing assumptions and conservatisms and significantly enhancing the ability to recognize landing and ground exceedances. With improved gross weight and CG estimations, cargo and crew capacities can be maximized to published limits in order to get full benefit out of the aircraft, leading to life-cycle cost reductions and optimized mission capabilities. By being able to assess and check the condition of landing gear components (e.g., tires, brakes, pistons, oleos) individually, maintenance requirements and inspections intervals can be adjusted to ensure that maintenance actions are performed only when appropriate and necessary.

PHASE I: Develop and demonstrate the technical feasibility of a multifunction SHM prognostic/diagnostic system that will capture and assess the structural data required to accomplish fatigue damage tracking of landing gear systems, improve on weight and CG estimations, and provide a method to evaluate the condition of individual landing gear components.

PHASE II: Fully develop, demonstrate, and validate a multifunction SHM prognostic/diagnostic system in a laboratory environment and on representative landing gear systems and associated back-up structures that it is low cost and highly reliable and improves on existing methods for fatigue damage tracking of these systems. Demonstrate direct load monitoring capabilities at key locations in the landing gear's load path, taking into account any hardware, software, environmental, and weight sensitivities at these key locations. Develop and provide an overall cost�benefit analysis for the multifunction system developed.

PHASE III: Transition the multifunction SHM system for implementation by original equipment manufacturers or onto an existing aircraft platform.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Similar to naval aircraft, commercial aircraft experience fatigue, which can culminate in cracks and lead to complete fracture after a sufficient number of load cycles. Structural deterioration in aging aircraft increases the maintenance workload, reduces aircraft readiness, and potentially increases safety risks. A multifunction structural health monitoring prognostic/diagnostic system that captures and assesses the structural data required to accomplish fatigue damage tracking of landing gear systems can ensure that maintenance requirements and inspection intervals can be adjusted so that maintenance actions are indeed appropriate and necessary.

REFERENCES:
1. Yool, K., Sheehy, S., & Lenhardt, D. (2006). A survey of aircraft structural-life management programs in the U.S. Navy, the Canadian Forces, and the U.S. Air Force. Santa Monica, CA: RAND Corporation. Available at http://www.rand.org/pubs/monographs/2006/RAND_MG370.pdf

2. Sohn, H., Farrar, C. R., Hemez, F. M., Shunk, D. D., Stinemates, D. W., Nadler, B. R., & Czarnecki, J. J. (2004). A review of structural health monitoring literature. Los Alamos, NM: Los Alamos National Laboratory. Available at http://institute.lanl.gov/ei/shm/pubs/LA_13976_MSa.pdf

KEYWORDS: landing gear; prognostic; diagnostic; structural health; fatigue tracking

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