Mary S. Reveley

Glenn Research Center, Cleveland, Ohio

Tolga Kurtoglu

Mission Critical Technologies, Moffett Field, California

Karen M. Leone

ASRC Aerospace Corporation, Cleveland, Ohio

Jeffrey L. Briggs and Colleen A. Withrow

Glenn Research Center, Cleveland, Ohio

Assessment of the State of the Art of Integrated 

Vehicle Health Management Technologies as 

Applicable to Damage Conditions

NASA/TM—2010-216911

December 2010
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Mary S. Reveley

Glenn Research Center, Cleveland, Ohio

Tolga Kurtoglu

Mission Critical Technologies, Moffett Field, California

Karen M. Leone

ASRC Aerospace Corporation, Cleveland, Ohio

Jeffrey L. Briggs and Colleen A. Withrow

Glenn Research Center, Cleveland, Ohio

Assessment of the State of the Art of Integrated 

Vehicle Health Management Technologies as 

Applicable to Damage Conditions

NASA/TM—2010-216911

December 2010

National Aeronautics and

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NASA/TM—2010-216911 1

Assessment of the State of the Art of Integrated Vehicle Health 

Management Technologies as Applicable to Damage Conditions

Mary S. Reveley

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Tolga Kurtoglu

Mission Critical Technologies

Moffett Field, California 94035

Karen M. Leone

ASRC Aerospace Corporation

Cleveland, Ohio 44135

Jeffrey L. Briggs and Colleen A. Withrow

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Summary

A survey of literature from academia, industry, and other Government agencies assessed the state of 

the art in current Integrated Vehicle Health Management (IVHM) aircraft technologies. These are the 

technologies that are used for assessing vehicle health at the system and subsystem level. This study 

reports on how these technologies are employed by major military and commercial platforms for 

detection, diagnosis, prognosis, and mitigation. Over 200 papers from five conferences from the time 

period of 2004 to 2009 were reviewed. Over 30 of these IVHM technologies are then mapped into the 17 

different adverse event damage conditions identified in a previous study. This study illustrates existing 

gaps and opportunities for additional research by the NASA IVHM Project. 

Introduction

Purpose of Study

NASA’s Integrated Vehicle Health Management (IVHM) Project is one of four projects within the 

Agency’s Aviation Safety Program (AvSafe) in the Aeronautics Research Mission Directorate (ARMD). 

The IVHM Project conducts research to develop validated tools and technologies for automated detection, 

diagnosis, and prognosis to mitigate adverse events during flight. Adverse events include those that arise 

from system, subsystem, or component faults or failures due to damage, degradation, or environmental 

hazards that occur during flight (Ref. 1). The purpose of this study is to review literature from academia, 

industry, and other Government agencies to assess the state of the art of IVHM research and technologies 

applicable to the specific adverse event conditions documented in the adverse events table of the project 

plan (Ref. 1).

Overview of Study Contents

• This report presents results of a survey of the literature from academia, industry, and other 

Government agencies to assess the state of the art in current IVHM technologies and to discuss 
NASA/TM—2010-216911 2

how they are employed by major military and commercial platforms for detection, diagnosis, 

prognosis, and mitigation. The results are summarized into the 17 documented adverse event 

example damage conditions. A brief explanation of these example damage conditions is given. In 

addition, a summary illustrates existing gaps and opportunities for the NASA IVHM Project. 

Adverse Event Example Damage Conditions Related to Integrated

Vehicle Health Management (IVHM) Fault Type

A list of example damage conditions as they relate to given adverse event fault types are presented in 

Table 1. It was built upon the results of an examination of the most recent statistical and prognostic 

incident and accident data that was available. In the analysis, publicly available National Transportation 

Safety Board (NTSB) accident, Federal Aviation Administration (FAA) accident and incident data, and 

Aviation Safety Reporting System (ASRS) incident data were examined, and 17 example damage 

conditions were documented (Ref. 2).

TABLE 1.—INTEGRATED VEHICLE HEALTH MANAGEMENT (IVHM)

ADVERSE EVENTS TABLE

Adverse event type Example damage condition

Incipient faults 1. Icing conditions in propulsion system

2. Fault of power electronics

3. Turbine engine bearings

Slow progression faults 4. Fatigue cracks on metallic airframe structure

5. Delamination in composites

6. Hydraulic failures

7. Air conditioning and pressurization faults

8. Oil and/or lubrication failures

Intermittent faults 9. Wire chafing failures

Cascading faults 10. Power system faults

11. Control surface faults (aileron, rudder, or elevator)

12. Instrumentation, communication, and navigation failures

13. Fuel system faults

Fast progression faults 14. Engine stall or faults in turbomachinery

15. Landing gear faults

16. Brake and/or anti-skid system faults

17. Lightning- and radiation-related avionics faults

The purpose of the adverse event table (Table 1) is to provide guidance to the IVHM Project about 

critical research areas in health management technologies. It may be noted, that this table is expected to 

change as the IVHM Project matures and future technologies and trends become clear. The first version 

of the initial adverse events table was constructed by collecting data gleaned from findings within the 

ASRS, FAA, and NTSB databases and is shown in Table 1 (Ref. 2).

The adverse event types are categorized into five classes based on the overall remaining useful life of 

the affected system, subsystem, or component defined as follows:

• Incipient fault.—Faults that are hard to detect and differentiate due to extremely slow degradation 

in performance.

• Slow progression fault.—Faults that are very hard to detect; gradual degradation in performance.

• Intermittent fault.—Faults that do not degrade but instead manifest themselves in a recurring 

fashion. 
NASA/TM—2010-216911 3

• Cascading fault.—Faults that may have a single-root cause yet progress to create faults in other 

systems, subsystems, or components.

• Fast progression fault.—Faults that have a limited precursor signature but show rapid 

degradation.

Depending on the nature of the damage condition, there are scenarios in which more than one of these 

classes applies to a particular adverse event example damage condition type. For example, a software 

fault could lead to a fast progression fault as in the case of a stack overflow or a slow progression fault, as 

in the case of a memory leak. Acknowledging this, each adverse event type is categorized based on the 

most typical progression behavior.

State-of-the-Art Assessment of Integrated Vehicle Health Management 

(IVHM) Research and Technologies

This section contains the results of the literature survey conducted to address this expected outcome. 

The findings for each adverse event example damage condition are summarized below.

1. Icing conditions in propulsion systems

Icing is one of the adverse events that has attracted a significant amount of research ranging across

IVHM (Ref. 3). Starting with detection, the state of the art includes both the detection of icing conditions 

in the airspace using ground-based models, as well as in situ detection of ice accretion on the aircraft.

Researchers at the National Center for Atmospheric Research have developed the forecast icing potential 

algorithm, which uses data from the National Center for Environmental Prediction to calculate icing 

potential in the three-dimensional environment (Ref. 4). If integrated into the flight deck, this could be 

used by pilots to avoid areas of high icing potential. Another method of detection is a joint venture 

between NASA and the U.S. Army to use ground-based Ka- and X-band radar to locate icing conditions 

in cloud formations, but this is also not yet integrated into any flight system (Ref. 5). 

One method currently being investigated for detection of ice accretion on the aircraft is time domain 

reflectometry (TDR). With this method, electromagnetic waves are transmitted, and when they are 

received, the presence of ice is detected based on the dispersion of the waves. Currently, this technology

is still in the feasibility study stage (Ref. 6). In terms of prognosis, there are numerous computational 

models that are used to predict the growth of ice both on the airframe and on surfaces in the propulsion 

system (Ref. 7). In addition to prognostic models of ice accretion, models also exist to predict the 

degradation of flight controls as icing conditions develop (Ref. 8). In terms of mitigation, one method that 

has been found effective was introduced by General Electric (GE) in 1996. They found that flameout due 

to icing frequently occurred on descent, between an altitude of 20 000 and 10 000 ft. GE changed the 

engine control unit variable bypass valves logic to increase ice extraction from booster core and inlet in 

this range of altitudes, and no events have occurred on aircraft where this change was made (Ref. 9).

2. Fault of power electronics

Monitoring the deterioration of components in electronic systems is much harder than mechanical 

systems (Ref. 10). There has been a recent increase in developing methods and technologies for 

prognostics and health management of electronic components such as power system components, 

avionics, and solder joints. For diagnostics of electronic components expert systems, neural networks and 

fuzzy logic systems are used (Ref. 11). Bayesian diagnostic models are also used for diagnosing faults in 

electronic systems (Ref. 12). Solder joint faults are proving to be one of the main failure sources of 

electronic components. Thus, solder fatigue modeling and monitoring technologies are used to monitor 

small cracks that develop and propagate in the microstructure of solder joints due to thermal loading 

and/or aging. The changes in the material microstructure due to aging are used by some researchers to 

predict remaining useful life of the solder joints of electronic components (Refs. 13 and 14). Other 
NASA/TM—2010-216911 4

prognostic methods have been proposed for various critical electronic components including digital 

electronic boards (Ref. 15), silicon carbide packaging (Ref. 16), integrated circuits (Ref. 17), and power 

actuators (Ref. 18).

3. Turbine engine bearings

Bearing health monitoring is critical to the performance of turbomachinery. Diagnostic technologies 

for rolling elements are relatively well developed. Among these are stress wave analysis, which provides 

real-time measurement of friction and mechanical shock in bearings. This high-frequency acoustic 

sensing technology filters out background levels of vibration and audible noise, and provides a graphic 

representation of machine health (Ref. 19). Acoustic emission technologies also indicate bearing stress. 

Robust laser interferometer (RLI), an alternative technology to acoustic emissions, has also demonstrated

use for bearing health monitoring (Ref. 20). Alternatively, accurate prediction of remaining life of 

bearings remains a challenge. Novel techniques for remaining useful life (RUL) estimation for bearings 

are in development especially those targeting incipient faults (Refs. 21 to 24). Smart bearings, although

still at development stage, is another novel technology. If successfully developed, these bearings will 

have a number of unique features such as having all sensory and telemetry data built into the bearing, 

durability for higher temperatures, long-life, and compatibility with the operating environments (Refs. 25

to 27).

4. Fatigue cracks on metallic airframe structure

The current state of the art in detection methods for fatigue cracks on the metallic airframe structure 

mostly involves onground inspections. In terms of current fleet operation, this is usually done manually as 

a part of routine maintenance, which means a crack might be present for a while before it is identified. 

Various probe technologies are currently being investigated that would allow technicians to detect cracks 

that would not otherwise be visible. One such technology is eddy current inspection (ECI), which is a new 

method that produces a magnetic field on a material’s surface to detect cracks, corrosion, heat effects, and 

thicknesses. ECI is effective on metallic surfaces, but is limited to the area directly below the 

instrumentation point (Ref. 19). For example, one probe uses eddy current to detect fatigue cracks 

underneath fasteners (Ref. 28). For onboard detection, piezoelectric transducers (PZTs) can detect crack 

via disruptions in stress waves sent through particular airframe structures in flight (Ref. 19).The 

feasibility of integrating this technology onto an aircraft is yet to be determined. Acoustic emission is

another technology that is used for surface and inner structure crack detection and localization. This 

nondestructive evaluation technique involves monitoring for the emission of high-frequency vibration 

(>100 000 Hz) as an existing structural defect (crack) is stressed from the static loading of the system

(Refs. 26, 27, and 30). In addition, induced positron analysis technologies can reliably detect and quantify 

tensile plastic strain damage induced by simulated and operational conditions in aerospace material 

specimens and components including metallic surfaces (Refs. 31 and 32). In the area of prognostics, 

numerous algorithms have been developed to predict the growth of cracks under various flight conditions

(Ref. 33). Once a crack has been discovered, the current state of the art in mitigation normally involves 

patching or bonding with epoxy or selective reinforcement (Ref. 29). Some additional research has been 

done in terms of effective selective reinforcement and the types of fixes that can be implemented to slow 

the crack (Ref. 34). Still, none of these methods involves onboard solutions. Some condition-based 

mitigation methods exist that use materials through which cracks naturally propagate more slowly

(Ref. 35). Another method of condition-based mitigation is dynamic controls that change the load spectra 

to lighten loads on fractured structures. This does not fix the crack but could help slow its progression 

until it could be fixed on the ground (Ref. 36).

5. Delamination in composites

The use of composite materials in structural design is becoming increasingly popular in aerospace 

applications because of the benefit of reduced weight without much compromise in strength and stiffness 

performance. Understanding composite material’s principal mode of failure, delamination, has become 
NASA/TM—2010-216911 5

increasingly important (Ref. 37). Delamination may occur in the form of microcracks and voids, usually

leading to macroscopic loss of stiffness and strength, followed by a catastrophic structural collapse. 

IVHM technologies for delamination encompass sensors that detect aging, unanticipated events such as 

impacts, and nondestructive inspection (NDI) tools for flaw identification and damage characterization. 

Specific sensors developed for onboard impact detection include PZT and fiber Bragg grating ultrasonic

sensors, which can locate the point of impact (Refs. 19 and 38). Once impact is detected, NDI tool images 

are used to study the damage region, and modally selective sensors (i.e., lamb wave sensors (Ref. 39)) are 

used to monitor further damage growth. Different damage growth prediction methods are finally 

employed to estimate the remaining lifetime of the structure. These methods include the linear elastic 

fracture mechanics method, the cohesive fracture model, and delamination threshold load method

(Ref. 40). Feature-based signal processing methods, and data-driven classification techniques are also 

proposed for damage detection and prediction (Refs. 41 and 42).

6. Hydraulic failure

Hydraulic systems are extremely important in aircraft, especially for positioning control surfaces and 

for operating landing gear. Linked closely with hydraulic systems that maneuver the control surfaces are 

actuators, as they are often driven by hydraulics. One problem that can occur in the hydraulic system is 

called cavitation. This phenomenon occurs when the pressure at the pump inlet is lower than the vapor 

pressure of the fluid, and it causes bubbles to form in the system. It is ideal to avoid this situation if 

possible, as it increases wear on the system. Researchers have developed a neural-network-based 

algorithm to detect cavitation using as inputs the electric current supplied to the pump motor and a 

voltage that can be used to determine the operational speed and output flow of the pump (Ref. 43). The 

cavitation data could possibly be fed to an adaptive control system to help increase the operational life of 

the hydraulic system. Another method of monitoring the health of hydraulic systems is vibration analysis 

of the hydraulic pumps. The algorithm uses the fundamental frequencies and harmonics of the pump and 

determines if it is operating normally or in a degraded state (Ref. 44). There are many types of hydraulic 

system faults that result in a small leak somewhere in the system. Small leaks can be present before they 

cause larger problems. If they are detected early, it is possible to prevent these problems. One method for 

detecting these faults involves online, model-based fault detection. Using the operational speed of the 

pumps, it is possible to determine the pressure that should exist at various points in the system. Models 

have been developed that use pump data and pressure sensors to determine if there are leaks in the system 

and where those leaks are located (Ref. 45).

Hydraulic actuators are being replaced with electromechanical actuators (EMAs) in future aircraft 

designs. In normal operation these are driven by electric power, but in high stress situations, during a jam, 

or when electric power to the actuator is disrupted, the hydraulic system supplies the power needed to 

operate the actuator. One problem that can occur in these actuators is called ball-jam where the moving 

part containing the ball bearing gets stuck somehow, preventing the control surface from moving. The 

majority of detection, diagnosis, and prognosis technologies in regards to ball-jam in EMAs are model 

based. Various modeling techniques are used to detect malfunctioning EMAs, as well as for the prediction 

of time to fail for the entire system (Ref. 46). Although models have been created, the majority of the 

work is still done by technicians on the ground. Technologies are being developed that use flight control 

data and will be able to deliver gray-scale, as opposed to pass-fail feedback to maintenance technicians. 

Another technology for in-flight mitigation of ball-jam is the development of hybrid electromechanical/

hydraulic actuators. These actuators run mechanically in normal operation, but a parallel hydraulic system 

kicks in when extra power is needed because of heavy loads or a ball-jam. It is not certain that the 

hydraulic system will fix the jam, but it is one method of in-flight mitigation (Ref. 47).

7. Air conditioning and pressurization faults

Researchers have produced fairly extensive prognostic models predicting the failure of various 

components in the pressurization system using Weibull distributions. These distributions are based on a 

power law relationship in which the probability of failure increases with flight hours (Ref. 48). It is also 
NASA/TM—2010-216911 6

noted that 61 percent of component failures in this system are related to the water separator, so a 

significant amount of risk can be reduced by focusing on this component (Ref. 49). However, in the future 

this failure in particular may have a decreased relevance. Currently, pressurization systems use either 

preconditioned air from the ground or bleed air from the engine, which requires a water separator. As 

aircraft manufacturers move towards more electric aircraft, such as the Boeing 787, they may use electric 

fans to pressurize the cabin where no water separators are required. Reasons for the transition to an 

electric architecture include significant maintenance cost as well as reliability advantages (Ref. 84). Even 

if these faults are eliminated, they would then need to focus more attention on electrical and wiring faults, 

and the powerplant will need to produce four times as much electricity. 

8. Oil and lubrication system failures

The main problems associated with aircraft oil and lubrication systems arise from clogged filters, 

pressure anomalies, and water in the oil. Pressure anomalies can result from multiple causes including 

low levels of lubricant and clogged filters. The health of this aspect of the lubrication system is monitored 

by pressure sensors that relay information to the pilot. Pilots are trained to recognize and adjust for 

various situations. Another method that is used to monitor the loss of lubricant in an engine involves a 

vibration analysis of moving engine components. Different vibration signatures arise as the level of 

lubricant in the system varies. This method is currently used by the U.S. Army for light armored vehicles, 

but might also be applied to aircraft (Ref. 30). Faults in the oil system, such as a clogged filter, are often 

indicators of failures in components lubricated by the system. For example, as a moving part is worn 

down over time, particulates accumulate in the oil that lubricates it, and this can clog the filter. Also, more 

than just getting worn down, structural failures in components might cause larger pieces of debris to 

accumulate in the system. By analyzing the oil in the system for composition and size of particulates, 

information about the health of lubricated components and of the oil system itself can be obtained. In 

1995, the U.S. Army mandated that its aircraft undergo this type of analysis on a regular basis (Ref. 50).

Jet-Care has also commercialized the process by analyzing oil and filter samples for operators. Results of

the analyses include system health diagnosis and a prognosis for suggested maintenance (Ref. 51).

9. Wire chafing failures

Visual inspection is the most common technique to detect wire chafing in current aircraft. This failure 

increasingly occurs as aircraft fleets age; 43 percent of electrical system mishaps are related to connectors 

and wiring (Ref. 52). As we move towards more electric aircraft, the increased amount of wiring will only 

make this failure more frequent. Wire placement is a big driver of these problems, because wire bundles 

closer to hydraulics or airframe structures are more prone to chafing. One simple method of mitigation is 

to use standoff clamps to hold wire bundles away from these structures (Ref. 53). Some more advanced 

methods of detection use differences in impedance to detect flaws in insulation before the conductor 

inside is harmed and shorts occur. However, one drawback to this method is that it does not help locate 

the area of the wire failure (Ref. 54). Innovative Dynamics planned an integrated test of a wire chafing 

sensor in a Goodrich wiring harness. As the sensor became chafed, it made maintenance technicians 

aware of the problem before the actual wires were harmed (Ref. 55). Another detection method that is 

especially promising because it is used to both detect and locate faults in a wire harness is called the 

pulse-arrested spark discharge. This method can find faults that would only produce undetectable changes 

in impedance, and it can be used for multiple other purposes in addition to detection of wire chafing 

(Ref. 56). Another new type of sensing technology involves wrapping fiber optic cables around a wire 

bundle. As the fiber becomes chafed or breaks, the decreased transmission or short is easily detectable, as 

well as the location on the wire bundle (Ref. 57). As electric aircraft technology advances, wireless 

networks may be able to eliminate a lot of these failures (Ref. 58). Previously introduced RLI technology 

also has demonstrated potential for electrical wiring health monitoring (Refs. 20 and 59). Finally, 

previously introduced TDR is another method that can be used to measure changes in electronic wiring 

interconnect system characteristic impedance for detection of chafe, nicks, and corrosion defects 

(Ref. 60).
NASA/TM—2010-216911 7

10. Power system faults

Both military and commercial aerospace systems are becoming increasingly dependent on electrical 

power as systems move towards the more electric aircraft concept. This novel architecture relies on 

digitally controlled power distribution to provide power to flight critical subsystems such as avionics and 

fuel. This increasing dependence on electrical power necessitates the development of new technologies 

for autonomous health management of electrical power systems. Power system faults cover a wide range 

of problems, some of which is covered under power electronics faults and wiring chafing sections of this 

study. The main focus of this section is technologies developed for arc prevention and power 

management. 

The arcing of electrical-powered systems is a major safety concern to both new and legacy aircraft. 

The advent of high-voltage direct current systems accentuates this problem. Arc fault prevention methods 

and algorithms address this critical need for electrical power system health management. Arcing faults 

occur as a result of chafing and cracking of insulation, dielectric breakdown, and looseness at terminal 

connections. Once arcing is initiated, damage may propagate to other conductors in the wire bundle. The 

discharge of arcing energy results in insulation damage, smoke events, the loss of adjacent wires in a wire 

bundle, and ignition of flammable materials and vapors. The use of thermal circuit breakers is a common 

technique used for arc fault prevention. An arc fault circuit interrupter is a new technology developed for 

arc prevention (Ref. 61) that relies on arc fault circuit breakers as a supplemental protection against arc 

fault conditions and thermal overload. Current circuit breakers only provide for thermal overload 

protection. Solid-state devices are also proposed as a potential replacement for traditional thermal circuit 

breakers for arc fault prevention. These devices typically have longer life-cycle, faster response time for 

overloads, and lower power dissipation when compared to current circuit breakers (Ref. 62).

Technologies have been developed for management of digital power distribution and arc fault 

management. An example is the Aircraft Electrical Power Systems Prognostics and Health Management 

Program (Ref. 63) sponsored by the Air Force Research Laboratory. The architecture developed in this 

program enables the digital data to provide component operating signatures to the system, thereby 

facilitating advanced diagnostic and prognostic capabilities. Other arc fault management systems can 

monitor the operation of the loads. Examples include marker lights to flag failures, motors and actuators 

for acceptable current levels, and operations of loads for time and duration (Ref. 62). A distributed power 

system is another new technology that is an alternative to traditional centralized power systems. It is 

based on locating the power control devices near the electrical loads to reduce the amount of power 

wiring. The control commands for this type configuration are sent to the power control devices with a 

data bus. Use of a distributed power distribution unit not only significantly reduces wire weight, but can 

lower installation and maintenance costs just based on the reduction in the number of connections.

11. Control surface faults (aileron, rudder, or elevator)

There is quite a lot of technology currently in use in the area of control surface faults. Frequently, the 

faults associated with control surfaces come about because of problems with the actuators or hydraulic 

systems that move the control surfaces and keep them in position. Excessive loads can also cause 

structural damage including cracks and fractures. The presence of one of these faults can mean that the 

control surface gets stuck at a certain trim, is more difficult to position, or becomes completely 

ineffective. Algorithms have been developed that use flight variables to detect these types of faults

(Ref. 64). Once a fault is detected, flight control software can diagnose the problem by identifying the 

specific faulty component. Some systems have been designed that give this information to the pilot. More 

advanced algorithms have been developed that estimate the performance degradation and new flight 

envelope in the presence of the failure. Additional research focuses on the performance of autoland 

systems in the presence of a control surface fault. Because of the precision needed at landing, this phase 

of flight is particularly susceptible to loss of control due to control surface faults (Ref. 65). Numerous 

groups have designed flight control software packages that incorporate neural networks for additional 

capabilities to be used in landing with control surface faults. 
NASA/TM—2010-216911 8

12. Instrumentation, communication, and navigation failures

One common problem related to instrumentation, communication, and navigation faults is that these 

faults are often noticed by the flight crew, but not by detection systems (Ref. 66). Current detection 

methods are similar to those of wire chafing, in which impedance is measured for various electronics, and 

variations from the norm are detected as faults. Often faults are not detected because the variation in 

impedance is too small. Pulse-arrested spark discharge is one promising fault detection method that can 

be used to detect electronics faults that cannot be detected using impedance methods (Ref. 67). Onboard 

navigation faults are becoming less relevant due to the increased use of the global positioning system

(GPS), because it uses data transmitted via satellites as opposed to electronics integrated with the rest of 

the aircraft, although the use of GPS could introduce a new set of faults. Similarly, utilization of wireless 

networks onboard aircraft could reduce the reliance on physical wiring in the future, eliminating many of 

the current failure modes (Ref. 58).

13. Fuel system faults

The majority of fuel system faults occur because of water condensation. This condensation can be 

especially harmful in freezing temperatures causing degraded performance and clogged filters. There are 

very few diagnostic systems specifically for fuel systems and even fewer systems that can mitigate 

detected faults. The conventional method of fuel system management is via a central computer system 

that controls fuel distribution among various tanks according to the stage of flight. Researchers at Penn 

State University have developed an adaptive fuel filtering system with parallel pumps. Data is collected 

via pressure sensors and can be used to identify faults or clogged filters. Integrated logic can adjust the 

flow or use water from the water separator to back flush clogged filters. A successful integrated test of 

this system was performed using a test rig constructed with diesel engines (Ref. 68). Multiple groups are 

researching alternatives to the use of the central computer systems for fuel management that use 

distributed networks of sensors and microcontrollers. The microcontrollers are used to control the fuel 

pumps using input from the sensor network to achieve the desired distribution of fuel in the tanks. This 

network of smart sensors eliminates the need for a central computer (Ref. 69). One method of decreasing 

fire risk in the fuel tanks is to replace electromechanical sensors, which have a higher risk of initiating 

combustion in flammable environments, with fiber optic sensors (Ref. 70). Another technology to 

decrease fuel-system-related fires involves detection of electrical faults in the fuel pumps. A retrofit 

device called a universal fault interrupter was approved for use on some aircraft in 2008. It is an electrical 

box that is installed adjacent to the electrical relay. It can detect electrical faults in the fuel pump, and it 

shuts down the pump when the inlet is uncovered or in times of uncommanded pump operation (Ref. 71).

14. Engine stall and/or faults in turbomachinery

Technologies developed to monitor and prevent engine stall and faults in turbomachinery cover a 

wide variety of techniques. These include monitoring of specific vibrations in an individual blade to 

prevent the potential of catastrophic failure and prevent turbine downtime. Specific technologies for blade 

vibration monitoring include eddy current, optical, and capacitive sensors as well as algorithms that can 

process and fuse data from these sensors. Eddy current sensors are used for the purpose of gas turbine 

engine stability monitoring including stall detection and blade harmonics (Ref. 19). Microwave blade tip 

sensors have considerable promise as a state awareness technique for the monitoring of rotating blades 

and disks (Refs. 72 and 73). These sensors produce information-rich waveforms of the blade-end 

geometry. Foreign object damage (FOD) detection systems comprise a suite of new technologies. These 

include systems for detecting and analyzing ultrasound or stress waves emitted when an object enters the 

intake of a turbine engine and impacts one or more of the blades in the engine. Upon detection, the FOD 

detection system can immediately inform the operator, inform another electronic device (computer, etc.),

and/or latch the event for review by maintenance personnel (Ref. 23).

Debris monitoring is another technology applicable to engine health monitoring (Ref. 74). The 

fundamental principle of gas path debris monitoring is to sense the electrostatic charge associated with 

debris present in the gas path of jet engines or gas turbines. The engine distress monitoring system and the 
NASA/TM—2010-216911 9

ingested debris monitoring system are two technologies demonstrated on the United States’ Joint Strike 

Fighter Pro Seeded Fault Engine Test program. Tomography is another technology based on 

hyperspectral absorption spectroscopy for temporally and spatially resolved temperature and water 

concentration measurements in practical combustion devices.

Surface acoustic wave sensors are another technology with demonstrated application to turbine 

engines (Ref. 75). These sensors can be used as multifunctional temperature and pressure sensing devices 

for turbine engine test validation. Finally, developing harsh environment sensors is also critical to engine 

health management. Notable technologies in this class include silicon carbide sensor devices that can 

work at high temperatures up to 500 °C that can be used for sensing motion, acceleration and gas flow, 

gas composition, and radiation detection. Another harsh environment sensor development for propulsion 

system applications is ceramic sensors that are not limited thermally when compared to traditional metal 

thin film sensors.

15. Landing gear faults

The current methods used by commercial airlines for landing gear health management mostly relate 

to routine maintenance and visual inspections. Additional maintenance may also be performed based on 

flight data or pilot observations. For example, a perceived hard landing by the pilot might warrant some 

extra maintenance procedures. Unfortunately, some operational scenarios that put larger than normal 

loads on the landing gear may not be reported if the pilot does not think of the scenario as a hard landing.

Research is being conducted to design a load sensor to be able to monitor this information (Ref. 76).

Currently, structural sensors are not integrated into any landing gear systems. Even if a scenario causes 

some sort of damage to the structure, and it does get reported, it is possible that a visual inspection will 

not find the problem. For example, some stress fractures to the structure may not be visible without 

disassembling the main gear. Acoustic emission is a technique being developed that could detect faults in 

structures and breakage of seals (Ref. 30). During inspection, the technician uses a device to propagate 

acoustic waves. Sensors pick up potential variations in the waves caused by faults (Ref. 77). Another area 

of concern in landing gear is shimmy during taxiing and landing. Shimmy is an oscillation of the structure 

that is normally caused by rough runways, usually around 10 to 30 Hz. Although shimmy does not

usually cause catastrophic failures, it can cause excessive wear over time. A current method for dealing 

with shimmy is physical dampers that cannot adapt to changing situations. A new adaptive model uses

sensors to detect shimmy, calculates the taxiing velocity and yaw angle to minimize the shimmy, and 

gives this information to the pilot (Ref. 78). Currently, no production aircraft have this technology.

16. Brake and/or anti-skid system faults

Conventional anti-skid systems consist of sensors that are used to detect when wheels lock, directing

the brakes to pulse. Better algorithms in the anti-skid system control unit could help decrease skids and 

stopping distance and increase reliability. Research is being conducted to improve algorithms for use in 

anti-skid controllers (Ref. 79). There are also units that are designed to detect potential faults in the 

braking system. By utilizing braking data, an anti-skid control unit produced by General Atomics has the 

capability to detect anomalies in actual braking performance in contrast to expected performance. These 

anomalies act as a red flag for potential faults in the system (Ref. 80). Although this assists in the 

detection of potential failures, there is very little research addressing detection of the early stages of 

failure. The products on the market, such as the controller from General Atomics, are backward-looking,

and the mitigation is based on maintenance. Some research is being conducted in a brake-monitoring 

system that will offer some prognostic capabilities. This system involves using sensors to monitor wear 

on friction surfaces and pressure sensors to monitor conditions in hydraulics. As anomalies from normal 

sensor data become apparent, the system reports to a user interface that the risk of failure is high for a 

specific component (Ref. 81). Although this technology does not include fault mitigation, it can help 

increase operator awareness of conditions and decrease costs by reducing the need for frequent routine 

maintenance.
NASA/TM—2010-216911 10

17. Lightning- and radiation-related avionics faults

There is not a great deal of research in the area of lightning- and radiation-related avionics faults, but 

there are some models that show the electrical and thermal effects on the airframe after a lightning strike 

occurs. For example, if a lightning strike hits the nose of an airplane, models predict the intensity and 

time progression of voltage and thermal effects across the airframe. This prediction might be used to 

estimate the probability that avionics are damaged. The worldwide distribution of lightning strikes can be 

detected using satellite data, and this technology is used to alert aircraft operators when and where high 

lightning concentrations will be present. Lightning damage to metallic aircraft structures is not a problem; 

however, composites are being increasingly used in modern aircraft. Composite materials do not conduct 

and dissipate electricity. Lightning strikes could cause metal control cables to vaporize, metal hinges to 

weld, and vapors within fuel tanks to explode. A number of lightning strike protection materials have 

been developed (Ref. 82). Improved methods are needed for the detection of lightning strikes to 

composite materials as well as ways of diagnosing the impacts of the lightning strikes (Ref. 83).

Summary

A previous study conducted under the NASA Integrated Vehicle Health Management (IVHM) Project 

identified example damage conditions related to a set of adverse event fault types. One of the products of 

this previous study was Table 1 that identified 17 example damage conditions spanning across the five 

different adverse event fault types. The current study serves as a survey of the state of the art in IVHM 

technologies that relate to the 17 example damage conditions. As the scope of this study would make an 

in depth discussion of each technology infeasible, readers are encouraged to examine the references for 

more detailed information on the technologies summarized in the various sections. 

As a result of this survey, we have found that certain gaps still exist that create barriers for effective 

development and deployment of integrated health management technologies that address the documented 

example damage conditions. These are summarized below.

System-level perspective.—A large obstacle to the development of IVHM technologies is the lack of 

a system-level perspective. This is often encountered with technology development efforts devoted to 

detection of a specific phenomenon or diagnostics and prognostics of a particular subsystem. 

Researchers usually develop technologies that successfully meet the requirements at the subsystem 

level, but fail to meet the verification and validation at the system level. 

Systems integration.—Systems integration presents a big technical challenge. Most technologies are 

developed and matured only to a certain level without being effectively tested and integrated within

the broader system. Our results show that, although promising, most of the IVHM technologies

reported in this survey are yet to be integrated into a flight system.

Mitigation challenges.—IVHM technologies that fall under the mitigation theme often are yet to fully 

benefit from automation. In most subsystems and example damage condition domains, mitigation 

technologies lag behind detection, diagnosis, and prognosis. One reason for this deficiency is the fact 

that the decision systems employing mitigation strategies are typically not directly considered during 

the development of diagnostics and prognostic systems; however, the mitigation strategies are direct 

users of the information that is generated by such systems.

Flight safety.—Not all of the example damage conditions documented in the adverse events table 

have the same level of maturity when it comes to the technologies developed to address them. This 

has implications for flight safety. Damage conditions such as “icing conditions in propulsion 

systems,” “lightning-related avionic faults,” “delamination in composites,” and “fault of power 
NASA/TM—2010-216911 11

electronics” still demand a better understanding of the physical phenomenon that cause faulty 

behavior in flight systems. Models and methods that would facilitate the detection, diagnosis, 

prognosis, and mitigation of these damage conditions during flight are yet to mature to levels 

demonstrated by technologies in other domains. 
NASA/TM—2010-216911 12
NASA/TM—2010-216911 13

Appendix A.—Acronyms

ASRS Aviation Safety Reporting System

AvSafe Agency’s Aviation Safety Program 

ECI eddy current inspection

EMA electromechanical actuator

FAA Federal Aviation Administration

FOD foreign object damage

GE General Electric

GPS global positioning system

IVHM Integrated Vehicle Health Management

NDI nondestructive inspection

NTSB National Transportation Safety Board

PZT piezoelectric transducer

RLI robust laser interferometer

RUL remaining useful life

TDR time domain reflectometry
NASA/TM—2010-216911 14

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A survey of literature from academia, industry, and other Government agencies assessed the state of the art in current integrated vehicle 

health management (IVHM) aircraft technologies. These are the technologies that are used for assessing vehicle health at the system and 

subsystem level. This study reports on how these technologies are employed by major military and commercial platforms for detection, 

diagnosis, prognosis, and mitigation. Over 200 papers from five conferences from the time period of 2004 to 2009 were reviewed. Over 30 

of these IVHM technologies are then mapped into the 17 different adverse event damage conditions identified in a previous study. This 

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