DOT/FAA/AR-04/43 

Office of Aviation Research 

Washington, D.C. 20591 

Demonstration and Validation of 

the Excited Dielectric TestTM

Method to Detect and Locate 

Defects in Aircraft Wiring Systems

January 2005 

Final Report 

This document is available to the U.S. public 

through the National Technical Information 

Service (NTIS), Springfield, Virginia 22161. 

U.S. Department of Transportation 

Federal Aviation Administration
NOTICE 

This document is disseminated under the sponsorship of the U.S. 

Department of Transportation in the interest of information exchange. The 

United States Government assumes no liability for the contents or use 

thereof. The United States Government does not endorse products or 

manufacturer’s. Trade or manufacturer's names appear herein solely 

because they are considered essential to the objective of this report. This 

document does not constitute FAA certification policy. Consult your local 

FAA aircraft certification office as to its use. 

This report was prepared by CM Technologies Corporation as an account 

of work sponsored by U.S. Department of Transportation. Neither CM 

Technologies Corporation, nor any person acting on behalf of them (a) 

makes any warranty, expressed or implied, with respect to the use of any 

information included in this report or (b) assumes any liabilities with 

respect to the use of, or for damages resulting from the use of, the 

information included in this report. 

The Excited Dielectric TestTM method, including associated enabling 

instrumentation and software developed during the course of this work, is 

the subject of both patent and trademark applications currently under 

review by the U.S. Patent and Trademark Office. 

This report is available at the Federal Aviation Administration William J. 

Hughes Technical Center's Full-Text Technical Reports page: 

actlibrary.tc.faa.gov in Adobe Acrobat portable document format (PDF). 
 Technical Report Documentation Page

1. Report No.

DOT/FAA/AR-04/43

2. Government Accession No. 3. Recipient's Catalog No.

5. Report Date

January 2005

4. Title and Subtitle 

DEMONSTRATION AND VALIDATION OF THE EXCITED DIELECTRIC 

TESTTM METHOD TO DETECT AND LOCATE DEFECTS IN AIRCRAFT 

WIRING SYSTEMS

6. Performing Organization Code

7. Author(s)

Rollin Van Alstine and Gregory Allan

8. Performing Organization Report No. 

 

9. Performing Organization Name and Address

CM Technologies Corporation 

10. Work Unit No. (TRAIS)

1026 Fourth Avenue 

Coraopolis, Pennsylvania 15108

11. Contract or Grant No.

DTFA03-00-C-00024 

13. Type of Report and Period Covered

Final Report 

12. Sponsoring Agency Name and Address

U.S. Department of Transportation 

Federal Aviation Administration 

Office of Aviation Research 

Washington, DC 20591 

14. Sponsoring Agency Code

 AAR-480

15. Supplementary Notes

The FAA William J. Hughes Technical Center Technical Monitor was Robert Pappas.

16. Abstract

This report describes the development of an advanced test technique for assessing the condition of installed aircraft wiring. The 

Excited Dielectric TestTM (EDT) method is an electrical test that exploits a basic property of polar insulation materials, that is, 

some small amount of current will flow through the insulation material when the wire is exposed to an alternating electric field. 

The magnitude of this current varies with the frequency of the applied field and, in polar insulation materials, there is a specific 

frequency at which a maximum current occurs. This current is often expressed in terms of the phase angle, δ, between the applied 

voltage and the resultant current. The tangent of δ is known as the dissipation factor (DF). 

The other aspect of the EDT method is the time domain reflectometry (TDR). Aircraft wiring can be modeled as a classical 

transmission line (i.e., a continuous structure of resistors, inductors, and capacitors). Applying this model, the electrical 

characteristics of the wire (e.g., impedance, capacitance, DF, resistance) can be analyzed as distributed elements represented by a 

voltage waveform. TDR has been shown to be extremely effective in measuring the distributed characteristics of wiring. 

The EDT method applies established principles of both DF and TDR measurement theory. An alternating electric field, known as 

a forcing function, is applied to the wire under test at a frequency designed to induce the maximum value of DF. A TDR 

signature is acquired and stored under this excitation. The frequency of the forcing function is then changed, and a second TDR 

signature is acquired and stored. The two signatures are then compared. Points where the signatures do not overlay are indicative 

of localized areas where the insulation material is in the poorest condition. Polyimide and cross-linked ETFE were the insulation 

materials studied under this research effort. 

17. Key Words 

Excited dielectric test, Dissipation factor, Time domain 

reflectometry, Aircraft wiring

18. Distribution Statement 

This document is available to the public through the National 

Technical Information Service (NTIS), Springfield, Virginia 

22161. 

19. Security Classif. (of this report)

 Unclassified 

20. Security Classif. (of this page)

 Unclassified

21. No. of Pages

 57

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
ACKNOWLEDGMENTS

CM Technologies would like to thank the management of AirTran Airways for their support of 

this Federal Aviation Administration sponsored research. In particular, we would like to 

acknowledge the personal efforts of Mr. Chris Nichols of the AirTran Airways Engineering 

Group for his help in securing access to a DC9 test aircraft for use as a test bed during the field 

validation phase of this project. 

iii/iv 
TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY xi 

1. INTRODUCTION 1-1 

2. THEORY 2-1 

2.1 Modeling Aircraft Wiring as a Transmission Line 2-1 

2.2 Physics of a Capacitor 2-2 

2.3 Material Changes and Dissipation Factor 2-4 

2.4 Time Domain Reflectometry and Distributed Impedance 2-5 

2.5 Merging TDR and Dissipation Factor Manipulation 2-6 

2.6 Implementation of Theory 2-8 

2.6.1 Coupling Requirements and Design Details 2-9 

2.6.2 Forcing Function Source Considerations 2-10 

2.6.3 Time Domain Reflectometry Instrument Considerations 2-10 

3. EVALUATION OF COMMERCIALLY AVAILABLE EQUIPMENT 3-1 

3.1 Coupling Network 3-1 

3.2 Forcing Function Sources 3-2 

3.2.1 Hewlett-Packard LF Impedance Analyzer, Model 4192A 3-2 

3.2.2 Tabor Electronics Model 8020 Programmable Function Generator 3-2 

3.3 Time Domain Reflectometers 3-2 

3.3.1 Agilent 54754A TDR Module 3-2 

3.3.2 CM Technologies PCI-3100 TDR 3-3 

3.3.3 Final Equipment Configuration 3-3 

4. WIRE SAMPLE SELECTION, CONDITIONING, AND TESTING 4-1 

4.1 Wire Sample Test Fixture 4-1 

4.2 Initial Testing 4-3 

4.3 Conditioning of Wire Samples 4-4 

4.3.1 Nonbreached Samples 4-4 

4.3.2 Polyimide Control 4-5 

4.3.3 Polyimide Thermally Conditioned 4-7 

4.3.4 Polyimide Conditioned With Tap Water 4-8 

4.3.5 Polyimide Conditioned With Natural Saltwater 4-8 

4.3.6 Polyimide Conditioned With Jet Fuel 4-8 

v
4.3.7 Polyimide Conditioned With Hydrazine 4-9 

4.3.8 Polyimide Conditioned by Boiling in Water 4-9 

4.3.9 Cross-Linked ETFE Conditioned in Water 4-10 

4.3.10 Cross-Linked ETFE Conditioned in Saltwater 4-10 

4.3.11 Cross-Linked ETFE Conditioned in Jet Fuel 4-11 

5. FIELD TESTS 5-1 

5.1 First Field Test—PCI-3100 TDR 5-1 

5.2 Second Field Test—Aglient 54754A TDR 5-4 

6. VALIDATION 6-1 

6.1 Requirements for Validation 6-1 

6.2 Validation Details 6-1 

6.3 Validation Test 6-6 

6.4 Data Analysis 6-6 

6.5 Results Summary 6-9 

6.6 Supplemental Tests and Analysis 6-10 

7. CONCLUSIONS AND RECOMMENDATIONS 7-1 

APPENDIX A—SENSITIVITY OF TDR TO CABLE DEFECTS CAUSED BY

SIMULATED ARC FAULTS 

vi
LIST OF FIGURES

Figure Page 

2-1 Incremental Components of a Real Transmission Line 2-1 

2-2 Incremental Components of a Lossless Transmission Line 2-2 

2-3 Current vs Voltage in a Capacitor 2-3 

2-4 Dissipation Factor vs Frequency in Polar and Nonpolar Materials 2-4 

2-5 Functional Block Diagram of a TDR System 2-5 

2-6 General TDR Responses for Resistive Loads 2-6 

2-7 Time Domain Reflectometry Signature of a Degraded Portion of an 

Insulated Wire 2-7 

2-8 Time Domain Reflectometry Signature of a Degraded Wire at Frequency F2 2-8 

2-9 Comparison of TDR Signatures of Degraded Wire at Frequencies F1 and F2 2-8 

2-10 A Basic EDT Coupling Network 2-9 

3-1 Sample TDR Signature With and Without Filter Network 3-1 

3-2 Final Test Configuration 3-4 

3-3 Prototype EDT System in Use in the Field 3-4 

4-1 The EDT Test Fixture 4-1 

4-2 Grounding Connection Used With all Enclosures 4-2 

4-3 Inside Enclosure 1 4-2 

4-4 Results of EDT Applied to a Conditioned Coaxial Cable 4-4 

4-5 Excited Dielectric Test Results From the Polyimide Control Sample 4-5 

4-6 Excited Dielectric Test Results Acquired Using Different Forcing Functions 4-6 

4-7 Excited Dielectric Test Results From Polyimide Control Sample Using Forcing 

Functions of Different Amplitude 4-7 

4-8 Results From a Thermally Conditioned Polyimide Sample 4-8 

vii
4-9 Time Domain Reflectometry Signatures at 200 MHz, 2 Hz, and 20 Hz of 

Hydrazine-Conditioned Polyimide 4-9 

4-10 Excited Dielectric Test Signatures of Boiled Polyimide 4-10 

4-11 Signature of Cross-Linked ETFE Conditioned in Jet Fuel 4-11 

4-12 Zoom of Middle Section of Jet Fuel-Conditioned Sample 4-12 

5-1 Excited Dielectric Test Signatures of the Left Inboard Wheel Speed Transducer 5-2 

5-2 Excited Dielectric Test Signatures of the Left Inboard Transducer 5-3 

5-3 Excited Dielectric Test Signatures of the End of the Left Inboard Transducer 

Circuit 5-3

6-1 Validation Testing at the FAA William J. Hughes Technical Center 6-1 

6-2 Polyimide Sample 1—Unconditioned Section 6-3 

6-3 Polyimide Sample 3—Abraded not Through Electrically, not Contaminated 6-3 

6-4 Polyimide Sample 5—Abraded Through to Conductor, Saltwater Contaminated 6-3 

6-5 Polyimide Sample 6—Abraded not Through to Conductor, Jet Fuel Contaminated 6-4 

6-6 Polyimide Sample 7—Abraded Through to the Conductor, Hydraulic Fluid 

Contaminated 6-4

6-7 Cross-Linked ETFE Sample 1—Control Sample, No Condition 6-5 

6-8 Cross-Linked ETFE Sample 2—Abraded Through to Conductor, Saltwater 

Contaminated 6-5

6-9 Cross-Linked ETFE Sample 3—Abraded Through to Conductor, not 

Contaminated 6-5

6-10 Cross-Linked ETFE Sample 4—Abraded Through to Conductor, Hydraulic Fluid 

Contaminated 6-6

6-11 Cross-Linked ETFE Sample 5—Abraded Through to Conductor, Jet Fuel 

Contaminated 6-6

6-12 Results From Supplemental Test 1 6-10 

6-13 Results From Supplemental Test 2 6-11 

viii
LIST OF TABLES 

Table Page 

5-1 Systems and Components Tested on DC-9 5-1 

6-1 Sample Numbers and Installed Conditions Used for Final Validation 6-2 

6-2 Results of the Blind Analysis 6-7 

6-3 Correlation of Sample Letters to Sample Numbers 6-8 

6-4 Analysis Results for Distance From the Test Connection to the Condition 6-8 

ix
LIST OF ACRONYMS AND SYMBOLS 

ac Alternating current

C Capacitance 

c Speed of light 

CM CM Technologies Corporation 

dx Incremental length 

EDT Excited Dielectric Test

EMT Electromagnetic tubing 

ETFE Ethylene-tetrafluoroethylene 

FAA Federal Aviation Administration 

G Conductance 

GPIB General-purpose information bus 

L Inductance 

R Resistance 

TDR Time domain reflectometry 

Vp Velocity of propagation 

ZT Load impedance 

x
EXECUTIVE SUMMARY

In October 2000, the Federal Aviation Administration (FAA), as part of the Aging Aircraft 

Research Program, contracted with CM Technologies Corporation to develop an advanced 

electrical test method to assess the condition of installed aircraft wiring. One of the FAA’s goals 

under this program was to validate advanced test methods that could be used in the commercial 

aviation industry to detect the presence of damaged or defective wire insulation. Of specific

interest to the FAA and industry are test methods that can be used to detect the presence of 

nicked and chafed insulation. 

This report describes the development of an advanced test technique for assessing the condition 

of installed aircraft wiring. The Excited Dielectric TestTM (EDT) method is an electrical test that 

exploits a basic property of polar insulation materials, that is, some small amount of current will 

flow through the insulation material when the wire is exposed to an alternating electric field. 

The magnitude of this current varies with the frequency of the applied field and, in polar 

insulation materials, there is a specific frequency at which a maximum current occurs. This 

current is often expressed in terms of the phase angle, δ, between the applied voltage and the 

resultant current. The tangent of δ is known as the dissipation factor (DF). 

The other aspect of the EDT test method is the time domain reflectometry (TDR). Aircraft

wiring can be modeled as a classic transmission line (i.e., a continuous structure of resistors, 

inductors, and capacitors). By applying this model, the electrical characteristics of the wire (e.g., 

impedance, capacitance, DF, resistance) can be analyzed as distributed elements. TDR has been 

shown to be extremely effective in measuring the distributed characteristics of wiring. 

The EDT method applies established principles of both DF and TDR measurement theory. An 

alternating electric field, known as a forcing function, is applied to the wire under test at a 

frequency designed to induce the maximum DF. A TDR signature is acquired and stored under 

this excitation. The frequency of the forcing function is then changed and a second TDR 

signature is acquired and stored. The two signatures are then compared. Points where the

signatures do not overlay represent areas of weak insulation. Polyimide and cross-linked 

ethylene-tetrafluoroethylene were the insulation materials studied under this research effort. 

The underlying theory associated with the EDT method was validated in the laboratory and in 

the field. The EDT was demonstrated to be effective in detecting and locating common defects 

that may be present in aircraft wiring systems, including abrasions, fluid contamination, and 

thermal degradation. 

xi/xii
1. INTRODUCTION.

In April 2000, the Federal Aviation Administration (FAA) issued a Broad Agency 

Announcement to support the development of testing and inspection systems, technology, and 

techniques that characterize and identify material flaws in aircraft wiring. 

In response, CM Technologies Corporation (CM) proposed the development of a new inspection

technology, known as the Excited Dielectric TestTM (EDT) method. CM’s original proposal

postulated that the value of lumped impedance associated with a material flaw in aircraft wiring 

could be purposely altered (i.e., forced to increase or decrease) using an alternating current (ac) 

stimulus or an electrical forcing function. Furthermore, if the impedance of a material flaw were 

altered during a time domain reflectometery (TDR) test, it was postulated that the material flaw

would appear more pronounced on the resultant TDR waveform (signature). 

This report describes the research sponsored by the FAA, including 

• a discussion of the theoretical basis associated with the EDT 

• the results of an industry survey of suitable test hardware 

• a summary of the test results from a laboratory testing matrix 

1-1/1-2
2. THEORY. 

2.1 MODELING AIRCRAFT WIRING AS A TRANSMISSION LINE. 

Aircraft wiring can be modeled as a parallel wire transmission line possessing distributed 

resistance, capacitance, inductance, and conductance. The parallel wire model can be used to 

represent any two-conductor transmission line, such as a coaxial wire, two wires in a harness, or 

a single wire and a ground plane. 

Figure 2-1 shows the equivalent circuit for a small incremental length of a parallel wire

transmission line. Each incremental length (dx) has a series resistance (Rdx) associated with the 

conductors, a series inductance (Ldx) associated with the conductors, a shunt capacitance (Cdx) 

associated with the dielectric, and shunt conductance (Gdx) associated with the dielectric. These 

R, L, C, and G components are all defined per unit length and are considered as a single lumped 

element whose value is dependent upon the total length of the wire. 

FIGURE 2-1. INCREMENTAL COMPONENTS OF A REAL TRANSMISSION LINE 

For small lengths of wire, like those found on aircraft, a lossless transmission line model can be 

assumed. Under this simplified case, the series resistance of the conductor is assumed to be 

0 ohms, and the shunt conductance of the dielectric is assumed to be infinite (∞ ohms), see

figure 2-2. 

2-1
FIGURE 2-2. INCREMENTAL COMPONENTS OF A LOSSLESS TRANSMISSION LINE 

The inductance, (Ldx), is a function of the physical properties of the wire and, for installed 

wiring, can be assumed to be constant. The capacitance of a wire, (Cdx), is a function of 

geometry, the insulation materials, and the presence of any contamination. For example, consider 

an insulated wire and a ground plane (the airframe). The insulated wire and airframe form a

parallel plate capacitor where the wire’s conductor represents one of the plates, the ground plane 

or airframe serves as the other plate, and the wire’s insulation acts as the capacitor’s dielectric 

media. Considering that a wire’s insulation material is directly exposed to the environment and is

most susceptible to service wear or accidental damage, capacitance is the electrical parameter of

greatest interest in the model shown in figure 2-2. 

2.2 PHYSICS OF A CAPACITOR. 

A capacitor stores electrical energy in the dielectric by aligning the charge distribution of its 

molecules in one direction. This alignment is caused when an applied electric field is placed 

across the plates of the capacitor. While this molecular alignment is being made, a current

supplied by a source, such as a battery, flows between the plates and through the capacitor. The 

magnitude of this current, known as absorption current, is initially very large and decays to 

practically zero in a period of time that is exponentially proportional to the loop resistance of the 

circuit and the value of the capacitor. In an ideal capacitor, the absorption current will decay to 

exactly zero when the voltage of the capacitor equals the voltage of the source and no current

flows through the dielectric. 

When an electric field is applied and as the absorption current deposits charge in the capacitor,

the voltage across the capacitor plates will increase to source voltage and the absorption current 

will decrease to its minimum value. Because the current flows before the voltage builds, it is

said that the current leads the voltage in a capacitor. In a pure capacitor, the current would lead

the voltage by exactly 90°. Figure 2-3 shows a graph of the voltage and current for a capacitor.

In the ideal case, the red vector would overlay on the vertical axis indicating a 90° phase angle. 

2-2
In a real capacitor, however, there is some small current flow through the dielectric at all times. 

Because of this, the current leads the voltage by an angle slightly less than 90°. 

FIGURE 2-3. CURRENT VS VOLTAGE IN A CAPACITOR 

If the direction of the field imposed on the capacitor is reversed, as in the case of an alternating 

current electric field, the energy stored in the capacitor is returned to the source, causing the

absorption current to once again flow in the opposite direction. These charge and discharge 

cycles are seen as an impedance of current flow by the source. The impedance offered by a 

capacitor is inversely proportional to the value of capacitance and the frequency of electric field

alternations. 

When a high-frequency electric field is placed across a capacitor, the direction of molecular 

charge for most dielectric materials cannot keep up with the alternations of the electric field. As

the frequency of the applied field approaches 0 Hz (or direct current (dc)), however, there exists

a frequency where the direction of the molecular charge can follow the alternations of the 

applied field. At this frequency, the ability of the capacitor to store a charge is at a minimum and 

the apparent impedance of the capacitor will be at a maximum (i.e., it appears that the capacitor 

is dissipating energy). The term dissipation factor is applied to the measurement of this effect. 

Materials that behave in this manner are also known as polar. Non-polar-insulating materials

such as glass, ceramics, and oil do not demonstrate a dissipation factor peak because the 

molecules of these materials have little or no charge distribution. 

The project focused on two specific polar insulation types. The FAA required that polyimide

insulation be considered as a target for the test technology and choose from PVC/Glass/Nylon, 

cross-linked ethylene-tetrafluoroethylene (ETFE), Poly-X, and composite materials. CM chose 

cross-linked ETFE because of its polar characteristics and the widespread use throughout the

commercial aviation industry. 

2-3
2.3 MATERIAL CHANGES AND DISSIPATION FACTOR. 

Installed aircraft wiring is exposed to a variety of environmental influences that can cause a shift 

in the dissipation factor versus frequency curve. Some of the environmental influences that can 

affect the dissipation factor of a dielectric include 

• Temperature causing the dielectric to become brittle (less viscous) over time

• Moisture or other fluid contamination causing the dielectric to become more lossy 

• Nicked or abraded insulation resulting in the original dielectric being partially or 

completely replaced by air 

• Radiation causing the dielectric to become brittle (less viscous) over time

• Combinations of these factors 

Figure 2-4 illustrates the direction of the expected dissipation factor peak shift caused by a 

change in brittleness (viscosity) and temperature for a typical polar dielectric material. In

general terms, the shape of curve (i.e., a change in maximum value) can also be affected by

material changes in the dielectric. For example, moisture intrusion causes the maximum 

dissipation factor value to increase significantly when compared to a dry dielectric. 

Polar 

Material 

Nonpolar 

Material 

FIGURE 2-4. DISSIPATION FACTOR VS FREQUENCY IN POLAR AND 

NONPOLAR MATERIALS 

2-4
2.4 TIME DOMAIN REFLECTOMETRY AND DISTRIBUTED IMPEDANCE. 

As discussed earlier, aircraft and other types of wiring are unique circuit elements in that they

have both lumped (i.e., capacitance and dissipation factor) and distributed electrical 

characteristics. TDR is a well-established measurement technique that can be used to determine

the distributed electrical proprieties of a wire. Appendix A shows the results of time domain

reflectometer sensitivity test. 

A typical TDR measurement employs a step generator and an oscilloscope in a system best

described as a closed-loop radar. A voltage step is injected into the wire under test, and the

incident and reflected voltage waves are monitored by the oscilloscope at a particular point on 

the wire. TDR reveals at a glance the distributed characteristic impedance of the wire, and it 

shows both the position and the nature (i.e., resistive, capacitive, or inductive) of each 

discontinuity, or defect, within the wire. A typical TDR setup is shown in figure 2-5. 

Pulse Generator

Oscilloscope 

Er

ZT

FIGURE 2-5. FUNCTIONAL BLOCK DIAGRAM OF A TDR SYSTEM

The pulse generator produces a positive going pulse, Ei, which is applied to the wire under test.

The incident voltage pulse Ei travels down the wire at the velocity of propagation (Vp) that is

associated with the wire under test. Vp is a function of several physical characteristics of the 

wire including: 

• the dielectric constant of the insulation; 

• the geometry of the wire (i.e., coaxial, multiconductor, shielded twisted pair); 

• and the spacing between conductors. 

The maximum theoretical value of Vp is the speed of light (c). As a practical matter, most 

aircraft wires have a Vp ranging from 50% c to 80% c.

If the load impedance, ZT, is equal to the characteristic impedance of the wire, all of the incident

pulse are absorbed by the load impedance and no reflection occurs. If ZT is not equal to the 

characteristic impedance of the wire under test, a portion of the incident pulse, Ei, will be

absorbed and a portion will be reflected, Er. 

2-5
The ratio of the incident and reflected voltage waveforms is used to determine the efficiency of 

the transmission line (i.e., how effective is the wire in transmitting energy from the source to the 

load). This ratio is called the voltage reflection coefficient, or ρ, and is calculated by taking the

ratio of the magnitude of the reflected voltage pulse and the magnitude of the incident voltage 

pulse, see equation 2-1. 

ρ = Reflected voltage (Er) / Incident voltage (Ei) = (ZT – Zo)/ (ZT + Zo) (2-1) 

ρ varies from -1 for a wire terminated with zero impedance (short circuit) to +1 for a wire 

terminated with infinite impedance (open circuit). Some general TDR responses for various 

values of ZT are shown in figure 2-6. 

FIGURE 2-6. GENERAL TDR RESPONSES FOR RESISTIVE LOADS 

2.5 MERGING TDR AND DISSIPATION FACTOR MANIPULATION.

EDT is based on the combination of TDR and dissipation factor manipulation. If an ac forcing 

function is applied to a wire pair (i.e., single wire and airframe, multiple wires in a harness), the 

dissipation factor amplitude can be controlled by varying the frequency of the forcing function. 

This assertion is based on the frequency dependence of the dissipation factor for polar insulation 

2-6
materials such as polyimide insulation. TDR is sensitive to changes in capacitance and 

dissipation factor. An increase or decrease in the dissipation factor will produce an area of 

decreased or increased impedance in the area of the poorest quality capacitor, or section of wire. 

In the example shown in figure 2-7, the wire under test is a single conductor and a ground plane. 

The wire has an area of localized degradation—the nondegraded sections are shown in black, 

and the degraded section is shown in red. The ground plane is shown in green. 

FIGURE 2-7. TIME DOMAIN REFLECTOMETRY SIGNATURE OF A DEGRADED 

PORTION OF AN INSULATED WIRE 

If a section of this wire were degraded (e.g., aged, or exposed to a harsh environment), the 

dissipation factor peak for this section will occur at a different frequency when compared to the 

nondegraded portions of the wire. That is, the normal and degraded sections of this wire will 

appear as having different impedances to a certain excitation frequency (F1). If a TDR test was 

performed while an excitation signal of frequency F1 is applied, the difference in the dissipation 

factor will be seen as an area of increased impedance in the TDR signature. 

The TDR test sees the wire as a transmission line with three characteristic impedances with the 

degraded area appearing as an outlier. As a practical matter with just a single TDR signature, it 

would be difficult to determine if the difference in impedance in the degraded section was due to 

a defector flaw, or if the difference is related to some other factor (i.e., the spacing between the

wire and the airframe). For this to be known, a baseline TDR signature would be needed for

comparison. 

Figure 2-8 shows the TDR signature from the same wire with a forcing function applied at a

frequency F2, where F2>F1. At frequency F2, the apparent impedances of the normal and 

degraded sections change, but in opposite directions. The normal sections become better quality

capacitors at F2 (the dissipation factor moves away from the peak value), and the degraded

section becomes a poorer quality capacitor (the dissipation factor approaches the peak value). 

2-7
FIGURE 2-8. TIME DOMAIN REFLECTOMETRY SIGNATURE OF A DEGRADED 

WIRE AT FREQUENCY F2 

Figure 2-9 shows a comparison of the two TDR signatures acquired at forcing function 

frequencies F1 and F2. The difference of impedance between the two signatures indicates the 

location of the degraded section. These figures are general for illustrative purposes and are not 

to scale. Experience and observations suggest that the differences in impedance are, in reality, 

much more subtle. 

FIGURE 2-9. COMPARISON OF TDR SIGNATURES OF DEGRADED WIRE AT 

FREQUENCIES F1 AND F2 

2.6 IMPLEMENTATION OF THEORY. 

Implementation of EDT technique requires that two different electrical waveforms, a TDR pulse

and an ac forcing function, be applied to the wire under test simultaneously. 

2-8
The ac forcing function, which provides the excitation of the dielectric, is a low-frequency ac 

waveform with a frequency range of 1 MHz to 100 Hz—the frequency range is important 

because the naturally occurring peak dissipation factor for polyimide and cross-linked ETFE 

insulation falls within this range. Also, it was assumed that this frequency range was sufficient 

to bound a shift in peak dissipation factor caused by the degradation mechanisms considered in 

this project (e.g., fluid contamination, abrasions). 

The TDR pulse should be a fast risetime, step-function pulse as opposed to another pulse type 

such as a half-sine pulse. A step-function pulse provides the best horizontal (distance) resolution 

compared to other pulse types. In addition, because a step pulse is composed of a series of sine 

waves, the frequency content of a step pulse is very broad. It was assumed that by providing a

broadband TDR signal, the probability of detecting subtle defects in a wire’s insulation would be 

maximized. 

To apply the ac forcing function and TDR waveform simultaneously, a coupling network is 

needed. The EDT coupler must be able to pass both the low-frequency forcing function and the 

high-frequency TDR pulse with minimal loss and attenuation. 

2.6.1 Coupling Requirements and Design Details. 

Ideally, the EDT coupling device will: 

• allow a low-frequency ac signal, the forcing function, to pass, 

• protect the TDR instrument from seeing the forcing function source so that the waveform 

is not adversely affected,

• provide the largest bandwidth for the TDR to realize the greatest resolution, and 

• permit the two-way travel of the TDR pulse without attenuation. 

CM chose an LC “T” filter network composed of a capacitor and an inductor for the EDT 

coupling device. Figure 2-10 shows the basic configuration of the coupling network. 

FIGURE 2-10. A BASIC EDT COUPLING NETWORK 

2-9
2.6.2 Forcing Function Source Considerations. 

As mentioned earlier, the forcing function source is required to provide an ac waveform with 

frequencies ranging from 1 MHz to 100 Hz. This frequency range was selected to be bounding. 

In other words, the forcing function source needed sufficient range to excite the dielectric at or

near the natural peak dissipation factor frequency. 

As a question to be researched, the amplitude and form factor of the forcing function should be 

selectable. To protect the TDR and to limit the power delivered to the wire under test, the

amplitude should be adjustable from 0 to 5 volts. To investigate the effect that the forcing 

function form factor has on a wire sample, the source is required to supply a sine wave, a 

sawtooth wave, a square wave, and pulse trains. 

2.6.3 Time Domain Reflectometry Instrument Considerations. 

Some of the important considerations for the TDR instrument include: 

• The TDR is required to be a digital system to allow the digital representation of the

signatures. This is necessary because several signatures are to be overlaid to detect subtle

differences in impedance. 

• The TDR is required to have sufficient resolution at wire lengths found in commercial 

aircraft, up to 1000 feet (approximately 300 m). 

• The electrical energy the TDR pulse delivers to the circuit under test is required to be 

very small. This is an important consideration given the environments that wires are

subjected to in a commercial aircraft and the thickness of the insulation. For example, if 

a nicked wire were installed in a fuel tank, the energy of the TDR pulse would need to be 

adjustable to evaluate the wire in a safe manner. 

2-10
3. EVALUATION OF COMMERCIALLY AVAILABLE EQUIPMENT. 

3.1 COUPLING NETWORK. 

Several options were considered for coupling the TDR pulse and the ac forcing function onto the 

wire under test. Beyond the basic desired performance characteristics size, weight, ruggedness, 

availability, and the ability to withstand voltage were also considered. CM felt that the

Picosecond Pulse Laboratory, Bias Tee Network, Model 5541A, was the best choice for this 

project. 

 

This coupling network passed the forcing function and the TDR pulse efficiently. The network 

adds a noticeable amount of series impedance to the circuit under test. For the purposes of this 

project, however, this added impedance is not a problem because only a qualitative, not 

quantitative, indication of impedance is required. 

Figure 3-1 shows the performance of the selected filter network. The cyan trace shows the TDR

signature without the selected coupling network. The green trace is the signature of the same

circuit with the coupling network installed. The impedance of the wire under test is not affected 

by the coupling network. 

Impedance of the wire under 

test is identical except for 

resistive offset

Green signature was acquired 

with the coupling network

Cyan signature was acquired 

without the coupling network

FIGURE 3-1. SAMPLE TDR SIGNATURE WITH AND WITHOUT FILTER NETWORK 

3-1
3.2 FORCING FUNCTION SOURCES. 

3.2.1 Hewlett-Packard LF Impedance Analyzer, Model 4192A. 

The Hewlett-Packard LF Impedance Analyzer, Model 4192A was investigated as a possible 

excitation source for several reasons. The 4192A can sweep frequencies from 5 Hz to 13 MHz 

and acquire impedance data, including dissipation factor. This unit can be programmed to

perform swept frequency measurements and can be interfaced with a controller via a generalpurpose information bus (GPIB). 

While the 4192A is very versatile, several limitations precluded this unit from further study. 

Specifically: 

• The output waveforms available from the 4192A are limited—only sine waves with 

amplitudes up to 1 volt are possible. 

• When the instrument was programmed for very low frequencies, 5 to 20 Hz, the resulting 

impedance measurements were unstable and not repeatable in the laboratory. 

• Using the 4192A would require subtracting the impedance values of the coupler from the

data. There was a concern that subtle shifts in impedance or dissipation factor may have 

been masked by the presence of the coupler. 

No data presented in this report was collected with the 4192A because repeatability could not be 

obtained at very low frequencies. 

3.2.2 Tabor Electronics Model 8020 Programmable Function Generator. 

The Tabor Electronics Model 8020 function generator was ultimately selected for use in this

project. 

The 8020 can source waveforms with frequencies from 2 to 20 MHz with amplitudes as large as 

30 volts. The 8020 produces a variety of waveforms, including sine, square, sawtooth, and pulse 

trains, both negative and positive. The model 8020 also has a GPIB port to allow interfacing 

with a controller for automated control. All the data that are presented in this report was taken

using the Tabor Electronics Model 8020. 

3.3 TIME DOMAIN REFLECTOMETERS. 

The performance of two TDR instruments was evaluated for this application. Both instruments

met the requirement of being digital and allowing comparison of multiple signatures. 

3.3.1 Agilent 54754A TDR Module. 

The Agilent 54754A is a high-end, laboratory-grade TDR. The system operates under a 

Windows® operating system and has the ability to store thousands of signatures internally. The

3-2
54754A has an adjustable pulse width to allow testing of various wire lengths ranging from less

than 1 inch to several hundred feet long. 

The 54754A produces a continuous train of output pulses for a real-time measurement. This

instrument requires that the user have a certain amount of knowledge about the wire under test in 

order to adjust the pulse width for the required length. The presentation of the TDR signature is 

displayed with the horizontal axis in units of time rather than actual distance. Therefore, the user 

must multiply the measure time by the velocity of propagation of the wire to separately calculate

the distance. 

The software controlling the 54754A has a very convenient zoom feature, which in conjunction

with a touchscreen operation, allows rapid zooming in on TDR signatures. The feature, 

however, will not zoom in or out on stored signatures. 

An important practical limitation of the 54754A TDR module is its sensitivity to electrostatic

damage. A slight static charge induced across the input will destroy the module. Static charges 

are quite common on installed aircraft wiring, so care needs to be taken when acquiring field 

data with the 54754A TDR module. 

3.3.2 CM Technologies PCI-3100 TDR. 

The PCI-3100 TDR is implemented in an ISA expansion card form factor and operates within an 

IBM®-compatible personal computer. The PCI-3100 can test circuits with lengths up to 3000 

feet. The control software allows the user to compare up to eight waveforms, and stores as many 

signatures as the computer hard drive can accommodate. The PCI-3100 control software can

magnify acquired and stored signatures up to 64x in steps of 2 (e.g., 2x, 4x, 8x). The control 

software also allows manipulation of the velocity of propagation so that an accurate 

measurement of length can be made between any two points along the waveform. 

 

The pulse generator used in the PCI-3100 is very robust. Extensive field use of the PCI-3100 

suggests that its pulser, while not as fast as the pulser used in the Agilent 54754A TDR module, 

is more tolerant of electrostatic discharge. 

The majority of the data presented in this report were acquired with the PCI-3100 TDR. 

3.3.3 Final Equipment Configuration. 

Figure 3-2 is a block diagram of the final equipment configuration used in this project. This is

the configuration with which all EDT data in this report were obtained. 

3-3
FIGURE 3-2. FINAL TEST CONFIGURATION 

Figure 3-3 shows the prototype EDT equipment (labeled 1-4) used during a field test evaluation. 

1. CM Technologies PCI-3100 TDR installed in a field computer 

2. Tabor Electronics 8020 function generator 

3. Agilent 54754A TDR Module installed in the Agilent Model 86100 Oscilloscope 

Mainframe 

4. Picosecond Pulse Laboratory, Bias Tee Network, Model 5541A 

FIGURE 3-3. PROTOTYPE EDT SYSTEM IN USE IN THE FIELD 

3-4
4. WIRE SAMPLE SELECTION, CONDITIONING, AND TESTING. 

All the wire samples used in the project started as new material. The samples were exposed to a 

variety of conditions in known locations and in a manner such that they may be repeated in other 

samples (e.g., the amount of heating used to condition a polyimide sample was the same amount 

used to condition the cross-linked ETFE samples). 

The FAA required that two insulation materials be studied: polyimide insulation and another 

type used in the industry. The other type selected for this project was cross-linked ETFE. 

Spools of wire for both insulation types were procured from A.E. Petsche Company. Each spool 

consisted of an unshielded single conductor, 16 AWG. 

The polyimide sample is Mil Spec M81381/11-20-N and was manufactured by Barcel/CBT. The

cross-linked ETFE sample is Mil Spec BMS14-48T2C1G20 and was manufactured by Raychem 

Corporation. 

4.1 WIRE SAMPLE TEST FIXTURE. 

The test fixture used in this project consisted of three sections of 3/4-inch electromagnetic tubing 

(EMT) connected to four watertight metallic enclosures spaced 20 feet apart. Terminal strips are

located inside each enclosure to install conditioned wire samples. The intention of this project

was to only condition the center wire sample to show response of conditioned samples and 

nondegraded samples in the same test (this is the same model used in the theoretical discussion 

in section 2.5). Figure 4-1 shows the test fixture in its entirety. 

FIGURE 4-1. THE EDT TEST FIXTURE 

4-1
Each metallic enclosure is grounded to a single-point earth ground. Figure 4-2 shows the ground 

connection on enclosure 1, which is typical of all enclosures. 

FIGURE 4-2. GROUNDING CONNECTION USED WITH ALL ENCLOSURES

As shown in figure 4-2, the coating of the enclosure mount was removed to bare metal to insure 

a good, consistent ground. 

Enclosure 1, as shown in figure 4-3, is the location of the EDT test lead connection (red and 

black alligator clips) to the wire sample. The inside of enclosure 1 is typical of the other 

enclosures, with the exception of the convenient ground connection in the lower portion of the 

panel and the PVC test lead tube. Coating on both sides of the lower right mount hole of all 

panels was removed to bare metal to insure a good and consistent ground. 

FIGURE 4-3. INSIDE ENCLOSURE 1 

4-2
4.2 INITIAL TESTING. 

The first goal of the project was to demonstrate the basic theory associated with the EDT 

method. To achieve this goal, the project began with the application of EDT to a cross-linked 

polyethylene (XLPE) insulated coaxial wire. Coaxial wire was selected as the starting point 

because it possesses well-understood impedance characteristics and has a uniform geometry. 

Two 30-foot lengths of coaxial cable were connected together to form the circuit and were 

installed in the test fixture. The barrel connector used to connect both samples together was 

insulated from ground so that the shield was floated. 

The first 30-foot length was not conditioned. The second 30-foot length was conditioned by 

introducing an abrasion to the wire jacket located approximately 18 inches from the intermediate 

barrel connector. The EDT was applied between the shield and the ground of the test fixture 

since this configuration would consider the condition of the jacket material. 

Figure 4-4 shows the first demonstration of the EDT method. Figure 4-4 is a 32x magnification 

of the coaxial sample excited at 1 Hz (cyan signature) and 15 Hz (red signature). The green 

signature was acquired with the coaxial cable open at the intermediate barrel connector to easily 

locate the end of the first 30-foot length and the location of the connector. 

• The blue signature was acquired using an excitation frequency of 1 Hz. The red signature 

was acquired at 15 Hz. 

• Item 1 shows the location of the connection between both samples. This signature was 

acquired with cable 2 disconnected from cable 1 with no excitation. 

• Item 2 shows the location of the connector on cable 1, which was insulated from ground 

with electrical tape. 

• Item 3 shows the location of the connector on cable 2, which was insulated from ground 

with electrical tape. 

• Item 4 shows the location of the beginning of the conditioned area (approximately 18 

inches from item 3). 

• Item 5 shows the location of the unconditioned areas of the two cables near the connector

area. 

4-3
FIGURE 4-4. RESULTS OF EDT APPLIED TO A CONDITIONED COAXIAL CABLE 

The change in insulation at the connectors is clearly indicated by the difference in impedance

seen when using two different forcing function frequencies. The conditioned area is also

indicated, although not uniformly. The lack of uniformity is due to the conditioned portion of 

the cable not lying uniformly with respect to the ground plane. 

4.3 CONDITIONING OF WIRE SAMPLES. 

Wire degradation was grouped into three general categories: breached insulation that had not 

become contaminated (i.e., a clean breach), breached insulation that had become contaminated, 

and nonbreached insulation. The nonbreached wire samples were subjected to conditions and

media that were believed to be realistic (e.g., fluids found on commercial aircraft) and would be 

representative of service wear in an aircraft. Generally, the samples were conditioned until a 

visible difference (e.g., color change) could be discerned. Breached samples included complete 

abrasion (abraded through until the conductor became exposed) and partially abraded (the

insulation was abraded but the conductor was not exposed). 

4.3.1 Nonbreached Samples. 

Twenty-foot samples of polyimide and cross-linked ETFE were placed in fluids and conditions

to simulate the environments seen by wiring installed on commercial aircraft. The samples were

placed in containers so that only the middle 1-foot section of the sample was immersed in the 

conditioning media. 

4-4
4.3.2 Polyimide Control. 

This sample was tested extensively with a number of different forcing function waveforms to 

determine if the forcing function form factor had any effect on the technique. 

Figure 4-5 contains the results of an EDT on the polyimide control sample with two forcing 

functions. In this case, the waveform characteristics of the forcing functions were identical, but 

the source frequency was varied. 

The results from these tests were as expected for the control sample—no appreciable difference 

when the frequency of the forcing function was changed. This was an important result to the 

project because the theory behind EDT assumes that the electrical characteristics of each 

incremental length of wire will change in a uniform fashion for a homogeneous wire without any 

defects (control). 

Applying EDT to the Polyimide 

Control Sample Using Different 

Forcing Functions Produced

Expected Results (i.e., no change)

FIGURE 4-5. EXCITED DIELECTRIC TEST RESULTS FROM THE POLYIMIDE 

CONTROL SAMPLE 

Another question that was considered early in the project was the shape of the forcing function. 

Would different waveform types cause the dielectric to act differently during an EDT? 

4-5
Figure 4-6 shows an example of the results. Figure 4-6 is a comparison of the four TDR 

signatures acquired using four different forcing function waveforms: sine wave, sawtooth, 

positive pulse, and square. All four TDR signatures are identical, suggesting that the shape of 

the forcing function (i.e., sine, triangle) does not have a significant effect on the EDT results. 

The shape of the 

Forcing Function 

waveform had 

little effect on the 

TDR signature

FIGURE 4-6. EXCITED DIELECTRIC TEST RESULTS ACQUIRED USING DIFFERENT 

FORCING FUNCTIONS 

Another variable that was examined during the early stages of the research was the effect, if any,

that the amplitude of the forcing function had on the EDT method. 

Figure 4-7 is a comparison of two EDT signatures: the green signature was acquired with a 

1 volt peak-to-peak (Vpp) sine wave forcing function, and the cyan signature was acquired with 

a 5 Vpp sine wave forcing function. The data suggest that the EDT method is not affected by the 

amplitude of the forcing function. 

This was an important finding since there is always a concern regarding the amount of energy 

placed on an installed wire in an aircraft. Since the amplitude of the forcing function does not 

appear to influence the EDT results, the energy introduced into the wire can be kept at a 

minimum. 

4-6
The amplitude of the 

Forcing Function 

waveform had little 

effect on the TDR 

signature 

FIGURE 4-7. EXCITED DIELECTRIC TEST RESULTS FROM POLYIMIDE CONTROL 

SAMPLE USING FORCING FUNCTIONS OF DIFFERENT AMPLITUDE 

4.3.3 Polyimide Thermally Conditioned. 

This sample was bent around a cylindrical steel mandrel for heating. The sample was maintained 

at approximately 450oF for 50 hours. After 50 hours of heating, the insulation appeared to be 

discolored. 

An example of the results from the thermally condition wire are shown in figure 4-8. Figure 4-8

compares an EDT with a 20-MHz sine wave forcing function (green) to an EDT with a 20-Hz 

sine wave forcing function (cyan). The area that was heated on the mandrel appeared differently

(i.e., increased/decreased impedance) to the TDR when exposed to a 20-MHz and 20-Hz forcing 

function. 

These results are an excellent illustration of the EDT method. The localized heating caused the

wire sample to become brittle (less viscous) and the dissipation factor curve to shift. This shift 

was seen as a change in impedance on the TDR signature. 

4-7
The area that was heated 

appeared as an

impedance change with a 

20 MHz and 20 Hz 

forcing function 

FIGURE 4-8. RESULTS FROM A THERMALLY CONDITIONED POLYIMIDE SAMPLE 

4.3.4 Polyimide Conditioned With Tap Water. 

This sample was immersed in ordinary tap water. The sample was tested at 6 months and 13 

months. The contaminated length of the sample had a white powder-like film that was easily

removed and was slightly lighter in color than the unconditioned areas. When tested with the

EDT method, there was no indication of degradation. 

4.3.5 Polyimide Conditioned With Natural Saltwater. 

This sample was immersed in natural saltwater and was tested at 6 months and 13 months. The 

conditioned portion of the sample was lighter in color than the unconditioned length of wire. 

When tested with the EDT method, there was no indication of the condition. 

4.3.6 Polyimide Conditioned With Jet Fuel. 

This sample was immersed in jet fuel and was tested at 6 months and 13 months. The 

conditioned portion of the sample became much darker than the unconditioned length of wire 

and the jet fuel developed a jelly-like deposit on the bottom of the container. When tested with 

the EDT method, there was no indication of the condition. 

4-8
4.3.7 Polyimide Conditioned With Hydrazine. 

During one of the project reviews, a National Aeronautics and Space Administration 

representative suggested that hydrazine be considered as a possible contaminate. A 1-foot

section of a sample was immersed in a 35% solution of hydrazine. After 72 hours, the outer 

layer of the insulation appeared to be bleached nearly white and a yellowish precipitate was

deposited on the bottom of the container. This conditioning was detected with the EDT. 

Figure 4-9 shows (at 32x zoom) the EDT results of hydrazine contamination using forcing 

functions of 200 MHz, 2 Hz, and 20 Hz. Item 1 indicates the conditioned area where the 2-Hz 

signature exhibits increased impedance compared to the 20-Hz signature. 

FIGURE 4-9. TIME DOMAIN REFLECTOMETRY SIGNATURES AT 200 MHz, 2 Hz, AND 

20 Hz OF HYDRAZINE-CONDITIONED POLYIMIDE 

4.3.8 Polyimide Conditioned by Boiling in Water. 

During one of the project reviews, a U.S. Air Force participant suggested that boiling a

polyimide wire would produce an interesting test case. The middle 1-foot section of a polyimide 

insulated wire was boiled in tap water for 4 hours. This method of accelerated aging was found 

to yield a positive result with the EDT. Figure 4-10 shows the 200-mHz, 2-Hz, and 20-Hz TDR 

signatures of the boiled polyimide sample installed in the test fixture. Items 1-5 in figure 4-10 

indicate: 

• Item 1—Test point connection in box 1. 

• Item 2—End of first 20-foot section at box 2. 

4-9
• Item 3—Middle of second 20-foot section between boxes 2 and 3. 

• Item 4—End of second 20-foot section at box 3. 

• Item 5—End of third 20-foot section at box 4. 

FIGURE 4-10. EXCITED DIELECTRIC TEST SIGNATURES OF BOILED POLYIMIDE 

This figure is only 8x zoom of the signatures. The difference in the low-frequency traces (green 

200 MHz, cyan 2 Hz, and red 20 Hz) is very apparent. 

4.3.9 Cross-Linked ETFE Conditioned in Water. 

A 1-foot section of a cross-linked ETFE sample was immersed in ordinary tap water for 13 

months. The sample was new material and had no other defects present. For this reason, the

EDT detected no condition. 

4.3.10 Cross-Linked ETFE Conditioned in Saltwater. 

A 1-foot section of a cross-linked ETFE sample was also soaked in natural saltwater for 13

months. The sample was also new material and had no other defects present. For this reason, 

the EDT detected no condition in this sample. 

4-10
4.3.11 Cross-Linked ETFE Conditioned in Jet Fuel. 

A 1-foot section of a cross-linked ETFE sample was immersed in jet fuel for 13 months. Unlike 

the polyimide sample soaked in jet fuel, EDT detected a condition in the center area of the crosslinked ETFE sample when it was installed in the test fixture.

Figure 4-11 is a comparison, at 4x zoom, of the 200-MHz, 2-Hz and 20-Hz signatures of the 

cross-linked ETFE sample conditioned with jet fuel. Items 1-5 in figure 4-11 indicate: 

• Item 1—Test connection at box 1. 

• Item 2—End of first 20-foot section at box 2. 

• Item 3—Middle of second 20-foot section between boxes 2 and 3. 

• Item 4—End of second 20-foot section at box 3. 

• Item 5—End of third 20-foot section at box 4. 

FIGURE 4-11. SIGNATURE OF CROSS-LINKED ETFE CONDITIONED IN JET FUEL 

This condition is visually detectable near item 3 at 4x zoom. The condition becomes more 

distinct at 32x zoom, see figure 4-12. Items 1 and 2 in figure 4-12 indicate: 

• Item 1—Indication of condition. 

• Item 2—End of second 20-foot section at box 3. 

4-11
FIGURE 4-12. ZOOM OF MIDDLE SECTION OF JET FUEL-CONDITIONED SAMPLE 

The green, cyan, and red signatures were acquired using forcing functions of 200 MHz, 2 Hz, 

and 20 Hz, respectively. 

4-12
5. FIELD TESTS. 

Two field test demonstrations were performed in conjunction with AirTran Airways. The first 

demonstration used the PCI-3100 TDR system configuration while the second demonstration 

used the Agilent 54754A TDR. The reason for using different system configurations was to 

demonstrate that both hardware versions could be taken to the field and acquire data. 

5.1 FIRST FIELD TEST—PCI-3100 TDR. 

The first demonstration was on a DC-9 aircraft during a heavy maintenance check in Lake City, 

Florida. The circuits and associated systems that were tested during this field test are shown in 

table 5-1. The airline partner formulated the list of candidate circuits based on their experience 

in maintaining the DC-9 aircraft and the installed environments. 

TABLE 5-1. SYSTEMS AND COMPONENTS TESTED ON DC-9 

System Component 

AC Power Left Generator 

AC Power Right Generator 

AC Power Aux Power Unit 

Anti-Skid Left Inboard Wheel Speed Transducer 

Anti-Skid Left Outboard Wheel Speed Transducer 

Anti-Skid Right Inboard Wheel Speed Transducer 

Anti-Skid Right Outboard Wheel Speed Transducer 

Anti-Skid Primary Left I/O Solenoid Valve 

Anti-Skid Secondary Left I/O Solenoid Valve 

Anti-Skid Primary Right I/O Solenoid Valve 

Anti-Skid Secondary Right I/O Solenoid Valve 

Ram Air Temperature Ram Air Temperature Detector 

Ram Air Temperature Standby Ram Air Temperature Detector 

Fire Detection Left Engine Loop A 

Fire Detection Left Engine Loop B 

Fire Detection Right Engine Loop A 

Fire Detection Right Engine Loop B 

Fire Detection Auxiliary Power Unit Loop A 

Fire Detection Auxiliary Power Unit Loop B 

No modifications were made to the circuits to facilitate testing because the aircraft was in active 

service. Therefore, all the circuits were tested with their respective loads left connected. 

In addition, ac power was applied to some aircraft systems during the testing. This resulted in a

significant amount of induced electrical noise in the plane and made most of the circuits that 

5-1
were terminated with large inductances, such as generators, too noisy for effective EDT analysis 

of the TDR signatures. 

Of the circuits tested during this demonstration, the anti-skid left inboard wheel speed transducer 

is the only circuit that indicated a possible condition. Figure 5-1 is a comparison of the 4x 

magnification of the 200-MHz (green), 2-Hz (cyan), and 20-Hz (red) TDR signatures from this

circuit. Items 1-7 in figure 5-1 indicate: 

• Item 1—Test lead connection. 

• Item 2—Terminal board. 

• Item 3—Terminal board. 

• Item 4—Pressurized to unpressurized connector. 

• Item 5—Section of possible degradation. 

• Item 6—Section of possible degradation in wheel well. 

• Item 7—End of circuit. 

FIGURE 5-1. EXCITED DIELECTRIC TEST SIGNATURES OF THE LEFT INBOARD 

WHEEL SPEED TRANSDUCER 

The impedance of the cyan signature near item 5 is slightly lower than the other signatures. 

When this area is magnified, see figure 5-2, the areas on the cyan signature near the pressurized

and unpressurized connector appear to have lower impedance compared to the other signatures. 

• Item 1—Section of possible degradation before pressurized and unpressurized connector. 

• Item 2—Section of possible degradation at the pressurized and unpressurized connector. 

5-2
FIGURE 5-2. EXCITED DIELECTRIC TEST SIGNATURES OF THE LEFT 

INBOARD TRANSDUCER 

In addition, the cyan signature indicated an area of low impedance near item 6 in figure 5-1 

which corresponded to the location of the wheel well on the aircraft. When this area was 

magnified, see figure 5-3, the cyan signature had lower impedance in the wheel well area 

compared to the other signatures. This response could be related to the environment associated 

with this circuit (i.e., the wheel well represents a harsher environment compared to other areas on 

the aircraft). The cyan signature was acquired using a 2-Hz forcing function. 

• Item 1—Possible degraded area at transducer connector. 

• Item 2—Possible degraded area of wire to transducer. 

FIGURE 5-3. EXCITED DIELECTRIC TEST SIGNATURES OF THE END OF THE 

LEFT INBOARD TRANSDUCER CIRCUIT 

5-3
5.2 SECOND FIELD TEST—AGLIENT 54754A TDR. 

This field testing demonstration was performed with the Agilent 54754A TDR on a DC-9 aircraft

that had recently been removed from service and was scheduled to be scrapped. 

The circuits selected for testing included generator power cables and all of the fuel boost pumps. 

In general, all the circuits that were tested appeared to be in very good condition, and the realtime analysis performed in the hanger found no areas of degradation. These test files are not 

included in this report. 

5-4
6. VALIDATION. 

The final validation test was performed at the FAA William J. Hughes Technical Center on 

March 26 and 27, 2002. 

6.1 REQUIREMENTS FOR VALIDATION. 

The final validation of this project required a double-blind test. To ensure a double-blind 

procedure, samples of known conditions would be prepared and inserted into a metal conduit test 

fixture that was setup at the FAA William J. Hughes Technical Center. 

FIGURE 6-1. VALIDATION TESTING AT THE FAA WILLIAM J. HUGHES 

TECHNICAL CENTER 

6.2 VALIDATION DETAILS.

Twenty-foot wire samples were prepared so that the ends of each sample appeared identical. A

crimp type termination was attached to an end of the sample and insulating heat shrink was 

applied to the other end. Each sample was then marked by placing a number in the middle. The 

conditions, with the exception of the ultraviolet (UV) conditioned polyimide sample, were 

inserted on new wires at randomly selected locations at least 1 foot from either end and far 

enough away from the middle so as not to interfere with the sample numbering. The UVconditioned sample was conditioned by exposing the middle foot length to natural sunlight for 13 

months. 

The EMT test fixture was prepared to be 19.5 feet long, allowing approximately 6 inches of the 

samples to extend out for attaching alligator-style test clips. After the samples were inserted in 

the test fixture, the wire ends were randomly assigned a letter. Since the numbers for each 

sample were hidden by the EMT, the operator would only know if the sample was polyimide or

6-1
cross-linked ETFE by the color of the insulation, but would not know how a particular sample 

had been conditioned. 

The purpose of the EMT was twofold. First, it provided a convenient mechanism for hiding the 

sample numbers during the double-blind test. Second, and more importantly, it served as a 

simulated airframe or ground for the test in a conference room environment. Since all the wire 

samples used in the validation were unshielded single conductor, another conductor was needed. 

The diameter of the EMT used during the validation was arbitrary. It was large enough to ensure

that the distance between each wire and ground was not constricted and made artificially uniform 

by the test fixture. 

Table 6-1 contains the description of each sample. Again, as mentioned earlier, each sample 

began as a brand new wire. 

TABLE 6-1. SAMPLE NUMBERS AND INSTALLED CONDITIONS USED FOR 

FINAL VALIDATION 

Sample Number Condition 

Polyimide 1 Control-no condition 

Polyimide 2 Natural UV exposed for 13 months 

Polyimide 3 Abraded not through to conductor not contaminated

Polyimide 4 Boiled in water for 4 hours 

Polyimide 5 Abraded through to conductor, saltwater 

contaminated for 72 hours 

Polyimide 6 Abraded not through to conductor, jet fuel 

contaminated for 72 hours 

Polyimide 7 Abraded through to conductor, hydraulic fluid 

contaminated for 72 hours 

Cross-linked ETFE 1 Control-no condition 

Cross-linked ETFE 2 Abraded through to conductor, saltwater 

contaminated for 72 hours 

Cross-linked ETFE 3 Abraded through to conductor not contaminated 

Cross-linked ETFE 4 Abraded through to conductor, hydraulic fluid 

contaminated for 72 hours 

Cross-linked ETFE 5 Abraded through to conductor, jet fuel 

contaminated for 72 hours 

Figures 6-2 through 6-6 show the conditioned sections of each polyimide wire sample. 

Figures 6-7 through 6-11 show 3x photographs of the conditioned sections of each wire sample. 

The polyimide boiled and UV samples are not included as there is no visually discernable 

difference between the nonconditioned and conditioned sections. The conditioned section of the 

sample is centered in the 0.5-inch target. 

6-2
1/2 in. target 

FIGURE 6-2. POLYIMIDE SAMPLE 1—UNCONDITIONED SECTION 

1/2 in. target 

FIGURE 6-3. POLYIMIDE SAMPLE 3—ABRADED NOT THROUGH ELECTRICALLY, 

NOT CONTAMINATED

1/2 in. target 

FIGURE 6-4. POLYIMIDE SAMPLE 5—ABRADED THROUGH TO CONDUCTOR, 

SALTWATER CONTAMINATED 

6-3
1/2 in. target 

FIGURE 6-5. POLYIMIDE SAMPLE 6—ABRADED NOT THROUGH TO CONDUCTOR, 

JET FUEL CONTAMINATED 

1/2 in. target 

FIGURE 6-6. POLYIMIDE SAMPLE 7—ABRADED THROUGH TO THE CONDUCTOR, 

HYDRAULIC FLUID CONTAMINATED 

Extreme care was required to abrade the polyimide samples. If the insulation was cut through 

completely, the entire section of insulation would unravel from the conductor and leave a large

section of bare wire. This condition was not desired and would not challenge the EDT method 

sufficiently.

6-4
1/2 in. target 

FIGURE 6-7. CROSS-LINKED ETFE SAMPLE 1—CONTROL SAMPLE, 

NO CONDITION 

1/2 in. target 

FIGURE 6-8. CROSS-LINKED ETFE SAMPLE 2—ABRADED THROUGH TO 

CONDUCTOR, SALTWATER CONTAMINATED 

1/2 in. target 

FIGURE 6-9. CROSS-LINKED ETFE SAMPLE 3—ABRADED THROUGH TO 

CONDUCTOR, NOT CONTAMINATED 

6-5
1/2 in. target 

FIGURE 6-10. CROSS-LINKED ETFE SAMPLE 4—ABRADED THROUGH TO 

CONDUCTOR, HYDRAULIC FLUID CONTAMINATED

1/2 in. target 

FIGURE 6-11. CROSS-LINKED ETFE SAMPLE 5—ABRADED THROUGH TO 

CONDUCTOR, JET FUEL CONTAMINATED 

The cross-linked ETFE samples were scraped until a connected electrical continuity meter 

indicated that the abrasion was through to the conductor. 

6.3 VALIDATION TEST. 

Two sets of data were collected on each sample using two different equipment operators.

Testing was done with a forcing function with a 5-volt amplitude and a sine wave form factor. 

Data were collected at 2 MHz, 20 MHz, 200 MHz, 2 Hz, and 20 Hz. 

6.4 DATA ANALYSIS.

Prior to the removal of the samples from the test fixture, both sets of data were analyzed by CM 

Technologies. The level of analysis was kept realistic by implementing the following rules: 

• Limit the analysis time to approximately 5 minutes, the amount of time an average 

technician would be willing to spend analyzing data. 

6-6
• Limit comparisons to different forcing functions. No attempt was made to compare 

results from one sample to another. 

• Geometry effects were accepted. No attempt was made to make the TDR signatures 

appear uniform by fixing the distance between each wire and ground. 

The results of the analysis for each set of data are summarized in table 6-2. 

TABLE 6-2. RESULTS OF THE BLIND ANALYSIS 

Sample 

Letter Test 1 Test 2 

A Condition at 13.7′ from the test 

connection 

No condition 

B Condition at 4′ from the test connection, 

may be a contaminated breach. May be 

a condition at 13.5′ from test condition. 

Condition at 5′ from the test 

connection. 

C No condition. Condition at 4.8′ from the test 

connection. 

D No condition. No condition. 

E No condition. Condition at 9′ from the test 

connection. 

F Condition at 10′ from test connection. 

Possibly a noncontaminated breach.

No condition. 

G No condition. Condition at 12.5′ from the test 

connection. 

H Condition at 12.4′ from the test 

connection. May be boiled. 

No condition. 

I Condition at 9′ from 

the test connection. 

No condition. 

J Condition at 3′ from the test connection. 

May be a contaminated breach. 

Condition at 8.3′ from the test 

connection. 

K Condition at 9′ to 10′ from the test 

connection. Could be UV. 

Condition at 14.7′ from the test 

connection. 

L No condition. Condition at 7.5′ from the test 

connection, possible contaminated 

breach. 

The samples were then removed from the test fixture to match the sample number and letters. 

Table 6-3 correlates the blind sample letters to the sample numbers. 

6-7
TABLE 6-3. CORRELATION OF SAMPLE LETTERS TO SAMPLE NUMBERS 

Sample Letter Sample Number Sample Condition 

A Polyimide 7 Abraded through to conductor, hydraulic fluid contaminated for 72

hours. 

B Cross-linked ETFE 2 Abraded through to conductor, saltwater contaminated for 72 hours. 

C Cross-linked ETFE 5 Abraded through to conductor, hydraulic fluid contaminated for 72

hours. 

D Cross-linked ETFE 1 Control-no condition. 

E Cross-linked ETFE 4 Abraded through to conductor, hydraulic fluid contaminated for 72

hours. 

F Cross-linked ETFE 3 Abraded through to conductor not contaminated. 

G Polyimide 2 Natural UV exposed for 13 months. 

H Polyimide 4 Boiled in water for 4 hours. 

I Polyimide 5 Abraded through to conductor, saltwater contaminated for 72 hours. 

J Polyimide 1 Control-no condition.

K Polyimide 3 Abraded not through to conductor not contaminated. 

L Polyimide 6 Abraded not through to conductor, jet fuel contaminated for 72 hours. 

The distance from the test connection to each condition was then measured. Table 6-4 contains

the distance, sample number, condition, and analysis results for each set of data. 

TABLE 6-4. ANALYSIS RESULTS FOR DISTANCE FROM THE TEST CONNECTION TO

THE CONDITION 

Sample

Number 

Blind 

Test 

Letter 

Measured 

Distance to

Condition Condition 

Data Set 1 

Result Data Set 2 Result

Polyimide 7 A 8′ 8′′ from test 

connection. 

Abraded through to 

conductor, hydraulic fluid

contaminated for 72 hours 

Condition at 13.7′ from 

the test connection. 

No condition. 

Cross-linked 

ETFE 2 

B 9′ 9′′ from test 

connection. 

Abraded through to 

conductor, saltwater

contaminated for 72 hours 

Condition at 4′ from 

the test connection, 

may be a contaminated 

breach. May be a 

condition at 13.5′ from 

the test connection. 

Condition at 5′

from the test 

connection. 

Cross-linked 

ETFE 5 

C 10′ 3′′ from test 

connection 

Abraded through to 

conductor, hydraulic fluid

contaminated for 72 hours 

No condition. Condition at 4.8′

from the test 

connection 

Cross-linked 

ETFE 1 

D N/A Controlno condition No condition. No condition. 

Cross-linked 

ETFE 4 

E 10′ 4′′ from test 

connection. 

Abraded through to 

conductor, hydraulic fluid

contaminated for 72 hours 

No condition. Condition at 9′

from the test 

connection. 

Cross-linked 

ETFE 3 

F 9′ 9′′ from test 

connection. 

Abraded through to 

conductor not contaminated 

Condition at 10′ from 

the test connection. 

Possibly a noncontaminated breach. 

No condition. 

6-8
TABLE 6-4. ANALYSIS RESULTS FOR DISTANCE FROM THE TEST CONNECTION TO

THE CONDITION (Continued) 

Sample

Number 

Blind 

Test 

Letter 

Measured 

Distance to

Condition Condition 

Data Set 1 

Result Data Set 2 Result

Polyimide 2 G 9′ 9′′ (Distance to 

the greatest 

visually affected

section) 

Natural UV exposed for 13

months 

No condition. Condition at 12.5′

from the test 

connection. 

Polyimide 4 H 12′ 11′′ from test 

connection. 

Boiled in water for 4 hours Condition at 12.4′ from 

the test connection. 

May be boiled. 

No condition. 

Polyimide 5 I 12′ 7′′ from test 

connection. 

Abraded through to 

conductor, saltwater

contaminated for 72 hours 

Condition at 9′ from 

the test connection. 

No condition. 

Polyimide 1 J N/A Controlno condition 

Condition at 3′ from 

the test connection. 

May be contaminated

breach. 

Condition at 8.3′

from the test 

connection 

Polyimide 3 K 7′ 6′′ From test 

connection. 

Abraded not through to 

conductor not contaminated 

Condition at 9′ to 10′

from the test 

connection. Could be 

UV.

Condition at 14.7′

from the test 

connection. 

Polyimide 6 L 8′ 8′′ from test 

connection. 

Abraded not through to 

conductor, jet fuel 

contaminated for 72 hours 

No condition. Condition at 7.5′

from the test 

connection. 

Possibly a 

contaminated

breach. 

6.5 RESULTS SUMMARY. 

In only one case, blind test J, did the analysis fail for both test runs. In this case, the wire under 

test was a control sample with no defects. In both cases, from blind test J, the analysis identified

a condition when no condition was present, which could be considered a false positive 

indication. In all the other tests, at least one of the two tests identified the presence or absence of

a condition, resulting in no false negative indications. 

Some of the reasons for the false positive indication and the differences between the analysis of

both data sets include: 

• The results were evaluated on a very small computer monitor typical of with a fieldhardened computer. Since the differences in the TDR signatures are visually very small, 

a larger monitor could have improved the analysis. 

• The data collection techniques demonstrated by the two operators and the differences in 

the resultant signatures show that the EDT technique must be automated so that soak 

times at the lower frequencies are consistent. Soak time is the amount of time that the 

forcing function is applied to the wire under test. Computer-controlled acquisition would 

eliminate these differences and ensure more consistency in the results. 

6-9
• As mentioned earlier, the wires were loosely installed in the EMT and no attempt was 

made to create a uniform distance between each wire and ground. If the wire samples 

attached to the outside conduit to simulate a typical installation in an aircraft were placed 

in a bundle, the analysis may have been more accurate. If each wire had a more uniform

geometry, the TDR signatures may have been flatter, making subtle impedance changes 

more easily detected. 

6.6 SUPPLEMENTAL TESTS AND ANALYSIS. 

Two supplemental tests, requested by FAA personnel, were performed during the verification at 

the FAA William J. Hughes Technical Center. 

The first supplemental test configuration consisted of electrically connecting all the wire samples 

together, except one wire. The EDT was performed between the single wire and the other wires 

in the bundle. The single wire was validation sample L, polyimide sample 6, which was abraded 

not through to the conductor, and jet fuel contaminated for 72 hours. The jet fuel contaminated 

abrasion was located 8 feet 8 inches from the test connection. 

Figure 6-12 shows a 64x magnification of an area of possible degradation located 12.2 feet from

the test connection. Figure 6-12 is a comparison of four TDR signatures acquired with a 5 volt, 

sine wave forcing function at frequencies of 20 MHz (green trace), 200 MHz (cyan trace), 2 Hz 

(magenta trace), and 20 Hz (red trace). 

Area of Increased 

Impedance 

Located at 12.2 

feet 

FIGURE 6-12. RESULTS FROM SUPPLEMENTAL TEST 1 

6-10
The second supplemental test requested by FAA personnel consisted of testing one wire sample 

with the other wires electrically connected together outside the EMT. The wire bundle was taped 

to the outside of the test fixture to maintain the geometry. Figure 6-3 is a comparison of TDR

signatures acquired for this configuration. This figure is a comparison of four TDR signatures 

acquired with a 5 volt, sine wave forcing function at frequencies of 20 MHz (green trace),

200 MHz (cyan trace), 2 Hz (magenta trace), and 20 Hz (red trace). 

The section of possible degradation was at the same approximate distance from the test point as 

determined from the previous test, 12.2 feet for the first supplemental test, and 13.6 feet for the 

second supplemental test. This difference may be due to geometry effects and a difference in Vp. 

Slight Separation of

TDR Signatures 

Located at 13.6 feet

FIGURE 6-13. RESULTS FROM SUPPLEMENTAL TEST 2 

6-11/6-12
7. CONCLUSIONS AND RECOMMENDATIONS. 

The primary goal of this research effort was to demonstrate and validate the underlying theory of 

the new excited dielectric test (EDT) test method developed by CM Technologies. Overall, the 

results demonstrated how the EDT test method detected and located known wiring defects under 

controlled test conditions. 

The EDT found common defects that may be present in aircraft wiring systems, including 

abrasions, fluid contamination, and thermal degradation. 

At its core, the new EDT method stimulates the wire under test to reveal a greater sensitivity to 

defect detection and location than traditional time domain reflectometry (TDR) test methods. 

Further, application of the EDT technique currently uses a qualitative comparison methodology 

to identify localized areas of insulation degradation; these appear as readily discernable 

differences in the resulting TDR signatures. This unique feature provides technicians with a 

valuable tool to characterize localized insulation conditions without the need for historical 

baseline data; i.e., implementation of the EDT test methodology generates a comparative data set 

using global insulation quality as a reference against which localized qualities are evaluated. 

The response of various defects and various materials to the EDT method is a strongly related to 

the frequency of the forcing function applied during the test. Laboratory data developed during 

this effort shows that this response is repeatable and predictable for the narrow set of materials 

and defects studied. Based on these results, other polar insulation materials are likely to exhibit 

similar responses to the EDT method. 

The research demonstrated that the EDT method represents a fundamentally sound approach to 

defect detection and location. However, there were small defects that went undetected during the 

development and validation effort. Based upon prior experience with traditional TDR and other 

test methods, it is believed that the threshold for detection for these small defects will be lowered 

substantially when applied to installed aircraft wiring due to the presence of moisture, dust, 

lubricants, and other contaminates. 

7-1/7-2
APPENDIX A—SENSITIVITY OF TDR TO CABLE DEFECTS CAUSED BY

SIMULATED ARC FAULTS

A.1 RESULTS OF TIME DOMAIN REFLECTOMETER SENSITIVITY TEST. 

A.1.1 PURPOSE. 

The purpose of this experiment was to determine the general sensitivity of the time domain 

reflectometry (TDR) to detect the presence of damaged insulation—the type of damage that may 

be associated with an arc-fault event. 

A.1.2 TEST SETUP. 

Two 3-foot lengths of polyimide insulated wire were used for the experiment. One length served 

as a baseline for comparison purposes. The second sample was used to introduce the simulated 

arc fault defects. Three defects were simulated: the first defect was approximately 1 mm long, 

the second was approximately 2 mm long, and the third defect was approximately 3 mm long 

(note that the 1, 2, and 3 mm were the target lengths, the actual lengths varied somewhat). The

defects were all placed in the same area along the wire (i.e., the 1-mm defect was introduced first 

and was widened to approximately 2 mm and was widened again to approximately 3 mm). 

Figure A-1 shows the 3-mm defect (in reality the defect measured 2.4 mm). 

FIGURE A-1. LENGTH OF SIMULATED ARC LOCATION DETECTABLE BY THE TDR 

A-1
Figure A-2 shows a side view of the 3-mm defect. Notice the insulation was only removed from 

one-half of the circumference to more realistically simulate an arcing event. 

Side View of 

Simulated Arc 

Fault Defect 

FIGURE A-2. SIDE VIEW OF THE SIMULATED ARC-FAULT DEFECT 

A.1.3 RESULTS. 

Since all the simulated defects were clean (i.e., no contamination), the expected response of the 

TDR to a simulated arc-fault defect was an increase in impedance (Z), where 

Impedance (Z) = SQRT(L/C) 

and 

Capacitance = (εA)/d

where ε = 2.0 to 3.0 for polyimide insulation and ε = 1.0 for air (or the absence of insulation). In 

the local area near the defect, the incremental capacitance would decrease because of the 

polyimide insulation being replaced by air. This decrease in capacitance would produce a 

corresponding increase in impedance. 

The 1-mm defect was not visible to the TDR. There was essentially no difference between the 

two signatures. The 2-mm defect did affect the TDR signature, but only very slightly. 

A-2
The signatures from the 3-mm defect, shown in figure A-3, produced a discernable separation. 

Turquoise signature 

indicates an area of 

decreased capacitance near

the 2.4 mm defect 

Yellow Signature is 

the baseline (i.e., no

faults)

FIGURE A-3. TME DOMAIN REFLECTOMETRY SIGNATURES FROM BASELINE AND 

2.4-mm DEFECT WIRE SAMPLES 

A.2 CONCLUSIONS. 

• Based on the results from this experiment, the length of detectable defects, with no 

contamination, is between 2 and 3 mm. 

• If any residue or contamination occurs as the result of an actual arcing event, the time 

domain reflectometry (TDR) may become more sensitive to shorter defect lengths.

Carbon residues would result in a larger capacitance change. 

• In a nonlaboratory environment, baseline data would be required. Comparison of TDR 

signatures between similar circuits or geometries on the aircraft would be of limited use

in detecting these subtle types of defects. 

A-3/A-4