DOT/FAA/AR-05/34 

Office of Aviation Research 

Washington, D.C. 20591 

High-Speed Bolthole Eddy-Current 

Scanning for In-Service Inspection 

of Engine Disks 

September 2005 

Final Report 

This document is available to the U.S. public 

through the National Technical Information Service (NTIS), 

Springfield, VA 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 

manufacturers. 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 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-05/34

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

5. Report Date 

September 2005

 4. Title and Subtitle 

HIGH-SPEED BOLTHOLE EDDY-CURRENT SCANNING FOR IN-SERVICE 

INSPECTION OF ENGINE DISKS 6. Performing Organization Code

7. Author(s)

Lisa Brasche2, Tony Mellors, Thadd Patton3, Rob Stephen, Kevin Smith4 , Andy 

Kinney, Waled Hassan, Jim Ohm, and Joel Schraan1

8. Performing Organization Report No. 

10. Work Unit No. (TRAIS) 

 

9. Performing Organization Name and Address

2

Iowa State University 4Pratt & Whitney 

 Center for NDE 400 Main Street 

 Ames, IA 50011 East Hartford, CT 06108 

3

General Electric Aircraft Engines 1Honeywell Engines, Systems & Services 

 Cincinnati, Ohio 111 S. 34th Street 

 Phoenix, Arizona 85072 

11. Contract or Grant No.

 

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

 ANE-100

15. Supplementary Notes 

The FAA William J. Hughes Technical Center Technical Monitors were Rick Micklos, Paul Swindell, and Cu Nguyen. 16. Abstract 

High-speed bolthole (HSBH) eddy-current inspection is the most popular inspection technique for fast and accurate inspection of

circular air- and boltholes. When HSBH techniques were introduced in the field, inspection differences among original 

equipment manufacturer (OEM) techniques were unclear. Also unavailable were quantification of inspection capability and 

identification of the main sources of variation for this inspection technique. A three-factor, two-level design of experiment was 

conducted to determine the main sources of variations to better control the inspection process. A semiautomated scanner 

prototype was designed and manufactured to minimize these variations. The scanner and associated fixturing were then tested to

compare the process capability and reliability with that of the typical manual field technique. Both approaches were found to 

exceed the program performance metric of detecting a 30-mil crack with 4:1 signal-to-noise ratio in a common bolthole geometry. 

The semiautomated scanner showed a 30% improvement in the 90% probability of detection point with 95% confidence (12-mil 

crack) compared to a manual inspection (17-mil crack) using experienced eddy-current inspectors and relatively short 0.25′′ long 

bolthole crack specimens. Scanner reliability testing resulted in a written inspection technique that was jointly used by the OEMs 

and served to further improve the inspection. Information gained in the process of improving this inspection was used to make 

recommendations for possible modification of the industry specification, SAE AS4787, “Eddy Current Inspection of Circular 

Holes in Nonferrous Metallic Hole Inspection in Aircraft Engines.” 

17. Key Words 

In-service inspection, Eddy-current, Bolthole, Jet engine 

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 

 63

22. Price

Form DOT F1700.7 (8-72) Reproduction of completed page authorized 
TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY vii 

1. INTRODUCTION 1 

1.1 Purpose 1 

1.2 Objectives 1 

1.3 Background 1 

1.4 Related Activities and Documents 2 

2. TECHNICAL APPROACH 2 

3. DISCUSSION OF RESULTS 3 

3.1 Survey of OEM Current HSBH Inspection Practices 3 

3.2 Determination of Key Inspection Parameters Contributing to Inspection 

Variability Through DOE Study 4 

3.3 Conclusions of DOE 6 

3.4 Design and Fabrication of Common Fixtures and Tooling 7 

3.5 Semiautomated vs Manual HSBH Eddy-Current Inspection Process Capability 

Evaluation 12 

3.6 Process Capability Results 14 

4. CONCLUSIONS 15 

5. REFERENCES 16 

APPENDICES 

A—Main Effects and Interaction Plots Generated in the DOE Study Designed to 

Determine the Important Parameters Used in HSBH Inspection 

B—Design Drawings of the Common Tooling and Fixtures 

C—Probability of Detection Inspection Technique and Data Collection Sheet 

D—Draft Revision of Industry Standard AS4787

iii
LIST OF FIGURES 

Figure Page 

1 Design of Experiment Overall Results (Units are Uncoded) 7 

2 FML100146 PoD Specimen Set Used in This Study 9 

3 Semiautomated Scanner on Top of Power Supply Box 10 

4 Adapter Plates Fabricated for the Semiautomated Scanner 10 

5 Specimen-Centering Pin and Bolthole Simulator Plate 11 

6 Overall PoD Results for HSBH Inspection Using Manual Scanner 14 

7 Overall PoD Results for HSBH Inspection Using Semiautomated Scanner 

Developed Under This Effort 15 

LIST OF TABLES 

Table Page 

1 High-Speed Bolthole Eddy-Current Scanning OEM Practices 4 

2 Design of Experiment Factors and Levels 6 

3 Crack Surface Lengths for the 0.328′′ Diameter IN718 Bolthole Specimens Used 

in the DOE Study 6 

4 Evaluation Metrics for the Analysis of the Results of the DOE Study 6 

5 Comparison of the Manual and Semiautomated HSBH Inspection Results to the 

Program Metric 14 

iv
LIST OF ACRONYMS 

DOE Design of experiments 

EDM Electron discharge machined 

ETC Engine Titanium Consortium 

FAA Federal Aviation Administration 

GE General Electric 

HSBH High-speed bolthole eddy-current inspection 

OEM Original equipment manufacturer 

P-P peak-to-peak 

rpm Revolutions per minute 

SNR Signal-to-noise ratio 

v/vi
EXECUTIVE SUMMARY 

Nondestructive inspection of in-service critical jet engine rotating parts is key to the success of 

the Federal Aviation Administration (FAA) enhanced inspection initiative. Typical jet engine 

disks contain numerous stress-concentrating geometries, such as dovetail slots, boltholes, and air 

holes. Many of these features are subjected to substantial stresses under operating conditions. 

Nondestructive inspection of these complex features is vital, since undetected cracks can grow to 

critical size and potentially result in disk failure. This program is focused on evaluating existing 

eddy-current procedures for boltholes and documenting a best practice for inspection of these 

features in critical rotating parts. 

High-speed bolthole (HSBH) eddy-current inspection is an effective, efficient, and commonly 

used technique for conducting fast and accurate inspection of air- and boltholes. Although 

several HSBH techniques have been introduced in the field by all the original equipment 

manufacturers, no studies of the comparative effectiveness and quantitative fatigue crack 

detection capabilities of the various techniques have been completed. 

This report describes an investigation into the main sources of variation of HSBH eddy-current 

inspection through the design and evaluation of a three-factor, two-level design of experiment 

test matrix. A semiautomated scanner prototype was designed and manufactured to minimize the 

impact of the identified inspection variations on inspection performance. The scanner and 

associated fixturing were tested, and probability of detection (PoD) curves were constructed to 

compare the process capability and reliability with that of a typical manual field technique. Both 

the manual and semiautomated inspection approaches were found to exceed the program 

performance metric of detecting a 0.030′′ long crack with a 4:1 signal-to-noise ratio in a typical 

bolthole geometry. The semiautomated scanner demonstrated a 30% improvement in both 90/50 

and 90/95 PoD flaw size over the manual inspection method using experienced eddy-current 

inspectors and relatively shallow (0.250′′ deep) bolthole crack specimens. 

In addition to the investigation into the main sources of variation, this study also provided data 

and procedures, enabling a standardized written inspection technique or best practice to be 

prepared for the industry specification, SAE AS4787, “Eddy-Current Inspection of Circular 

Holes in Nonferrous Metallic Hole Inspection in Aircraft Engines.” 

vii/viii
1. INTRODUCTION.

1.1 PURPOSE.

Application of high-speed bolthole (HSBH) eddy-current inspection has increased significantly 

as the aircraft engine industry continues to expand the scope and extent of their in-service 

inspections of critical rotating part features for fatigue cracks in support of the Federal Aviation 

Administration (FAA) Enhanced Inspection Initiative. The Engine Titanium Consortium (ETC), 

other original equipment manufacturers (OEM), the airlines, and the FAA have placed an 

emphasis on the importance of quantifying and improving the reliability of these inspections. 

This study has been initiated to (1) determine the primary sources of variation in a typical inservice manual eddy-current inspection, (2) rank these variations according to relative influence, 

(3) fabricate tooling and semiautomated test equipment to improve inspection reproducibility and 

reliability, and (4) quantify and compare the resulting process capability to that of the current 

manual field practice. 

1.2 OBJECTIVES. 

The primary objectives of this study were 

• to develop common fixtures to reduce inspection variables and arrive at a standardized 

inspection technique. 

• to measure the eddy-current bolthole inspection process capability. 

• to use this information to develop an industry best practices document. 

1.3 BACKGROUND. 

In 1998, the FAA the announced the Safer Skies initiative, which aimed to reduce the 

commercial aviation accident rate by 80 percent by 2007. Commercial aviation safety is one of 

the three areas of the Safer Skies initiative that addressed accident causes such as uncontained 

engine failures, one of the leading causes of commercial aviation fatalities. As part of this 

program, government and industry have agreed to use enhanced inspections for certain highenergy, rotating engine components. Critical features were identified by the OEMs for enhanced 

inspection at opportunities when the disks were available as part of routine overhaul and 

maintenance actions. Among the most significant and important features to be inspected were 

air- and boltholes, typical of those machined in critical engine components. 

Compared to fully automated bolthole inspection systems, the semiautomatic HSBH inspection 

evaluated in this study is a simple, low-cost technique that can achieve acceptable commercial 

crack detection sensitivity. Precision fully automated eddy-current equipment costs can exceed 

$200,000, while the cost of a hand-held probe rotator and instrumentation is typically less than 

$20,000. HSBH inspection is also preferred over fluorescent penetrant inspection for detection 

of cracks in critical engine features. This is particularly true for inspection of high length-todiameter (l/d > 1) holes often found in critical rotating components where line-of-site access is 

severely restricted or obstructed. 

1
OEM HSBH inspection practices and reliability data are usually maintained in field manuals or 

considered proprietary information and, therefore, are not publicly available. There are, for 

example, a variety of eddy-current coil configurations used by the different OEMs for bolthole 

inspection, including absolute (single coil), differential (dual coil), split-core differential (close 

proximity dual coils), and reflection (driver-pickup) in both absolute and differential 

configurations. Likewise, a variety of hand-held probe rotators operating at a number of 

rotational speeds are commonly used for commercial bolthole inspection. Probe translation 

through the bolthole length is performed manually or by using a semiautomated fixture. Manual 

inspection is generally expected to produce a higher level of variability than semiautomated 

systems. 

The industry specification for circular hole inspection, SAE AS4787 [1], was written in 1992. 

Since then, significant technical advances in inspection techniques of critical rotating parts were 

developed by OEMs. Part of this project was to draft a new SAE AS4787 to reflect these 

improvements. 

1.4 RELATED ACTIVITIES AND DOCUMENTS. 

The ETC was established in 1993 by Iowa State University in cooperation with General Electric 

(GE), Pratt & Whitney, and Honeywell. The primary goal of the ETC program was to provide 

improved inspection technology for engine materials and components. Several FAA reports 

have been generated that highlight the work [2 and 3] completed over the past 11 years. All 

FAA reports are available on-line at http://actlibrary.tc.faa.gov/. Related papers and publications 

are provided at http://www.cnde.iastate.edu/etc/. 

2. TECHNICAL APPROACH.

The initial work of this study focused on identification and evaluation of existing OEM 

inspection processes practiced by the ETC partners that manufacture large commercial engines. 

Eddy-current inspection has changed substantially over the years, and current OEM techniques, 

approaches, and experiences were used to define practices and establish parameters to be 

included in the experimental study and instrumentation design. 

The second part of this study involved a design of experiments (DOE) [4] exercise to determine 

if the major inspection parameters not under strict procedural control (i.e., subject to inspector 

variability) had a significant effect on inspection results. Inspection parameters, having the 

highest potential to adversely affect test results, were scan density (i.e., speed and feed) and offnormal response (angle). Variation of these parameters is typically accentuated during manual 

scanning. 

The third part of this study was to develop optimum eddy-current fixturing to permit improved 

control of the most critical inspection parameters identified in the second part of the study. 

Development of optimum inspection fixturing was completed and incorporated into a prototype 

portable semiautomated scanner. Although some OEM-manufactured eddy-current fixturing had 

been developed prior to this study, none addressed both the probe alignment and feed rate issues. 

2
The fourth part of this study was to use the common fixtures developed in the third part to arrive 

at a standardized best practice inspection technique and quantify process sensitivity. PoD testing 

of both the semiautomated scanner and manual hand-held inspections were conducted by four 

different inspectors at two different facilities. This also involved fabricating tooling to precisely 

and repeatably locate bolthole test specimens in a simulated disk pattern. 

The performance metric was detection of a 0.030′′ long crack with a 4:1 signal-to-noise ratio 

(SNR) in a common bolthole geometry. These metrics represent current state-of-the-art 

capability for a production eddy-current bolthole inspection. 

3. DISCUSSION OF RESULTS. 

3.1 SURVEY OF OEM CURRENT HSBH INSPECTION PRACTICES. 

Current in-service OEM practices for HSBH eddy-current inspection of engine disks were 

surveyed and found to be similar for the two primary users, with the major parameters 

summarized in table 1. A review of the manufacturer’s equipment showed OEM practices 

consistent with the range of available bolthole scanners, instruments, and probes. 

The eddy-current hole inspection practice of OEM 2 was limited exclusively to a manual 

inspection with hand feed, while OEM 1 typically employed a semiautomated inspection system 

with hand feed. Both OEMs occasionally employed a simple fixture to assist in maintaining 

probe alignment and normality with the surface of the hole. 

Procedural guidelines are currently used to control the maximum feed rate in a particular scan, 

although adherence to the guidelines is totally dependent on the operator. For instance, an 

inspector may be instructed not to exceed 0.5 in./sec. when feeding the probe through the hole, 

but there is no documentation to verify that the feed rate limit was not exceeded during the test. 

Probe type and size show widespread use of reflection differential type probes with increasingly 

smaller nominal coil diameters in the range of 0.040′′ to 0.060′′. 

3
TABLE 1. HIGH-SPEED BOLTHOLE EDDY-CURRENT SCANNING 

OEM PRACTICES 

Inspection Variables OEM 1 OEM 2 

Parameter 

Frequency of operation 1-6 MHz 2 MHz 

Scan speed 800-3000 rpm 800-3000 rpm 

Filter 250-1000 Hz (rpm dependent) 200-300 Hz HP/500-1500 Hz LP 

Probe type Contact Contact 

Element type Reflection, differential, and absolute Driver—pickup (reflection) 

Element size 0.080′′ diameter coil Not specified 

Data recording Screen shot/digital archive None/digital archive 

Inspection method Hand-held and fixtured rotator with 

manual feed 

Hand-held rotator with manual 

feed 

Scan resolution (e.g., pitch or 

increment) 

0.025 max Not specified 

Other considerations 

Calibration feature size—

corner 

0.015′′ long by 0.015′′ deep EDM Not specified 

Calibration feature size—barrel 0.030′′ long by 0.015′′ deep EDM 0.030′′ long by 0.015′′ deep EDM 

Flaw rejection level 100 % of calibration slot response 50% of calibration response 

Hole diameter range 0.250′′-2.500′′ 0.175′′-1.00′′

Hole depth range 0.0625′′-4.00′′ 0.125′′-2.0′′

Materials Titanium/nickel alloys Ti 6-4/Inconel/R88 

Corner geometry Machined Machined 

Angled entry surfaces No Yes 

Other 3:1 min. SNR Not applicable 

EDM – Electron discharge machined 

3.2 DETERMINATION OF KEY INSPECTION PARAMETERS CONTRIBUTING TO 

INSPECTION VARIABILITY THROUGH DOE STUDY. 

The team identified three key test parameters that could have a significant effect on inspection 

sensitivity. 

• Probe rotational speed (i.e., revolutions per minute (rpm)) 

• Probe angle (i.e., deviation from normal) 

• Probe feed rate (i.e., increment per revolution) 

These parameters were selected because of their potential contributions to the inspection 

variability relative to scan data density (i.e., speed and feed) and off-normal response associated 

with manual scanning. Furthermore, the effects of other inspection parameters such as 

instrument, probe selection, signal phase, and filtering could be minimized through the use of 

standardized setup and calibration procedures. 

4
Evaluation of a full factorial DOE was identified as the most efficient method to measure process 

sensitivity to the key inspection parameters. The DOE design and summary results were as 

follows: 

a. The DOE was a three-factor, two-level full factorial design (two [4] or eight 

combinations) blocked for motor speed and randomized within each block. The limits 

associated with each parameter are shown in table 2. 

b. Eddy-current amplitude and noise levels were the response metric established for the 

DOE study. This information was collected during the study on several low-cycle fatigue 

cracks in 0.328′′ diameter, Inconel (IN718) bolthole specimens. Crack surface length-todepth ratio was nominally 2:1. Crack surface lengths used in the study are summarized in 

table 3. 

c. The evaluation metrics for the analysis of the results from the DOE study are summarized 

in table 4. These guidelines were used to establish the relative importance of the 

inspection parameters investigated in the study. 

d. The DOE study was conducted using a fully automated scanning system that provided 

total control of the inspection parameters under investigation. The following components 

and procedures were used: 

• A Rohmann Elotest instrument and bolthole scanners. 

• A fully automated ETC2000 scanner used for feed and angle control, and data 

acquisition and analysis. 

• A 2-MHz reflection differential coil, unshielded, 0.040′′ coil diameter served as 

the inspection probe. 

• Calibrations were performed on a 0.030′′ long by 0.015′′ deep electron discharge 

machined (EDM) notch located in a 0.341′′ diameter, IN718 bolthole specimen. 

Calibrations were optimized in phase and filter for each rpm with a 0° probe angle. 

5
TABLE 2. DESIGN OF EXPERIMENT FACTORS AND LEVELS 

Levels 

Factors (-1) (1) 

Probe (rpm) 1200 2200 

Probe feed rate (in./sec) 0.125 0.750 

Probe angle (degrees) 0 2 

TABLE 3. CRACK SURFACE LENGTHS FOR THE 0.328′′ DIAMETER 

IN718 BOLTHOLE SPECIMENS USED IN THE DOE STUDY 

Specimen 

ID 

Crack Length 

(inches) 

19 0.007 

04 0.016 

20 0.020 

07 0.034 

18 0.041 

14 0.050 

13 0.061 

TABLE 4. EVALUATION METRICS FOR THE ANALYSIS OF THE RESULTS 

OF THE DOE STUDY 

Metric Optimum Response 

Largest missed crack Minimum 

Amplitude of largest crack Maximum 

Noise Minimum 

SNR Maximum 

3.3 CONCLUSIONS OF DOE. 

Difficulties associated with characterization of noise levels resulted in measurement of only two 

noise variables (i.e., high and low noise). All measurements were made in the scan-up direction. 

Peak-to-peak (P-P) noise measurements were made in an approximate 0.100′′ long window on 

the lower side of the flaw location in each specimen. Two noise measurements (maximum P-P 

noise of the entire window and the baseline noise value excluding any discrete, noise spikes) 

were taken from this window location in each specimen. Only the maximum P-P noise value 

was evaluated, since it was judged as most representative of typical scan-generated noise from 

vibration, chatter, and hole irregularities. 

The data analysis tool used in the DOE study was MinitabTM, which was useful for determining 

the relative influence of the main effects (angle, feed, and rpm) as well as identifying parameter 

interaction effects. Resultant values have no associated units. The following is a summary of 

6
the important observations based on the main effects and interaction plots generated in this DOE 

study. Complete results are attached as appendix A. 

• Small flaw detection (i.e., largest missed crack) 

− Both main and interaction effects show a high sensitivity to all three factors (i.e., 

rpm, feed rate, and angle), with optimum conditions favoring a 0° probe angle, 

slow feed rate, and high rpm. 

• Signal amplitude (large crack) 

− Main effects plots show a high sensitivity to probe angle with the optimum 

condition favoring a 0° probe angle. The probe rotational speed (rpm) and feed 

rate effects are negligible for large flaw amplitudes. 

• Noise and SNR (high-noise case only) 

− Main effects plots show a high sensitivity to probe angle with the optimum 

condition favoring a 0° probe angle. Interaction plots show sensitivity, albeit to a 

lesser and sometimes conflicting level, to both probe rotational speed (rpm) and 

feed rate. 

The primary conclusions derived from the Minitab analysis of the DOE data are that probe angle, 

feed rate, and rotational speed (rpm), in that order, have significant effects on the results of 

HSBH inspection, as shown in figure 1. 

Feed

RPM

DOE Parameter

Tooling Importance ...

Angle

High

Low

FIGURE 1. DESIGN OF EXPERIMENT OVERALL RESULTS (UNITS ARE UNCODED) 

3.4 DESIGN AND FABRICATION OF COMMON FIXTURES AND TOOLING.

The DOE study showed very clearly that probe angle, probe translational speed, and rotational 

speed all significantly influence inspection sensitivity. Therefore, it was evident that a probe 

7
translation and alignment control device that would securely hold the probe rotator would be 

vital to the success of the inspection. The ETC technical team created detailed specifications for 

the tooling and fixtures needed to achieve the proper level of control over the probe angle, probe 

translational speed, and rotational speed. Allen Design, located in Tempe, Arizona, was selected 

to design and manufacture the following items. 

• Probe Translation Device. This device was required to precisely position and translate a 

rotating eddy-current probe through boltholes of various diameters. The probes were 

mounted to commercial off-the-shelf, high-speed bolthole eddy-current rotating drivers 

and used to detect fatigue cracks along the length of the boltholes. There were a number 

of different probe-rotating drivers available, so the device was designed to be compatible 

with many of these drivers. The probe translation device had to be adaptable to different 

engine parts. The following requirements were imposed during the design process: 

− Fixtured alignment to control wobble of probe while translating through a hole 

feature. 

− Fine X-Y plane adjustment to center probe within bolthole diameter. 

− Machine control of probe travel through boltholes by means of selectable, fixed, 

transverse speeds of 0, 0.100′′, and 0.200 in./sec in a helical scan pattern. 

− Rapid, adjustable, automatic translation retraction to a start next scan position. 

− Scan limit switch to prevent excess travel of probe. 

− Minimum of 3′′ of probe translation travel. 

− Slip clutch to prevent burnout of translation motor. 

− Use of exchangeable bolt-on adapter plates for fit to specific parts and bolthole 

patterns. 

− Adaptability to Uniwest probe rotating driver and also other drivers including 

Roman Elotest, Foerster, Zetec, etc., as provided. 

− No electrical interference to be caused by the probe translation device during the 

eddy-current inspection process (including data acquisition). 

− Manufacturing cost of less than $10,000 (This goal was not met for the prototype 

built under this task, but may be achievable for larger production volume). 

• Part Adapter Plate. To adapt the proposed bolthole-scanning system design to various 

parts and facilitate inspection of each hole, the instrument must be manually lifted and 

secured for each bolthole by an exchangeable adapter plate designed for that purpose. 

These adapter plates bolt to the bottom of the translation device. To precisely adjust the 

eddy-current probe within a bolthole, a fine X-Y adjustment was provided that also 

8
locked the probe into the correct position within the hole. The program included the 

design and manufacture of two such adapter plates, one to be used with the bolthole 

simulator fixture and the other to be used for the inspection of a preselected industry 

application. GE provided an actual engine rotating part for the inspection demonstration. 

Therefore, an adapter plate was fabricated for the GE component. 

• Bolthole Simulator Fixture. The bolthole simulator fixture was used to precisely position 

round bolthole specimens (see figure 2) in a circular layout as they would be in a typical 

rotating engine component. This fixture was capable of positioning 25, 1.875′′ diameter 

by 0.250′′ thick specimens. The bolthole diameter within these circular specimens is 

0.460′′. The layout aligns the specimens in a precise circular pattern, with the boltholes 

within 0.002′′ of the geometric center. When specimens are aligned by the fixture, they 

can then be locked into position to prevent rotation and lift-out during scanning. 

Φ 1.875" REF Final LCF Crack

Φ 0.460" REF

Drill point to be

used as locating

point

0.250" REF

FIGURE 2. FML100146 PoD SPECIMEN SET USED IN THIS STUDY 

The design drawings for the common tooling, fixtures, and the semiautomated scanner are 

attached as appendix B to this report. Figures 3, 4, and 5 illustrate the final components 

manufactured by Precision Gage, a tooling workshop located in Tempe, Arizona. 

9
Jog 

Toggle 

Translation 

Toggle 

Centering 

Knobs 

Rotator 

Holder 

FIGURE 3. SEMIAUTOMATED SCANNER ON TOP OF POWER SUPPLY BOX 

Adapter Plates 

(a) For the bolthole simulator 

(STE8388686-1 U.S.GOVT) 

 

For the GE disk, OD hole set 

(STE8388686-3 U.S. GOVT) 

(c) For the GE disk, ID hole set 

(STE8388686-2 U.S. GOVT) 

FIGURE 4. ADAPTER PLATES FABRICATED FOR THE SEMIAUTOMATED SCANNER 

10
FIGURE 5. SPECIMEN-CENTERING PIN AND BOLTHOLE SIMULATOR PLATE 

Scanner modifications were completed as necessary during initial testing. Modifications 

included changing the thumb toggles to easily peak-out indications, moving the speed control 

from the power supply to the scanner body, modifying the rotator bracket to better secure the 

Elotest rotator, and milling out the specimen adapter for precision fitting. Scanner drawings 

were then modified to show the speed selector on the scanner body, cable disconnect at the 

driver box, and the clearance change made to the adapter. The three-position motion control 

switch permitted a minimum of 0.008′′ to 0.010′′ of probe travel when toggled at a 0.1-in./sec 

speed and a minimum of 0.010′′ to 0.030′′ of probe travel when toggled at a 0.2-in./sec speed. 

The high-speed setting was useful for initial positioning of the eddy-current probe. The jog 

toggle is located on the right thumb toggle and moves 0.001′′ to 0.002′′ per jog or 0.006 in./sec 

when held down. Although these settings can be modified with a program change, they were 

proven effective in testing performed during this study. 

Two microlimit switches controlled the travel range of the probe. This provided economy of 

motion and reduced the total time of the bolthole inspection. Fine adjustment screws allowed for 

easy probe centering within a bolthole. With properly toleranced part fixtures, the scanner was 

rapidly positioned for the next bolthole. Probe alignment with the translating axis was precise, 

and no bolthole axes misalignment difficulties were experienced over the full extent of probe 

travel. These switches controlled the probe alignment and the probe feed rate and translation 

speed variables identified as important during the DOE portion of this study. 

The following is a summary of the principal features of the semiautomated prototype scanner. 

• Fixtured alignment to control wobble and alignment 

• No electrical interference from scanner to eddy-current instrumentation 

• Machine control of probe travel (feed) through bolthole 

11
• Use of exchangeable adapter plates (alignment templates) for precise fit to specific parts 

• Fine X-Y plane adjustment to center probes within bolthole diameter 

• Switch-selectable fixed transverse speeds of 0.100′′ or 0.200 in./sec 

• Jog control for 0.001′′ to 0.002′′ minimum or 0.006 in./sec when held down 

• Rapid retraction mode to increase inspection speed of 0.700 in./sec 

• Microlimit switches to prevent excess travel of probe 

• 3′′ of probe travel 

• Adaptability to multiple probe rotators including UniWest, Roman Elotest, and Foerster 

(requires custom adapter for each probe rotator) 

• Prototype price, which included initial design costs, was $23,000. Subsequent units 

would be priced at less than $14,000, depending upon volume. 

The precision adapter plate provided hole centering within ±0.002′′, and the centering tool helped 

achieve quick placement of the PoD specimens for testing the other half of the specimen set. 

3.5 SEMIAUTOMATED VS MANUAL HSBH EDDY-CURRENT INSPECTION PROCESS

CAPABILITY EVALUATION.

PoD tests of both the semiautomated and manual hand-held scanners were conducted using four 

different inspectors at two different facilities. All the inspectors were experienced in bolthole 

inspection. Each inspector completed two runs with the manual inspection and two runs with the 

semiautomated inspection. A single test protocol was followed to standardize the tests and keep 

it on schedule. Prior to the first inspection run, each inspector was briefed on the PoD test 

procedure and the data report form. A written inspection technique was available for set-up and 

reference (see appendix C). Two inspection runs could be completed in a day under optimal 

conditions. Each run included two specimen change outs since the test fixture was designed to 

contain only half of the PoD test specimens. 

A schematic of the bolthole PoD specimens used in this study is presented in figure 3. The 

specimens are 0.250′′ thick disks of titanium 6-2-4-6 from a U.S. Government-owned test set 

FML100146. The 1.875′′ diameter specimens each contained a 0.460′′ diameter center-aligned 

bolthole. Measurement of all bolthole diameters indicated they were within 0.001′′ of their 

nominal 0.460′′ diameter. This bolthole diameter is typical of those commonly inspected by the 

OEMs, as indicated in the OEM survey. It would have been desirable to have longer bolthole 

lengths (higher l/d) in the test specimens since they would have been more representative of 

those in critical rotating disks. However, higher l/d boltholes would have made it very difficult 

to produce a crack that extended through the entire length of the hole by cyclical bending. A 

deeper bolthole would also have been more effectively inspected by the semiautomated scanner 

since precise control of probe feed rate and translation velocity would have been critical to a 

12
successful inspection. Tooling fabricated earlier was used to precisely and repeatably locate test 

specimen boltholes in a simulated disk pattern (see figure 5). 

The United States Air Force bolthole set, FML100146, contained low-cycle fatigue cracks with a 

nominal 2:1 surface length to depth ratio. The set consisted of 50 titanium 6-2-4-6 specimens 

containing 44 cracks with surface lengths ranging from 0.012′′ to 0.050′′. Ten of the specimens 

did not contain any fatigue cracks. 

The calibration reference standard contained four EDM notches. Three corner notches of sizes 

0.003′′, 0.006′′, and 0.009′′ were used, along with one 0.030′′ long by 0.015′′ deep notch in the 

bolthole center. The notches were separated by 90 degrees around the bolthole. Only the 0.030′′

long by 0.015′′ deep center notch was used for the PoD tests to reduce the number of inspection 

repetitions. Calibrations were optimized in phase and amplitude for each translation run using a 

0° probe angle. 

The scanning equipment consisted of a Rohman Elotest B1-SDM/UC Version 5.11 instrument 

with a SR1-HF 96.1.610 probe rotator (1200 rpm). Instrument filters were identical for all 

inspections to eliminate this variable. A Rohman 2-MHz unshielded reflection differential, 

0.040′′ diameter coil in a 0.460′′ nominal fit probe was used for all tests. Instruments and probes 

selected for this study were considered typical for commercial aviation inspection, although their 

use does not imply endorsement of a particular product. Teflon tape, although commonly used 

during production inspections to minimize probe wear during calibration and inspection, was not 

applied to the specimen surface during the study evaluation to eliminate any variation due to tape 

wear throughout a run. 

Eddy-current amplitude, clock position, and depth position (top, middle, or bottom) were 

recorded for all crack indications. In addition, an average noise level was estimated for each run. 

Clock position was previously documented with respect to an indent on one of the specimen 

faces. Reported indications were verified to determine whether they were hits from any of the 

known cracks, or false calls. The results indicated only one false call was reported during all of 

the tests. 

A missed call on a larger size crack occurred with the semiautomated scanner on a specimen that 

had two cracks, one at the top and one at the bottom of the hole. The inspector reported the first 

crack but, for some unknown reason, did not report the second crack. This miss illustrated a 

limitation of current instrumentation when it was used with a fixtured inspection such as the 

semiautomated scanner. With the Y-T or amplitude-time display, an indication can appear split 

between the start and end of the display screen. If this occurs during manual inspection, the 

inspector would simply twist the rotator until the indication shifted to the middle of the display. 

However, this manipulation of the signal is not possible with the semiautomated system since the 

rotator is fixtured for proper alignment. Addition of an electronic adjustment that would change 

the rotator zero position would eliminate this potential problem. This would avoid the split-signal 

occurrence in fixtured inspections, but would also allow an inspector testing in the manual mode 

to hold the rotator in a preferred position and then adjust the zero circumferential reference start, 

or 0° point. An alternative solution would be to display more than 360° of inspection angle so 

that the entire signal would always be visible. 

13
3.6 PROCESS CAPABILITY RESULTS.

Both manual and semiautomated technique capabilities exceeded the program metrics. The 

semiautomated scanner provided a 30% improvement in the 90/50 and 90/95 PoD flaw sizes and 

almost a factor of 2 improvement in SNR over manual inspection of titanium 6-2-4-6 boltholes, 

as shown in table 5. The SNR was estimated by dividing the signal amplitude response from the 

defect by the average baseline noise observed during the tests. 

TABLE 5. COMPARISON OF THE MANUAL AND SEMIAUTOMATED HSBH 

INSPECTION RESULTS TO THE PROGRAM METRIC 

Length 90/50 and 90/95 

CL (mils) SNR 

Program metric 30 4:1 

Manual scanner 14/17 >7.5:1 

Semiautomatic scanner 10/12 >12.5:1 

A PoD analysis was completed for each inspector for both manual and semiautomated inspection 

[5]. The analysis indicated that the PoD capability for every inspector was improved through the 

use of the semiautomated scanner. The analytical method is based on the PoD methodology 

developed at the University of Dayton, Research Institute. The PoD data in figures 6 and 7 

represent a total of eight manual and eight semiautomated PoD inspection runs by four inspectors 

and includes a total of 352 crack detection opportunities. 

FIGURE 6. OVERALL PoD RESULTS FOR HSBH INSPECTION USING 

MANUAL SCANNER 

14
FIGURE 7. OVERALL PoD RESULTS FOR HSBH INSPECTION USING 

SEMIAUTOMATED SCANNER DEVELOPED UNDER THIS EFFORT 

4. CONCLUSIONS. 

High-speed bolthole (HSBH) eddy-current inspection has emerged as the leading inspection 

technique for fast and accurate inspection of circular air- and boltholes. This study investigated 

the main sources of variation in the HSBH inspection process by designing and conducting a 

three-factor, two-level design of experiments (DOE) study. 

A semiautomated scanner prototype was designed and manufactured to minimize variations in 

the inspection process, as identified by the DOE. This scanner and associated fixturing were 

then tested to evaluate the process capability and reliability compared to a current manual field 

technique. Both the manual and semiautomated inspection techniques successfully exceeded the 

program metric of demonstrating the capability to detect a 0.030′′ long crack at a 4:1 signal-tonoise ratio in a common bolthole geometry. 

Probability of detection curves were prepared for both techniques using titanium 6-2-4-2 bolthole 

test specimens containing fatigue cracks ranging in length from 0.012′′ to 0.050′′. The 

semiautomated scanner showed about 30% improvements in the 90/50 flaw size (0.010′′ versus 

0.014′′) and the 90/95 flaw size (0.012′′ versus 0.017′′) over the manual inspection method. All 

data was obtained using experienced eddy-current inspectors and relatively shallow (0.250′′

deep) bolthole crack specimens. 

15
Scanner reliability tests resulted in a written inspection technique that was jointly used by the 

original equipment manufacturers and served to further standardize the inspection process. 

Information gained during this process was used to draft a best practices proposed revision of the 

industry specification, SAE AS4787, “Eddy-Current Inspection of Circular Holes in Nonferrous 

Metallic Hole Inspection in Aircraft Engines.” 

5. REFERENCES.

1. SAE Aerospace Standard – AS4787 Issued 1992-10 and Reaffirmed 1996-07, “Eddy 

Current Inspection of Circular Holes in Nonferrous Metallic Aircraft Engine Hardware.” 

2. Burkel, R.H., Chiou, C.-P., Keyes, T.K., Meeker, W.Q., Rose, J.H., Sturges, D.J., 

Thompson, R.B., and Tucker, W.T., “A Methodology for the Assessment of the 

Capability of Inspection Systems for Detection of Subsurface Flaws in Aircraft Turbine 

Engine Components,” FAA report DOT/FAA/AR-01/96, September 2002. 

3. Margetan, F.J., Gigliotti, Brasche, L., and Leach, W., “Fundamental Studies: Inspection 

Properties for Engine Titanium Alloys,” FAA report DOT/FAA/AR-02/114, December 

2002. 

4. Design and Analysis of Experiments, 5th edition, by Douglas C. Montgomery. 

5. Berens, A.P. and Hovey, P.W., “Flaw Detection Reliability Criteria, Volume 1—Methods 

and Results,” AFWA:-TR-84-4022, April 1984. 

16
APPENDIX A—MAIN EFFECTS AND INTERACTION PLOTS GENERATED IN THE 

DOE STUDY DESIGNED TO DETERMINE THE IMPORTANT PARAMETERS USED 

IN HSBH INSPECTION

A three- factor, two-level full factorial design (two* or eight combinations) blocked for motor 

speed and randomized within each block design of experiment (DOE) was selected as the tool to 

measure the high-speed bolthole process sensitivity. The DOE design and summary results were 

discussed in this report. In this appendix, the data generated in this study are presented for 

completeness. All charts were generated using MinitabTM, a comprehensive statistical and 

graphical analysis software package. The following is an explanation of charts that appear in the 

appendix and could be used as guidance in the interpretation of these figures. 

• Pareto Chart of Effects. The Pareto chart illustrates both the magnitude and the 

importance of an effect. This chart displays the absolute value of the effects and draws a 

reference line on the chart. Any effect that extends past this reference line is potentially 

important. Minitab displays the 

− absolute value of the unstandardized effects when there is not an error term. 

− absolute value of the standardized effects when there is an error term. 

• Cube Plot. Cube plots in a DOE study can be used to show the relationship between up 

to eight factors, with or without a response measure. Viewing the factors without the 

response illustrates what a design looks like. If there are only two factors, Minitab 

displays a square plot. One can draw a cube plot for one of the following: 

− data means—the means of the raw response variable data for each factor level. 

− fitted means after analyzing the design—predicted values for each factor level. 

The plots in the appendix illustrate a three-factor cube plot without a response variable. These 

factors are probe angle, probe rotation speed, and probe feed rate. The range of these factors is 

also indicated on the figure. 

• Main Effects Plot. The main effects plot is used to plot data means with multiple factors. 

The points in the plot are the means of the response variable at the various levels of each 

factor, with a reference line drawn at the grand mean of the response data. The main 

effects plot can be used for comparing magnitudes of main effects. 

• Interaction Plot. Interactions Plot creates a single interaction plot for two factors, or a 

matrix of interaction plots for three to nine factors. An interactions plot is a plot of 

means for each level of a factor with the level of a second factor held constant. 

Interaction plots are useful for judging the presence of interaction between the various 

factors. Interaction is present when the response at a factor level depends upon the 

level(s) of other factors. Parallel lines in an interaction plot indicate no interaction. The 

 

*

 Berens, A.P. and Hovey, P.W., “Flaw Detection Reliability Criteria, Volume 1—Methods and Results,” AFWA:-

TR-84-4022, April 1984. 

A-1
greater the departure of the lines from the parallel state, the higher the degree of 

interaction. To use the interactions plot, data must be available from all combinations of 

levels. 

0 1 2 3 4

C

B

A

BC

AC

AB

ABC

Pareto Chart of the Effects

(response is FstLrgCr, Alpha = .10)

A: Probe An

B: Probe Fe

C: Probe RP

Probe Angle Probe Feed Probe RPM

2200

1200

0.750

0.125

0

2

19

18

17

16

15

FstLrgCrck

20

16

20

20

16

 7

20

16

0 2

Probe Angle

Probe Feed

Probe RPM

0.125

0.750

1200

2200

Cube Plot (data means) for FstLrgCrck

2200

1200

0.75

0.125

20

16

12

20

16

12

Probe Angle

Probe Feed

Probe RPM

0.75

0.125

2

0

Interaction Plot (data means) for FstLrgCrck

FIGURE A-1. LARGEST CRACK MISSED RESULTS

A-2
1.91

2.14

2.12

2.09

2.70

2.41

2.43

2.50

0 2

Probe Angle

Probe Feed

Probe RPM

0.125

0.750

1200

2200

Cube Plot (data means) for Signal

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

B

C

BC

AB

AC

ABC

A

Pareto Chart of the Effects

(response is Signal, Alpha = .10)

A: Probe An

B: Probe Fe

C: Probe RP

Probe Angle Probe Feed Probe RPM

2200

1200

0.750

0.125

0

2

2.5

2.4

2.3

2.2

2.1

Signal

Main Effects Plot (data means) for Signal

0.125

0.75

1200

2200

2.00

2.25

2.50

2.00

2.25

Probe Angle 2.50

Probe Feed

Probe RPM

0

2

0.125

0.75

Interaction Plot (data means) for Signal

FIGURE A-2. SIGNAL AMPLITUDE RESULTS 

A-3
0.00 0.05 0.10

A

BC

ABC

B

AB

AC

C

Pareto Chart of the Effects

(response is HighNois, Alpha = .10)

A: Probe An

B: Probe Fe

C: Probe RP

0.20

0.35

0.24

0.13

0.10

0.12

0.13

0.09

0 2

Probe Angle

Probe Feed

Probe RPM

0.125

0.750

1200

2200

Cube Plot (data means) for HighNoise

Probe Angle Probe Feed Probe RPM

0

2

0.125

0.750

1200

2200

0.120

0.145

0.170

0.195

0.220

HighNoise

0.125

0.75

1200

2200

0.10

0.18

0.26

0.10

0.18

0.26 Probe Angle

Probe Feed

Probe RPM

0

2

0.125

0.75

Interaction Plot (data means) for HighNoise

FIGURE A-3. HIGH-NOISE RESULTS 

A-4
0.00 0.01 0.02 0.03 0.04 0.05

AC

C

ABC

BC

AB

B

A

Pareto Chart of the Effects

(response is LowNoise, Alpha = .10)

A: Probe An

B: Probe Fe

C: Probe RP

0.14

0.19

0.10

0.07

0.07

0.09

0.07

0.07

0 2

Probe Angle

Probe Feed

Probe RPM

0.125

0.750

1200

2200

Cube Plot (data means) for LowNoise

Probe Angle Probe Feed Probe RPM

0

2

0.125

0.750

1200

2200

0.08

0.09

0.10

0.11

0.12

LowNoise

0.125

0.75

1200

2200

0.08

0.12

0.16

0.08

0.12

0.16 Probe Angle

Probe Feed

Probe RPM

0

2

0.125

0.75

Interaction Plot (data means) for LowNoise

FIGURE A-4. LOW-NOISE RESULTS 

A-5
16.07

 8.73

 6.14

 9.69

26.92

18.29

20.08

26.62

Probe RPM

Probe Feed

Probe Angle

0 2

0.750

0.125

2200

1200

Cube Plot (data means) for HiNoiseSNR

2200

1200

0.75

0.125

20

15

10

20

15

10

Probe Angle

Probe Feed

Probe RPM

0.75

0.125

2

0

Interaction Plot (data means) for HiNoiseSNR

Probe Angle Probe Feed Probe RPM

2200

1200

0.750

0.125

0

2

22

19

16

13

10

HiNoiseSNR

0 5 10

A

BC

AB

B

C

ABC

AC

Pareto Chart of the Effects

(response is HiNoiseS, Alpha = .10)

A: Probe An

B: Probe Fe

C: Probe RP

FIGURE A-5. HIGH-NOISE SNR RESULTS 

A-6
28.13

21.20

11.26

13.23

35.71

36.19

28.12

36.35

Probe RPM

Probe Feed

Probe Angle

0 2

0.750

0.125

2200

1200

Cube Plot (data means) for LoNoiseSNR

0.125

0.75

1200

2200

15

25

35

15

25

35 Probe Angle

Probe Feed

Probe RPM

0

2

0.125

0.75

Interaction Plot (data means) for LoNoiseSNR

Probe Angle Probe Feed Probe RPM

2200

1200

0.750

0.125

0

2

34

30

26

22

18

LoNoiseSNR

0 5 10 15

A

B

AB

BC

AC

C

ABC

Pareto Chart of the Effects

(response is LoNoiseS, Alpha = .10)

A: Probe An

B: Probe Fe

C: Probe RP

FIGURE A-6. LOW-NOISE SNR RESULTS 

A-7/A-8
APPENDIX B—DESIGN DRAWINGS FOR THE COMMON TOOLING 

AND FIXTURES 

B-1

FIGURE B-1. BOLTHOLE SPECIMEN SIMULATOR FIXTURE
FIGURE B-2. SEMIAUTOMATED SCANNER DESIGN 

B-2
B-3

FIGURE B-3. ADAPTER PLATES DESIGN FOR GE DISK USED IN DEMONSTRATING THE INSPECTION TECHNIQUE ON 

ACTUAL ENGINE HARDWARE ALONG WITH A SCHEMATIC OF THE DISK CROSS SECTION 
B-4

FIGURE B-4. ADAPTER PLATES DESIGN 
APPENDIX C—PROBABILITY OF DETECTION INSPECTION TECHNIQUE AND 

DATA COLLECTION SHEET 

C-1 
ETC Subtask 2.2.2 

HIGH SPEED BOLTHOLE EDDY CURRENT 

SCANNING 

POD TESTING INSTRUCTIONS FOR CAPABILITY 

TESTING OF SEMI-AUTOMATED BOLTHOLE 

SCANNER P/N STE8388601 

 

 

21-1 March 08, 2003 

C-2 
ETC SUBTASK 2.2.2 

HIGH SPEED BOLTHOLE EDDY CURRENT SCANNING

 

 

 21-1 March 08, 2003 

 

 Prepared by: Andy Kinney 

ET Level III 

 Approved by: _Rob Stephen______________ 

ETC 2.2.2 Team Rep. 

 

C-3 
REPORT NO. 21-1

TOTAL PAGES 8 

ATTACHMENTS: 

 

 

REV BY APPROVED DATE PAGES 

AFFECTED 

1 AK 2002-12-20 All 

2 AK 2003-02-04 1,3,5,6,7 

3 AK 2003-03-08 8 

 

C-4 
ETC SUBTASK 2.2.2 

HIGH SPEED BOLTHOLE EDDY CURRENT SCANNING

1.0 SCOPE

This procedure uses high-speed eddy current bolthole scanning to inspect and record results from 

the POD specimen testing set FML 101146 using manual (fixtured & handheld calibrations), 

semi-automated (ETC 2.2.2 scanner P/N ST8388601), and automated scanning (ETC2000). 

2.0 PERSONNEL QUALIFICATION

Personnel performing these inspections shall be certified to a minimum of Level II in 

eddy current per NAS-410 requirements with previous eddy current bolthole inspection 

experience. 

3.0 EQUIPMENT

Instrument: The eddy current instrument used for manual and semi-automated 

scanning shall be the Rohmann Elotest B1-SDM/UC Version 5.11instrument. 

Automated scanning will use the ETC2000 scanner and data acquisition. 

Equipment: 

Manual Un-fixtured Inspection-Rohmann Elotest probe rotator SR1-HF 96.1.610 

(1200 rpm) 

Manual Fixtured Inspection-GEAE fixture PN 8047G01 and Rohmann Elotest probe 

rotator SR1-HF 96.1.610 (1200 rpm) 

Semi-Automated Inspection-ETC Scanner P/N STE8388601 and Rohmann Elotest 

probe rotator SR1-HF 96.1.610 (1200 rpm) 

Automated Inspection-ETC2000 Scanner and Uniwest probe rotator ETC-5037 (1500 

rpm) 

Probes: Rohmann P/N R&MS2/0.460 Diameter 0.46 inches 

Reference Standard: Marked titanium 6-2-4-6 standard having a bolthole diameter 

0.460”and 0.030” long x 0.015” deep EDM notch. 

Specimen Fixturing: The POD specimen set will be fixtured in adapter plate 

STE8388602 that holds 24 specimens plus the calibration standard. The semi-auto 

C-5 
scanner shall use the flat adapter plate, P/N STE838860-1 mounted for calibration and 

inspection. 

4.0 PROBE PREPARATION 

The test probe shall utilize a split-core differential coil that is nominally flush with the 

probe. 

5.0 INSPECTION PROCEDURE 

Complete the Figure C1 calibration record sheet for each set of pre, post, and any 

additional calibrations occurring during one specimen POD run set. A run set is defined 

as a complete inspection of all 50 POD specimens by one inspector for a particular 

technique, either manual or semi-automated scanner. 

Set inspection instrument parameters as follows: 

Frequency: 2 MHz 

Phase: Set instrument phase while in the impedance plane (X-Y) display to maximize 

the vertical response (Y) of the calibration notch. Initial setting is 132 degrees. 

Gain: The Elotest B1 pre-amp shall be set to 30 dB. Initial gain shall then be set such 

that the response from the 0.030” L x 0.015” Dp EDM notch is 7.5 screen divisions. 

Probe Rotator Rate: 

1200 rpm - Rohmann Elotest probe rotator SR1-HF 96.1.610 

1500 rpm - Uniwest probe rotator ETC-5037 

Display: A time sweep (Y-T) display shall be used for inspection. The X-Y impedance 

plane display shall be used for probe centering and phase adjustment off the EDM 

calibration notch. 

Filter: Low-pass and hi-pass filtering shall be used to decrease unwanted noise. The 

filters are correctly set up when the response from the notch target shows as a distinct, 

sharply peaked 'W' shaped sinusoidal signal about the horizontal centerline (see Figure 

C2). 

C-6 
Alarm Gate: No alarms are to be used for POD testing. 

Translation Rate or Index: 

Manual – typical rate for bolthole testing 

Semi-automated – 0.1 inches/sec setting 

Full-automated – 0.008” index (equivalent to a 0.2 inches/sec helical scan at 1500 rpm) 

C-7 
FIGURE C1. EDDY CURRENT POD CALIBRATION RECORD 

Facility: XXX Date: 

ET Inspector Identification (Circle One): 

A B C D 

Time(Start): 

Specification: Engineering Report 21-1 Time(Finish): 

Eddy Current Instrument Mfg, Model & Ver.:Rohmann Elotest B1-SDM/UC Version 5.11

Equipment Mfg.and Model: Elotest SR1-HF 96.1.610 probe rotator; ETC P/N 

STE8388601(Semi) & ETC2000 with (Full Auto) 

Probe Mfg./Part No.:Rohmann Probe S/N: 

Gain: (preamp 30dB) + dB gain Frequency: 2 MHz 

Display: Sweep Y-T V/H: Probe Centering Only Filters: HP_____ LP_____ 

Filters: HP 200 LP 800

Alarm: Manual-N/R 

Semi-auto- ± 1 Div. 

Audible: On for semi

Translation Rate: 

0.1”/sec Semi-automatic 

Probe rotator RPM: 

Elotest 1200 

0.008”index Automatic Uniwest 1500 

Calibration Sensitivity: 0.030” L x 0.015” Dp EDM notch to 7.4 Divisions 

Alarm Level: Not used for manual inspections; ± 1 Division for Semi-Auto 

Reject Level: Any crack indication 

Post cal.• Y N 

Circle One: 

Manual-Unfixtured 

Manual-Fixtured Cal. 

Semi-Auto Inspection 

Automated Inspection 

Calibration Files 

Pre:________________

Mid:________________

Post:_______________ 

C-8 
6.0 REFERENCE STANDARD CALIBRATION 

Adjust instrument per section 5.0. 

Teflon tape shall not be be applied for calibration or for inspection of the POD set. 

Center the probe as follows: 

• Visually align probe over bolthole. 

• Turn filters off and place instrument in XY plane mode 

• Rotate probe down into the calibration standard away from notches 

• Minimize excursion of the flying dot using fixture adjustment wheels 

• Place instrument in YT trace mode, set up instrument filters, and make any final fine 

adjustments to reduce noise. 

Calibrate the inspection system on the 0.030” long x 0.015” deep EDM notch to 7.5 

divisions on the calibration standard using either the semi-automated or automated 

fixturing. Print out and label a pre-calibration report when using the Elotest B1. 

Refer to and complete an Eddy Current POD Calibration Record (separate copies 

available) including: Inspection date, start time, assigned inspector I.D., probe P/N, 

probe S/N, calibration gain, filter settings, inspection type (manual, semi-auto, auto.), 

calibration file(s) or pre-cal print-out. 

The display of the reference standard notch should be as shown in Fig. C2. when filter settings 

are set as follows: 

Set the high-pass filter to 200 Hz to produce a more symetrical “W” shape from the 

calibration notch. Set the low-pass filter to 800 Hz to eliminate any high frequency 

spikes without significantly reducing signal amplitude to produce the best signal-tonoise ratio. 

Verify that all parameters are correctly set and recorded before proceeding.

C-9 
FIGURE C2. CALIBRATION STANDARD 0.030 X 0.015 RESPONSE 

7.0 SPECIMEN SCANNING 

Calibrate per Section 6. 

For manual inspection of the POD specimens, the rotator will be hand held. Use the ETC 

scanner P/N STE8388601 for semi-automated scanning. Use the ETC 2000 scanner for fully 

automated scanning. 

Scan the POD specimens to detect cracks. 

Indications shall be peaked out to determine if they exceed two (2) screen divisions. For 

indications exceeding two (2) divisions, record amplitude and the temporary specimen 

I.D. For indication amplitudes greater than full screen, reduce the preamp gain by 6dB 

or 12 dB, whichever results in the highest on-screen signal. Record preamp gain. 

Return instrument preamp setting to 30dB.

C-10
8.0 POST CALIBRATION CHECK 

After inspecting the POD set, check and record post calibration. Print out and label a pre-calibration 

report when using the Elotest B1. Print out and label a post-calibration report showing the calibration 

standard peak amplitude. If the amplitude repeats within 5 minor divisions of the precalibration run, the 

test is acceptable. If the amplitude of the EDM notch has fallen outside this range, check probes fit, 

and repeat calibration check. See the POD test proctor for possible re-inspection of all specimens since 

the last valid calibration if the calibration is still invalid. 

Complete a Figure 1C Eddy Current POD Calibration Record by circling the post 

calibration check and finish time. 

9.0 EVALUATION 

Peak out and record amplitude and temporary specimen identity for any indication greater 

than two (2) major screen divisions using the eddy current POD recording sheet provided 

below. In addition for signals greater than full screen, also record the pre-amplifier gain used 

to bring indication on screen. Return instrument preamp setting to 30dB. 

10.0 ACCEPTANCE CRITERIA 

Reject any relevant indication. A relevant indication is any repeatable indication with greater 

than a 2:1 signal-to-noise ratio. 

Data will later be analyzed to determine the Probability of Detection (POD). Inspector 

identity will remain confidential. 

C-11
 Eddy Current POD Recording Sheet Inspector/Run____________________ 

Noise Level _______________ Date_________________________ 

Specimen 

I.D. 

Amplitude Pre-Amp Setting (if 

not 30 dB) 

Depth Location in 

Hole (Top, Middle, 

Bottom) 

Indication 

Clock Position 

C-12
 

D-1/D-2

APPENDIX D—DRAFT REVISION OF INDUSTRY STANDARD AS4787 
 

34

The Engineering Society 

For Advancing Mobility 

Land Sea Air and Space ® 

I N T E R N A T I O N A L 

400 Commonwealth Drive, Warrendale, PA 15096-0001 

Submitted for recognition as an American National Standard 

Issued 1992-10 

AS4787 

EDDY CURRENT INSPECTION OF 

CIRCULAR HOLES IN NONFERROUS METALLIC 

AIRCRAFT ENGINE HARDWARE 

TABLE OF CONTENTS 

1. SCOPE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

1.2 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

1.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 3 

2. APPLICABLE DOCUMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3 

2.1 U.S. Government Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 

2.2 ASNT Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

2.3 ATA Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..4 

3. TECHNICAL REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 

3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

3.1.1 Tape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

3.1.2 Cleaning Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

3.1.3 Marking Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

3.2 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

3.2.1 Eddy Current Probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 

3.2.2 Eddy Current Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 

3.2.3 Mechanical Probe Manipulation Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .9 

3.2.4 Data Recording Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 

3.2.5 Calibration Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 

3.3 Personnel Qualification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 

3.4 Inspection Procedure Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..10 

3.5 Part Preparation for Inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

3.6 Equipment Calibration - Manual Probe Manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 12 

3.7 Equipment Calibration - Mechanical Probe Manipulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

3.8 Part Inspection - General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

3.9 Posttest Calibration Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 

3.10 Indication Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..15 

AEROSPACE 

STANDARD 
33

TABLE OF CONTENTS (Continued) 

4. QUALITY ASSURANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1 Records. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

4.2 Facilities/Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

4.3 Calibration Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

4.4 Inspection Procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

4.5 Quality Assurance Plans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

4.6 Calibration System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

FIGURE 1 Typical Eddy Current Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

FIGURE 2 Eddy Current Displays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7 

FIGURE 3 Eddy Current and Meter Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

FIGURE 4 Typical Calibration Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 
34

EDDY CURRENT INSPECTION OF 

CIRCULAR HOLES IN NONFERROUS METALLIC 

AIRCRAFT ENGINE HARDWARE 

1. SCOPE: 

1.1 Purpose: 

This SAE Aerospace Standard (AS) establishes minimum requirements for eddy current 

inspection of circular holes in nonferrous, metallic, aircraft engine hardware with fasteners 

removed. The inspection is intended to be performed at maintenance and overhaul facilities 

on engine run hardware. 

1.2 Application: 

This standard has been typically used to inspect for service-induced surface and near surface 

cracks, but usage is not limited to such applications. 

1.3 Definitions: 

1.3.1 PURCHASER: The organization that issued the procurement document invoking this 

specification. 

1.3.2 SUPPLIER: Source, other than the purchaser, who provides inspection services. 

1.3.3 INSPECTION PROCEDURE: Document written by the purchaser detailing inspection 

requirements. 

1.3.4 QUALITY ASSURANCE PLAN: Document written by the supplier outlining specific 

requirements unique to his/her facility in accordance with inspection procedure. 

2. APPLICABLE DOCUMENTS: 

The following publications form a part of this specification to the extent specified herein. The 

applicable issue of referenced publications shall be the issue in effect on the date of the 

purchase order. 

2.1 U.S. Government Publications: 

Available from DODSSP, Subscription Services, Building 4D, 700 Robbins Avenue, 

Philadelphia, 

PA 19111-5094. 

MIL-I-23594A-A-59474 Insulation Tape, Electrical, High Temperature, Polytetrafluoroethylene, 

Pressure Sensitive 

MIL-STD-410 Nondestructive Testing Personnel Qualification and Certification (Eddy

Current, Liquid Penetrant, Magnetic Particle, Radiographic, and Ultrasonic)

MIL-STD-45662 
35

2.2 International Organization for Standardization

Available from ISO1, International Organization for Standardization, Case Postale 56, 

Geneva, Switzerland Ch-1211

ISO 10012 Calibration System Requirements 

2.3 AIA Document:

Available from Aerospace Industries Association (AIA), 1250 Eye St. N.W., Suite 1200, 

Washington, DC 20005. (www.aia-aerospace.org)

NAS-410 Nondestructive Testing Personnel Qualification and Certification

2.24 ASNT Publications: 

Available from ASNT, 1711 Arlington Lane, P.O. Box 28518, Columbus, OH 43228. 

SNT-TC-1A Recommended Practice, Personnel Qualification and Certification in 

Nondestructive 

Testing 

2.35 ATA Publications: 

Available from Air Transport Association of America, 1301 Pennsylvania Avenue, N.W., Suite 

1100, Washington, DC 20006. 

ATA 105 Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods 

3.TECHNICAL REQUIREMENTS: 

3.1Materials: 

The high temperature, high speed environment in which many aircraft engine components 

operate results in an increased sensitivity to mechanical or chemical damage during 

maintenance activities. Personnel performing eddy current inspections must be aware of the 

need to ensure that the materials they use which come in contact with the hardware must be 

carefully chosen to avoid introducing chemical contamination or causing physical damage. 

Engine manufacturers can provide information regarding the suitability of specific materials or 

procedures for use on their hardware. 

3.1.1 Tape:MIL-I-23594A-A-59474, Type I (smooth backing), Class 3 0.0036 to 0.0045 in 

(0.08 09 to 0.131 mm) thick. Alternate thicknesses (Class) and alternate tapes may be used if 

specified in the appropriate inspection procedure. 

3.1.2 Cleaning Materials: Mild abrasive cleaning products may be used, if allowed by the 

purchaser, for the removal of surface contamination. Such products shall not be aggressive 

enough to remove the base metal or otherwise alter or damage the part. If allowed, mild, 
36

nonmetallic abrasive cleaning pads can be torn in strips or circular nonmetallic brushes may 

be used to clean the inside diameter of holes. These materials shall be specified in the 

inspection procedure. 

3.1.3 Marking Devices: Approved marking device that are acceptable to the purchaser for 

numbering part features and identifying the locations of suspect test indications. that are 

acceptable to the purchaser. 

3.2 Equipment: 

3.2.1 Eddy Current Probes: 

3.2.1.1 Select probes based on the minimum diameter of the holes to be inspected. The hole 

diameter and appropriate probe for any particular inspection shall be as specified in the 

inspection procedure. Depending on the type of probe, it is recommended that the purchaser 

consider the hole diameter range for the probe being specified. 

3.2.1.2 Absolute (single coil), differential (dual coil), and reflection (driver-pickup) coil 

configurations, with or without magnetic shielding, shall be used as specified in the inspection 

procedure. Differential coils are the preferred coil configuration for circular hole inspection

and shall be oriented (scanned) normal to the crack direction.

3.2.1.3 Probes shall be designed to maintain contact with the inspection surface at all times 

during the inspection. Noncontact probes which operate with a gap between the probe and 

the inspection surface are not acceptable. 

3.2.1.4 Probes with operating frequencies of 500 KHz or higher shall be used, unless 

otherwise specified by the appropriate inspection procedure. 

3.2.1.5 All probes shall be identified with the manufacturer’s name, frequency or frequency 

range, and a unique serial number (reference Figure 1). 

3.2.1.6 Eddy current probe sensing coils shall be prevented from directly contacting the 

surface of the part being inspected. This may be accomplished by covering the coil surface 

with low-friction tape, by recessing the coil from the surface of the probe, by manufacturing 

the probe with a suitable wear surface over the coil, or by other means specified in the 

inspection procedure. 

3.2.1.7 Probes shall have a maximum sensing coil diameter of 0.060 inches (1.5 mm), unless 

otherwise specified in the inspection procedure.

3.2.1.8 The impedance of probes and adapters shall match the instrument being used.

3.2.1.7 9 Precautions shall be taken to protect the part being inspected from wear or abrasion 

from contact with any metallic portion of the probe or test equipment. 
37

3.2.1.8 10 Probes shall not give interfering responses from handling pressures, manipulation, 

or normal operating pressure variations on the sensing coil. 

3.2.2 Eddy Current Instruments: Eddy current instruments are available which have a wide 

range of capabilities. The features required for any particular inspection shall be specified in 

the inspection procedure. 

3.2.2.1 Display: Eddy current instruments can be classified into two basic categories 

according to the manner in which they display shall be capable of displaying eddy current 

information in the impedance plane format. These categories are specified in 

3.2.2.1.1 and 3.2.2.1.2. Either type of instrument may be acceptable for a particular inspection 

unless a specific type is required in the inspection procedure (reference Figures 2 and 3).

3.2.2.1.1 Instruments which use a one-dimensional meter display.

3.2.2.1.2 Instruments which use a two-dimensional impedance plane display. The 

"iImpedance plane" refers to an orthogonal display of both the inductive reactance and 

resistance components of the complex impedance of the eddy current probe (X,Y). This type 

of display may be accomplished using a cathode ray tube (CRT), liquid crystal display (LCD), 

or other suitable method. Some instruments of this type also have the ability to produce a 

time-based display where one dimension of the eddy current signal is displayed as a function 

of time (Y,T) (reference Figures 2 and 3). 

FIGURE 1 – Typical Eddy Current Probe 

* This figure is unchanged from previous revision.

FIGURE 2 – Eddy Current Displays

* This figure is unchanged from previous revision
38

FIGURE 3 Eddy Current Time Base (Y,T) Display

• Renumber FIGURES changing old FIGURE 4 (Typical Calibration Standard) to FIGURE 6

FIGURE 6 – Typical Calibration Standard

* Modify this figure to show a “half penny” shaped EDM calibration notch with the square 

notch as an alternative configuration.
39

Figure 4 - Examples of Fixtured Probe Rotators with Manual Probe Translation

Figure 5 – Semi-Automated Scanner with Axial Probe Translation (Probe rotator and part 

adapter not attached)
40

3.2.2.2 Frequency :Instruments with variable probe excitation frequencies shall be operated at 

500 KHz or higher for maximum crack detection sensitivity in materials which are typically 

used for aircraft engine components, unless otherwise specified in the inspection procedure.

Instruments shall be capable of operating between 100 kHz and 6.0 MHz.

3.2.2.3 Filters: Electronic filters are frequently useful for reducing noise or spurious geometric 

signals and improving the signal to noise ratio of an inspection. Some inspections may be 

dependent upon the use of appropriate filters for their success. Such requirements will be 

specified in the appropriate inspection procedure. Three general types of filters are described 

as follows: 

3.2.2.3.1 High Pass Filters: Eliminates or reduces the amplitude of signals which have a 

frequency below the frequency setting of the filter. 

3.2.2.3.2 Low Pass Filters: Eliminates or reduces the amplitude of signals which have a 

frequency above the frequency setting of the filter. 

3.2.2.3.3 Band Pass Filters: A combination of a high pass and a low pass filter which acts to 

eliminate or reduce the amplitude of signals which have a frequency above or below the 

frequency settings of the filter. 

3.2.2.4 Analog Signal Output: Inspections which specify external recording or monitoring of 

the test signals may require the use of an instrument which has provisions for the output of 

analog voltages which are proportional to the eddy current signals displayed on the 

instrument. 

3.2.2.5 Alarms: Instruments shall be equipped with an audible and/or visual alarm system.

3.2.2.6 Voltage Regulators: For other than battery-powered usage, a voltage regular shall be 

used on the power source if instrument internal voltage regulators are not adequate to prevent 

an eddy current signal amplitude variation of 10% or more.

3.2.3 Mechanical Probe Manipulation Devices: 

Eddy current inspections of holes may be greatly facilitated by the use of a mechanical device 

to rotate the probe and translate it axially through the hole. Such devices can reduce the time 

required to perform an inspection and ensure that complete coverage of the hole inside 

diameter is obtained. Before using a mechanical manipulator the user shall verify that the 

rotational and axial motion speeds are appropriate for the inspection procedure. Devices can 

be classified into three basic categories according to the degree on mechanization. These 

categories are specified in 3.2.3.1, 3.2.3.2 and 3.2.3.3

3.2.3.1 Fixtured Mechanical Probe Rotator with Manual Probe Translation: This type of device 

provides for alignment of the probe with respect to the axis of the circular hole. This provides 
41

a degree of reliability improvement in comparison to a handheld probe rotator. These may be 

centered manually from one hole to the next or adapted to fix a particular part. 

3.2.3.2 Semi-Automated: Devices of this type provide alignment of a mechanical rotator and

also translate the probe axially through the circular hole. In addition to reliability improvement 

due to probe alignment, this device can assure a constant inspection sampling-rate. Predesigned fixtures can adapt this device to engine hardware to provide rapid hole-to-hole 

placement and centering (ref. Figure X).

3.2.3.3 Automated: These devices can be programmed to provide automatic probe motion, 

translation, hole-to-hole indexing, instrument set-up, evaluation, etc. dependant upon the 

degree of automation, all of which provides increasing capability for improving inspection 

reliability. Typically, these are stationary devices requiring parts to be brought to them for 

inspection.

3.2.4 Data Recording Devices: Inspections which that require the recording or documentation 

of some or all of the test signals produced during the inspection may utilize recording devices. 

Such devices may be incorporated into the eddy current instrument or in a separate 

instrument. Such devices may consist of paper chart recorders, magnetic tape recorders, 

digital data acquisition instruments, or similar device. The recording device used shall be 

appropriate for the type and quantity of data to be recorded. Specific details regarding 

recording device requirements shall be contained in the inspection procedure. 

3.2.5 Calibration Standards: 

3.2.5.1 Calibration standards are required to establish equipment sensitivity prior to 

performing an inspection. Unless otherwise specified by the purchaser, standards shall be 

fabricated from the same material as the component to be inspected. 

3.2.5.2 Notches or similar artificial defects shall be created in the standards to provide a 

uniform, repeatable source of an eddy current indication during equipment calibration. The 

size of artificial defect needed for an inspection shall depend on the requirements of the 

particular inspection and will be specified in the inspection procedure. A 0.030-inch long by 

0.015-inch deep, half-penny profile EDM notch is recommended for calibration. EDM notch

width is typically 0.002-0.003 inches.

3.2.5.3 Calibration standards shall be identified with the material, part number, hole size, 

unique serial number, and notch size (reference Figure 4). 

3.2.5.4 Calibration standard dimensions, including hole sizes and notch sizes, shall be 

measured and documented. Tolerances shall be specified in the inspection procedure. 

Certification and traceability of these measurements may be required by the purchaser. 
42

3.2.5.5 The electrical conductivity of calibration standards shall be similar to that of the part to 

be inspected. The tolerance in conductivity variation between calibration standards and test 

parts shall be specified in the inspection procedure. If specifically approved by the purchaser,

calibration standards with electrical conductivities different from that of the part to be 

inspected may be used when appropriate correction factors are also provided. The tolerance 

in conductivity variation between calibration standards and test parts should be no greater 

than 2 and no less than 2/3 times the conductivity of the material to be inspected. 

The tolerance in conductivity variation between calibration standards and test parts shall be 

specified in the inspection procedure. It may be acceptable to use calibration standards with 

electrical conductivities different from that of the part to be inspected if this is specifically 

approved by the purchaser and appropriate correction factors are provided.

3.2.5.6 Calibration standard finish shall be 63 RMS or better.

3.3 Personnel Qualification: 

Personnel performing eddy current hole inspections shall be qualified and certified in 

accordance with MIL STDNAS-410 or SNT-TC-1A. Other procedures (ATA 105, etc) may be 

used with prior approval of the purchaser. 

3.4 Inspection Procedure Requirements: 

Eddy current hole inspections shall be performed in accordance with a detailed inspection 

procedure for the component to be tested. Each procedure shall be prepared and verified and 

approved by a certified eddy current Level III employed the purchaser or his designee. A copy 

of each applicable procedure shall be readily available to all inspection personnel for 

reference and use while performing the inspection. Procedures shall comply with the general 

requirements of this specification and shall provide all of the specific information required to 

set up the equipment and perform the test. Each procedure shall include not less than the 

following information: 

3.4.1 Name and part number to which the procedure applies (include serial number if 

applicable) 

3.4.2 A description and sketch identifying the holes to be inspected, if this is not readily 

apparent. 

3.4.3 Part cleaning and preparation instructions 

3.4.4 A description and/or sketch of the manner in which the holes are numbered or 

referenced from the geometry of the part for identification purposes. 

3.4.5 Inspector training and qualification requirements 

3.4.6 Inspection equipment, probe and fixture requirements, including manufacturer and 

model number where appropriate 
43

3.4.7 Calibration standard to be used 

3.4.8 Inspection equipment setup and operation parameters 

3.4.9 Pretest calibration procedure 

3.4.10 Part inspection procedure 

3.4.11 Post-test calibration procedure and calibration check intervals 

3.4.12 Instructions regarding the evaluation of suspect eddy current indications 

3.4.13 Acceptance criteria 

3.4.14 Instructions regarding inspection records which shall be completed and maintained 

3.4.15 Part disposition 

3.4.16 It is recommended that there is a method of marking parts that have been inspected 

and accepted. 

3.5 Part Preparation for Inspection: 

3.5.1 Verify that the inspection procedure is applicable to the part to be inspected. 

3.5.2 Identify the location, number, and size of holes to be inspected. 

3.5.3 Visually inspect the holes under a white light [recommended 100 ft-c (10762x)] for 

evidence of burrs, out of round condition, dirt, rubbing, fretting, foreign material, or other 

contamination which would interfere with the inspection process. Borescopes or other optical 

aids may be used to enhance the visual inspection. 

3.5.4 Use approved cleaning materials to clean the holes or contact cognizant engineering 

activity for corrective action if the holes cannot be cleaned. 

3.5.5 Number the holes for identification. 

3.6 Equipment Calibration - Manual Probe Manipulation:

3.6.1 Connect the probe and instrumentation according to the manufacturer’s directions.

3.6.2 Adjust the initial equipment settings as specified by the inspection procedure or quality 

assurance plan.
44

3.6.3 If the probe diameter is variable, adjust the probe to fit the holes to be inspected. The 

probe shall fit snugly but not so tight as to cause excessive wear of the probe. On inserting or 

withdrawing the probe, there should be a light positive drag.

3.6.4 Place the probe in the probe collar and on the surface of the calibration standard, away 

from the calibration notch.

3.6.5 Null (or balance) the eddy current instrument.

3.6.6 Rotate the coil across the calibration notch and identify the coil position which produces 

the largest notch signal amplitude.

3.6.7 Rotate the coil across the calibration notch at the location of the maximum response and 

adjust the instrument phase (or rotation) control to orient the defect signal as specified in the 

inspection procedure.

3.6.8 Adjust the gain (or sensitivity) controls to produce the notch signal amplitude specified in 

the inspection procedure.

3.6.9 Repeat this process until an accurate calibration is achieved and null the instrument 

after any changes in gain or phase/rotation.

3.6.10 Perform a scan of the notch at the index rate specified in the procedure to verify the 

accuracy of the calibration.

*Renumber to end

3.7 Equipment Calibration - Mechanical Probe Manipulation: 

3.7.1 Connect the probe and instrumentation according to the manufacturer’s directions. 

3.7.2 Adjust the initial equipment settings as specified by the inspection procedure or quality 

assurance plan. 

3.7.3 If the probe diameter is variable, adjust the probe to fit the holes to be inspected. The 

probe shall fit snugly but not so tight as to cause excessive wear of the probe. On inserting or 

withdrawing the probe there should be a light positive drag. 

3.7.4 Align the probe and manipulator with the calibration standard hole to be used for 

calibration. 

3.7.5 Drive the probe into the hole and stop it when the coil is positioned away from the 

calibration notch. 

3.7.6 Null (or balance) the eddy current instrument. 
45

3.7.7 While the probe is rotating at the desired speed, translate the probe slowly over the 

notch. Stop the probe translation when the point of maximum notch response is attained. 

Continue to rotate the probe. 

3.7.8 Adjust the instrument phase/rotation control to orient the notch signal as specified in the 

inspection procedure. 

3.7.9 Adjust the gain (or sensitivity) control to produce the notch signal amplitude specified in 

the inspection procedure. Null after any adjustments to gain or phase rotation. 

3.7.10 Repeat this process until an accurate calibration is achieved. 

3.7.11Perform a scan of the notch at the index rate specified in the inspection procedure to 

verify the accuracy of the calibration. In the case of helical “indexing” an equivalent index 

value may be determined the following: 

Equivalent Index for Helical Scans = Axial Velocity (in/sec.) x 60 ÷ Probe Rotator RPM

3.8 Part Inspection - General Requirements: 

3.8.1 The inspection parameters listed below shall not be adjusted after calibration as they 

may reduce the inspection system sensitivity and affect the inspection results. 

3.8.1.1 Test frequency 

3.8.1.2 Gain (or sensitivity) 

3.8.1.3 Phase (or rotation) 

3.8.1.4 Probe rotational speed 

3.8.1.5 Type and frequency of filtering used 

3.8.1.6 Probe drive signal voltage or current 

3.8.1.7 Test material 

3.8.2 If low-friction tape is used to prevent or reduce probe wear, it will be necessary for the 

inspector to check the tape condition periodically. If the tape has worn significantly, it shall be 

replaced before performing additional inspections. 

3.8.3 Any eddy current signals which that meet or exceed the rejection criteria specified in the 

inspection procedure shall be noted for later evaluation. 

3.8.4 The eddy current probe shall be properly aligned during inspection of all holes. 
46

3.8.5 Record all inspection results and instrument settings as specified in 4.1. 

3.9 Post tTest Calibration Check: 

3.9.1 The calibration of the inspection equipment shall be checked against the calibration 

standard at least every 4 hours or when any other critical component of the inspection system 

changes. It shall also be checked upon completion of an inspection. 

3.9.2 If the eddy current signal produced from the calibration standard notch is within the 

limits of the original calibration level as specified by the inspection procedure, the test shall be 

considered acceptable. 

3.9.3 If the notch signal has decreased by more than the specified limits from the original 

calibration level, the test shall be considered unacceptable and all parts inspected since the 

last acceptable calibration shall be reinspected. 

3.9.4 If the notch signal has increased by more than the specified limits from the original 

calibration level, the test shall be considered acceptable. All parts rejected since the last 

acceptable calibration check shall be reinspected and evaluated, if necessary per 3.10. 

3.10 Indication Evaluation: 

3.10.1 Evaluate any area where indications were obtained which met or exceeded the 

rejection criteria specified in the inspection procedure or other referenced document. 

3.10.2 Reclean the area where the indication was obtained. 

3.10.3 Check the inspection system calibration and reinspect the suspect area. 

3.10.4 If the indication has been reduced to below the inspection procedure rejection criteria, 

the indication shall not be cause for rejection of the part. 

3.10.5 If the indication remains above the rejection limit, but has reduced from its previous 

level, it may be caused by a surface condition. Additional cleaning may reduce the indication 

further. 

3.10.6 If the indication remains above the rejection threshold, the part shall be rejected. 

4. QUALITY ASSURANCE PROVISIONS: 

4.1 Records: 

Inspection records shall include the following information regarding the inspection: 

4.1.1 Date of inspection

4.1.2 Location where inspection is performed 
47

4.1.3 Inspector’s name and acceptance/rejection stamp 

4.1.4 Part name 

4.1.5 Part number 

4.1.6 Part serial number 

4.1.7 Inspection results 

4.2 Facilities/Equipment: 

Shall be subject to survey and approval by the purchaser and shall include, but not limited to, 

test equipment, calibration standards, probes, fixtures, reference documentation, personnel 

certification, and procedures. 

4.3 Calibration Standards: 

Shall be certified when required by the purchaser. The purchaser shall maintain master 

calibration standards and shall have the responsibility of establishing equivalent responses if 

allowed by the inspection procedure/quality assurance plan. 

4.4 Inspection Procedures: 

Shall be maintained by the purchaser. 

4.5 Quality Assurance Plans: 

Shall be maintained by the supplier. 

4.6 Calibration System: 

Shall be maintained in accordance with MIL-STD-45662 ISO 10012 if required by the 

purchaser or other method acceptable to the purchaser’s quality assurance.