CONSOLIDAmD REPRINT-FEBRUARY 1977 

(includes Changes 1 and 2) 

AC !6: 20-83 

DATE: 17 Jan 73 

ADVISORY 

CIRCULAR 

MAINTENANCE INSPECTION NOTES FOR BOEING B-737 SERIES AIRCRAFT 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

Initiated by: AFS-230 

At NO: 20- 83 

DATE: 1Z Jan 73 

ADVISORY 

CIRCULAR 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADhlINISTRATION 

SUBIE@T: MAINTENANCE INSPECTION NOTES FOR BOEING B-737 SERIES AIRCRAFT 

1. PURPOSE. This advisory circular provides maintenance inspection notes 

which can be used for the maintenance support program for certain 

structural parts of the B-737 series aircraft. 

2. DESCRIPTION. Maintenance inspection matters on the wing and fuselage 

are reviewed with a view toward supplementing information currently 

available. 

3. REFERENCES. 

a. 

b. 

C. 

d. 

e. 

4. HOW 

Advisory Circular 20-61, Nondestructive Testing Techniques for 

Aircraft. 

Advisory Circular 65-9, Airframe and Powerplant Mechanics General 

Handbook. 

Advisory Circular 43.13-1, Acceptable Methods, Techniques, and 

Practices - Aircraft Inspection and Repair. 

Defense Metals Information Center ('DMIC) Report, S-25, dated 

1 June 1968, Current Problems and Prevention of Fatigue. 

Advisory Circular 20-9, Personal Aircraft Inspection Handbook. 

TO GET THIS PUBLICATION. 

a. Order additional copies of this publication from: 

Department of Transportation 

Distribution Unit, TAD 484.3 

Washington, D. C. 20590 

Initiated by: ~~~-230 
AC 20-83 17 Jan 73 

b. Identify this publication as: Advisory Circular No. 20-83, ' 

Maintenance Inspection Notes for Boeing B-737 Series Aircraft. 

&&?qqjC. R. MELUGIN, JR. 

Acting Director, Flight Standards Service 

Page ii 
l/31/75 AC 20-83 CHG 2 

TABLE OF CONTENTS 

Page No. 

1. Introduction. 1 

2. Description. 1 

3. Background. 1 

4. General Discussion of Structure Surveillance. 1 

5. Type of Construction. 3 

6. Aircraft Station Diagrams. 4 

7. Abbreviations Used In This Document. 4 

8. Maintenance Reports. 4 

APPENDIX 1. CORROSION (2 pages) 1 

APPENDIX 2. STATION CHARTS (6 pages) 

Figure 1. Principal Dimensions 1 

Figure 2. Body Station Diagram 2 

Figure 3. Wing Station Diagram 3 

Figure 4. Engine and Nacelle Station Diagrams 4 

Figure 5. Horizontal Stabilizer and Elevator Station Diagrams 5 

Figure 6. Vertical Stabilizer and Rudder Station Diagrams , 6 

APPENDIX 3. NONDESTRUCTIVE TESTING (5 pages) 1 

Page iii 

B/8/74 AC 20-83 CHG 1 

3. 

2. 

3. 

* 

4. 

INTRODUCTION. This-advisory circular provides maintenance information 

which can be used, but is not required to be used, by mechanics, repair 

agencies, owners, and operators in developing maintenance programs, making improvements in existing programs, and conducting inspection and 

repairs on certain structural parts of Boeing B-737 airplanes. The 

material is based, in part, upon infocFtion made available through discussions with personnel who have maintained these types of airplanes for 

'thousands of hours of time in service. The intent of the circular is to 

impart some of this knowledge to other interested persons 30 that it is 

not lost. 

DESCRIPTION. This circlzlar contains guidance material for performing 

maintenance on wing, fuselage, and empennage structure. The information 

has been derived from service experience. It does not comprise a full 

and complete maintenance program for the subject aircraft but should be 

considered as supplemental maintenance data. Included in the circular 

is a listing of selected maintenance difficulties which have been reportecl 

since 1968 by air carrier operators. 

BACKGROUND. 

a. 

b. 

Used Aircraft. The Administrator realizes that several different types of transport aircraft are being phased out of service 

by some airlines and are being purchased by other operators who may 

not be familiar with the scope of required maintenance atid the means 

which have been used to keep the aircraft in a safe condition. * 

Maintenance "Know How." Since maintenance %now howtr is not transferred with the aircraft, the new operator generally goes through a 

learning cycle before he is-able to rapidly pinpoint the important/ 

critical problem areas of the aircraft. In this respect, identification of known areas where structural problems have been experienced will help in the preparation of an initial maintenance program 

by a new operator. It also can serve as a guide to other operators 

who have not accumulated sufficient service experience to have knowledge of all the problem areas of the aircraft. 

GENERAL DISCUSSION OF STRUCTURE SURVEILLANCE. 

a. Manufacturer's Service Bulletins. The manufacturer has published 

service bulletins containing its recommendations concerning the inspection, repair, and modification of aircraft. Most of these bulletins cover known areas; however, some are predictive in nature and 

have been issued even though no fatigue damage has been identified 

in the fleet. Because of differences in structural configuration, 

most service bulletins apply only to certain aircraft. Effectivity 

is shown in each bulletin. Additional bulletins may be published by 

the manufacturer, and a service bulletiqindex is available from the 

manufacturer which is updated periodically. 

Page 1 
AC 20-83 CHG 1 8/8/74 

b. Other Documentation. Further, the manufacturer has published the following documents to aid in maintaining the aircraft in serviceable 

condition: 

* Maintenance Planning Document (D6-17594) 

Maintenance Manual; @6-series different for each operator) 

Overhaul Manual, (D6-17370) 

Structural Repair Manual, (D6-15565). 

NIYIY Document (D6-7170) * 

The above documents are updated from time to time by the manufacturer. 

Structural Item Interim Advisories are published by the manufacturer 

to notify operators of newly found problems which may be of fleetwide significance and may or may not be followed by service bulletins. 

c. Maintenance Action. For adequate maintenance of the B-737 ai-z-plane 

structure, every operator sho,uld have in his possession and be conversant with the above documentation, including service bulletins applicable to his particular aircraft. He should also obtain complete 

service records from previous owners and become familiar with the 

struct&al history of his aircraf't, including information on maintenance procedures followed, major repairs made, and preventive modifications and/or repair work incorporated per service bulletins. This 

is essential to carry out the follow-up procedures required, and to 

avoid unnecessary work where corrective action has already been taken. 

(1) The new operator should contact the manufacturer regarding any 

areas requiring clarification. 

(2) The operator should keep himself informed of new developments 

and arrange to be supplied with revised and new documentation 

by the manufacturer. Consultation with the manufacturer and/or 

more experienced operators should take place from time to time 

as necessary, to establish which service bulletins have structural significance and when they would best be incorporated. 

(3) The maintenance program established by the new operator should 

reflect changes in environment and Gage of the aircraft (e.g., 

shorter flights, intermittent use, etc.). 

d. Airworthiness Directives. The material in this circular does not 

supersede requirements of airworthiness directives (AD's) issued by 

the FAA. Although manufacturers'service bulletins may be referred 

to in AD's, the bulletins do not supersede any requirements of the 

AD’S. 

e. Certification. Type certificate data sheet No. A16m was issued 

15 December 1967 for model 737-100 and 21 December 1967 for.model 

737-200 to the Boeing Company, Renton, Wa. The basis for certification was FAR 25, amendments 25-1 through 25-3, 25-7, 25-15; FAR 21; 

Page 2 Par 4 
17 Jan 73 AC 20-83 

5. 

PAR 1; and special conditions. (See type certificate data sheet A16WE 

for information relating to special conditions.) The latest revision 

of type certificate data sheet Al&Q, at the time this document was 

published, was revision No. 5 dated 1 April 1972. 

TYPE OF CONSTRUCTION. The aircraft structure can be classified into 

three major components; the fuselage, the wing, and the empennage. 

These components have been designed to provide maximum strength with 

minimum weight by providing multiple load paths so that the failure of 

any one structural element will not result in the failure of the complete 

structural component. The materials most corrmonly used throughout the 

structure are aluminum,' steel, and magnesium alloys. In secondary structural areas and many flight control surfaces, aluminum and fiberglas 

honeycomb core material is extensively used. 

a. The fuselage is a semimonocoque structure with the skin reinforced 

bgircumferential frames and longitudinal stringers. It consists 

of four section* ,--body sections 41, 43, 46., and 48; with section 41 

located between stations 178 and 360, section 43 between stations 360 

and 540, section 46 between stations 540 and 1016, and section 48 

between stations 1016 and lZ?l7. The entire fuselage shell between 

stations 178 and 1016 is pressurized except for the nose gear well 

and the cutout for the center wing box and main landing gear well. 

b. The wing is composed of a left wing box, center wing box, and a right 

wing box. The left and right wing boxes ar e cantilevered from the 

center wing box and the thickness and chord of each wing taper down 

toward the tip and sweep badk from the center wing box. The surfaces 

of the wing boxes consist of upper and lower skin panels and front 

and rear spars. Throughout the wing boxes, the skin panels are 

reinforced by spanwise stringers and the spars are reinforced by 

vertical stiffeners. 

(1) The left and right wing boxes are reinforced by chordwise ribs 

with the greatest portion of their enclosed volume sealed to 

serve as fuel tanks, The center wing 'box is reinforced by 

spanwise beams and &ontains bladder-type fuel cells. On each 

wing, the leading edge structure is cantilevered forward of the 

front spar. The trailing edge structure and control surfaces 

are cantilevered aft of the rear spar. 

(2) Two flaps and three extendable slats are supported by the wing 

leading edge on each wing. Along the trailing edge of each 

wing an I/B and O/B flap, an aileron, and four spoilers are 

installed. The leading edge of each wing contains a pressure 

relief panel to alleviate pressures due to leaking or ruptured 

ducts associated with the engine. The pressure relief panel is 

designed to open at a pressure of.one psi. 

Par 4 Page 3 
a. 

20-83 CfIG 1 8/8/74 

c. The empennage consists of a dorsal fin, a vertical fin, an adjustable 

horizontal stabilizer, a rudder, and elevators. The left and right 

horizontal stabilizers are removable from the center section truss 

located within the fuselage and have removable leading edges. The 

elevators are aluminum frame structures with leading and trailing 

edge spars, ribs, and a fiberglas skin. Each elevator contains three 

balance panels and a control tab. The rudder is a monospar aluminum 

alloy structure with chordwise ribs and honeycomb core fiberglas skin. 

It does not contain a tab. * 

AIRCRAFI' STATION DIAGRAMS. The wing, fuselage, and empennage station 

diagrams included in this document were developed for the B-737 aircraft 

and are used as a general reference,only. Several models of these aircraft were manufactured and have different station locator numbers based 

on the particular configuration. Since the defective areas generally 

apply to all models of aircraft, the referenced area can be compared with 

a similar area and locator on the appropriate station diagram for the particular model aircraft. 

ABBREYIATIO~S USED IN THIS DOCUMEZT. 

O/B 

0 

R/H 

TAT 

TSN 

TSO 

Federal Aviation Regulation 

Inboard 

Left Hand 

Main Landing Gear 

Nose Landing Gear 

Outboard 

Part Number 

Right Hand 

Total Aircraft Time 

Time Since New 

Time Since Overhaul 

MAINTrnANCE REPORTS. The following is a listing of selected maintenance 

difficulties experienced, representing examples of reports submitted by 

air carrier operators. This information may be useful in identifying 

structural problem areas. It should be noted that this is a partial list 

only and covers just a portion of time in the history of the B-737 series 

aircraft. 

a. Fuselage. 

(1) During routine inspection, on two occasions a two-inch crack 

was found at the fuselage vertical pressure web above the wing 

rear spar below the cabin floor, 30" left of the tiabin center 

line at station 663.75, TAT 4989 hours and 5113 hours. * 

Page 4 Par 5 
818174 

* (2) 

(3) 

(4) 

(5) 

(6) 

(7) 

(8) 

AC 20-83 CHG 1 

Routine inspection of the stub beam at the outboard lower 

chord, P/N 65-47995-2, disclosed a 4" spanwise crack just above 

the radius, R/H side of fuselage at station 559. TAT 5028 hours. 

During routine inspection, on two occasions skin cracks were 

found originating at the aft rivet hole of the L/H vortex 

generator at station 1114. TAT 3533 and 3634 hours. 

Routine service check disclosed a stringer cracked in two 

places at station 1115 L/H side. TAT 5462 hours. Associated with item (3) problem. 

Flight returned.due to the L/H sliding window outer pane being 

cracked, P/N 5-71762-25. TSN 4130 hours. The outer pane has 

no structural significance. 

During routine inspection a 3" crack was found in the beam-toframe attach clip, P/N 69-53023-2 in the R/H wheel well at 

station 663.75. TAT 6394 hours. 

An unscheduled landing was made due to a shattered outer pane 

of the L/H windshield. TAT 5195 hours. The outer pane has 

no structural significance. 

Routine inspection disclosed the lower wing to body attach 

angles cracked at left and right body station 540. * 

b. Wing. 

(1) A flight was interrupted due to loss of a 50" x 20" section of 

upper skin on the L/H I/B spoiler. TSN 1970 hours. 

(2) During routine inspection, an 11" x J-3/4" section of the R/H 

aileron lower skin was found delaminated from the honeycomb 

core at the leading edge. TSR 2270 hours. 

(3) The abnormal appearance of a wing flap in flight resulted in an 

unscheduled landing. Inspection after landing disclosed a broken 82 flap fairing arm, P/N 65-68269-7. 

(4) During flight-crew walkaround check, part of the top skin was 

found to be missing on the L/H I/B gound spoiler. TAT 5590 

hours. 

(5) A crew report of high frequency vibration inflight disclosed the 

cause to be loose top skin at the I/B end of #4 ground spoiler, 

P/N 65-46452-35. TAT 6221 hours. 

Par 8 Page 5 
AC 20-83 17 Jan 73 

(6) During routine inspection, three cracks were found in the skin 

of #2 engine nose cowl. The cracks were 2", 4%", and 5" in 

length. Nose cowl P/N 65-55750. Nose cowl TSN 4181 hours. 

(7) During routine inspection, on three occasions, gusset plate 

P/N 65-45402-60 was found cracked, R/H side rear spar lower wing 

joint at WBL 70.85~ 

(8) Scheduled maintenance check found the #5 slat I/B attach fitting 

broken, P/N 69-40609-2. TAT 5374 hours. 

(9) During scheduled inspections, the L/H and R/H I/B trailing edge 

flap ribs were found cracked at the O/B flanges at WBL 82.58 

and WBL 117.10. Rib P/N's 69-40664-3, 65-50757-7, and 65-40663-5. 

TAT ranged from 4021 hours to 6948 hours. 

(10) During scheduled inspection, the top skin of the R/H aileron was 

found to be delaminated at a skin joint at the front spar. 

Aileron TSN 5774 hours. 

(11) Routine service check disclosed the #l and #& leading edge slat 

torsion tubes were cracked, tube P/N 69-50753-2. TAT 6310 hours. 

(12) A 6" crack was found in #l engine nose cowl, cowl TSN 4332 hours. 

(13) Aircraft returned after leaving gate due to broken # leading 

edge flap arm P/N 69-37841-7. TAT 8379 hours. 

(14) Crew report that the L/H trailing edge flap would not retract 

led to detection of a broken cable, P/N EAC!C13AP~-772. TAT 

6130 hours. 

(15) During scheduled inspection, the #l leading edge slat O/B torsion tube was found broken at the I/B end of the detent arm, 

tube P/N 65-55514-29. TAT 2888 hours. 

(16) A crew report that #5 slat did not fully retract and # was 

cocked at an angle when flaps were extended disclosed that bracket 

P/N 69-40609-2 was broken at station 145.83. 

(17) Crew reported that aircraft rolled to right when flaps were extended. Inspection found the R/H I/B aft flat cable tensioner 

terminal fitting and cable broken. TAT 5227 hours. 

(18) During routine inspection, a crack was found in the L/H MLC 

beam O/B stabilizer link fitting, P/N 69-38823-5. TAT 3336 

hours. 

Page 6 Par 8 
a/8/74 AC 20-83 CHG 1 

* 

(19) Aircraft returned to ramp, crew reporting flaps inoperative. 

Inspection of the flap system revealed that the flap power 

unit assembly worm gear teeth were stripped. Flap power 

unit was replaced, aircraft released. 

(20) There is a fleetwide corrosion maintenance program in operation 

to watch for corrosion and stress corrosion cracking on the 

upper and lower spar caps (both forward and rear spars) from 

RSS 450 outboard to about station 499. The upper cap is 

7178-T6 material and is especially susceptible to stress 

corrosion cracking. The lower spar cap is 2024-T3 and is not 

quite so critical as far as stress corrosion is concerned. 

So far, only overseas operators have been severely affected. 

(21) During routine inspection, two cracks were detected in the web 

of BBL 41.0 lower left beam in the air conditioning bay. 

TAT 3117 hours. 

(22) During scheduled inspections, the left and right wheel well 

gUSSet at station 663.75 were found cracked on four aircraft, 

gusset P/N’s 65-45402-59 and -60. TAT ranged from 4869 hours 

to 6909 hours. * 

c. Empennage. 

(1) Routine inspection disclosed a cracked L/H horizontal 

stabilizer mounted vortex generator. The crack extended 

forward from the aft edge througti four fastener holes. 

TAT 5462 hours. 

(2) Five cracks were found in the skin at BS 1104. The cracks 

propagated from the upper fastener hold for R/H 112 vortex 

generator. TAT 2715 hours. 

(3) A crew report of a rumble during letdown resulted in finding 

a portion of the R/H horizontal stabilizer vortex generator 

missing. TSO 1269 hours. 

(4) During fleet campaigp the L/H and R/H horizontal stabilizer 

vortex generators were found cracked at fhe 90° bend radius. 

TSN 1940 hours. 

(5) On arrival, crew reported fuselage vibration of moderate 

intensity from 800 to 1200 CPS at all speeds above 240 knots. 

Inspectors found the elevator tab inboard hinge brackets, 

left and right, were vorn resulting in excessive play. Hinge 

brackets were replaced and no vibration noted on test flight. 

MRR 70-082-11 dated 17 April 1970. 

Par 8 Page 7 
AC 20-83 CHG 1 8/8/74 

d.’ Main Landing Gear Problems. 

(1) Extensive research and testing by Boeing have been performed 

to solve the MLG broken torque link and ensuing shimmy problem. 

The MU is responsive to the correct oil and air in the shock 

strut and proper tire pressure. Correct servicing results in 

improved service life. Service Bulletin 32-1064’describes 

the installation of a shimmy damper with increased stroke which 

provides a greater margin of resistance to shimmy. * 

(2) During routine replacement of the main landing gear brake 

assemblies for normal wear, three shear studs (P/N 69-59091-l) 

were found failed and two cracked at the left inboard brake 

installation. Further investigation revealed one shear stud 

failed and one cracked at the left outboard brake installation. 

All 18 replaced (MRR 70-135-047 dated 7 July 1970). Investigation revealed the studs had been installed with excessive torque. 

Affected operators have been advised of the correct torque by 

Service Bulletin 32-1068 Rl. In addition, Service Bulletin 32- 

1094 describes the installation of new improved studs and bolts.* 

Page 8 Par 8 
17 Jan 73 AC 20-83 

Appendix 1 

APPENDIX 1. CORROSION 

1. CAUSES OF CORROSION. 

a. Metal corrosion is the deterioration of the metal by chemical or 

electrochemical attack. Corrosion can take place internally as 

well as on the surface of the metal. This deterioration may change 

the smooth surface, weaken the interior or damage or loosen adjacent 

parts. 

b. Water or water vapor containing salt and combined with oxygen in the 

atmosphere produces main source of corrosion in aircraft. Thus, 

aircraft operating in a marine environment or in areas where the 

atmosphere contains industrial fumes which are corrosive are particularly susceptible to corrosive attacks. If unchecked, corrosion 

can cause eventual structural failure. 

2. TYPES OF CORROSION. There are two general classifications of corrosion, 

direct chemical attack and electrochemical attack. In both types, the 

metal is converted into an oxide, hydroxide, or sulfate. The corrosion 

process involves the anode which is oxidized and cathode (or the 

corrosive agent) which is reduced. 

a. Direct chemical attack. Corrosion by direct chemical attack results 

from direct exposure to caustic liquids or vapors. The anodic and 

cathodic change occurs at the same-point. Direct chemical attack in 

aircraft structure deposits are caused by (1) spilled battery acid 

or fumes, (2) residual flux deposits from welds, and (3) trapped 

caustic cleaning fluids. 

b. Electrochemical attack. An electrochemical attack is similar to 

the electrolytic reaction in electroplating or in a dry cell battery. 

The reaction requires a medium, like moisture, capable of conducting 

electricity. When a metal comes in contact with a corrosive agent 

(dissimilar metal) and is connected by a liquid path, the metal decays 

or corrodes. The electrochemical attack is responsible for most forma 

of corrosion on aircraft structure. 

3. FORMS OF CORRCSION. There are many forms of corrosion which depend on the 

metal involved, size, shape, atmospheric conditions and corrosion producing agents. 

a. Surface corrosion. This may be caused by either direct chemical or 

electrochemical attack. Surface corrosion appears as a general 

roughening or pitting of the surface of a metal accompanied by a 

powdery deposit of corrosion products. 

b. Dissimilar metal corrosion. Extensive pitting damage may result from 

contact between dissimilar metal parts in the presence of a conductor. 

A galvanic action like electroplating occurs at points of contact 

when insulation has broken down or was omitted. 

Par 1 Page 1 
AC 20.83 

Appendix 1 

17 Jan 73 

c. Intergranular corrosion. The grain boundaries of an alloy are 

attacked by this type of corrosion. Intergranular corrosion t&y 

exist without visible surface evidence. Severe intergranular corrosion may sometimes cause the surface of a metalto "exfoliate." 

This is a flaking of the metal at the surface caused by pressure of 

corrosion residual product buildup. 

d. Stress corrosion. This type corrosion occurs as the result of the 

combined effect of tensile stresses and corrosive environment. Stress 

corrosion is found in mostmetals. However, it is a particular characteristic of aluminum, certain stainless steels, and high strength 

steels. 

e. Fretting corrosion. This occurs when two mating surfaces are subject 

to relative motion although normally at rest with respect to each 

other. It is characterized by surface pitting and generation of 

finely divided debris. 

4. FACTORS AFFECTING CORROSION. Many factors affect the speed, cause, type, 

and seriousness of metal corrosion which include: 

a. Climate. 

b. Size and type of metal. 

c. Foreign material. 

5. CORROSION PREVENTION. Corrosion-preventive maintenance includes: 

a. 

b. 

C. 

d. 

e. 

f. 

h. 

An adequate cleaning. 

Thorough periodic lubrication. 

Detailed inspection for corrosion and failure of protective systems. 

Prompt treatment of corrosion and touchup of damaged paint areas. 

Keeping drainholes free of obstruction. 

Daily wipedown of exposed critical areas. 

Sealing of aircraft against water during foul weather and proper 

ventilation on warm, sunny days. 

Making maximum use of protective covers on parked aircraft. 

Page 2 Par 3 
818174 

AC 20-83 CHG 1 

APPENDIX 2, STATION CHARTS Appendix 2 

. 

z 

cv 

t 

m 

\ I 

. 

\ 

Figure 

. 

1. Principal Dimensi&s t-200) 

Page 1 
AC 20-83 CHG I 

Appendix 2 8/8/74 

‘T _ 

,; 

-L 

-- 

-_ 

-- ii 

I 

-- I 

-- 

I ’ 

I ‘ 

-.v. 

0 ’ _ _ - - _ . .’ 

F I’ 

_-__--. 0 __ 

0 

---z-- -’ P B -. t --g+- --’ 

t 9OL - I I 

2 SE9 - 

e---e I i 

SLOE99 - 

---e-e r, Ant 

Figure 2. Body Station Diagram 

Page 2 
l/31/75 AC 20-83 CHG 2 

Appendix 2 

,., CW. --- 

QOUTBOARD FRONT SPAR 531 *‘- 

wB~~M$!i&j~~ 1 y 

Figure 3. Wing Station Diagram 

Page 3 
AC 20-83 CHG 2 

Appendix 2 

l/31/75 

COWL SPLIT COWL SPLIT 

-LINE 

c FWD MOUNT RING 

PLAN--i’IEW 

AS SHOWN ENGINE NO. 1 

NAC 

Q WING CHORD STA 

PLANE1 139.a37 

NAC 

I 

QFRONT SPAR 

STA AT WBL 191 .OO 

63.47 

COY 

LINE 

fiWD 

MOUNT 

RING ‘I 

NiC ENG 

STA STA 

99.25 136.00 

NAC TR TR 

REAR SPAR TR 

STA 

15f.66 

4 COWL SPLIT 

LINE 

% ENGINE 

INTERSECTION OF ENGINE ( 

AND THRUST REVERSER 4 

INTERSECTION OF ENGINE q 

AND NACELLE e 

Q THRUST REVERSER 

AND TAILPIPE 

Figure 4. Engine And Nacelle Station Diagrams 

Page 4 
i-r 

0 

Y 

CA 

6 

l-t 

I POINT “E” , 

KJACK SCREW mn,At DfilNT NOTE: 

HORIZONTAL STABILIZER RIBS 

FROM STA 83.50 TO 249.10 HAVE 

9.20 INCH CENTERS 

LEADING EDGE 

STABILIZER CONNECT POINT ” 

ELEV STA 66.54 ELEV HING 
1. FIN STATIONS ARE PERPENDICULAR TO 

55% LINE 

2. LEADING EDGE STATIONS ARE PERPENDICULAR 

TO LEADING EDGE 

hi 

K 3. RUDDER STATIONS ARE PERPENDICULAR TO 

s 

75% LINE 

(D 

cn 4. VERTICAL STABLIZER RIBS FROM FIN STATION 81.10 

. TO FIN STATION 292.70 ON 9.2 INCH CENTERS 

5 5. TO CONVERT FIN STATION AT FRONT SPAR (10%) 

2 LE STA=FIN STA 1.01756521 + 3.59150051 

214.77 233.09 

Le;c RUDDER SPAR 1 

6. TANGENT A = .52293481 

TANGENT B = .78884390 

TANGENT C = .84793481 

41.52 

i, Y--RUD I 

a 

FIN STA 57-n\- ’ 

F 

2 fD 

. LE \\ ’ 

-,,NCE WEIGHT 

FIN WL 0 (BWL 300.50) ’ 

BODY 

CROWN 

ill UC 
8/8/74 

APPENDIX 3. NONDESTRUCTIVE TESTING 

AC 20-83 CHG 1 

Appendix 3 

1. MAINTENANCE TOOL. Simply stated nondestructive testing (NM') is a 

tool for performing maintenance inspections such as X-ray, ultrasonic, 

* magnetic particles, eddy current, dye Qenetrant, and visual with or 

without magnification. * 

a. Maintenance Inspection. NDT permits maintenance inspection without 

removing components from aircraft or tearing down complex assemblies. 

Defects in various aircraft systems which would escape detection 

through normal visual inspection will be identified by NDT. 

b. Training Required. Special NDT training is desirable to make sure 

that the technician is caQable of operating the equipment and 

interpreting the results. Also, many States require that an X-ray 

technician have an approved certificate for use of X-ray in 

industrial applications. This is to minimize improper use with 

attendant health hazard of X-ray equipment. 

2. METHODS AND APPROACHES FOR DAMAGE DETECTION. Information on the 

application of NDI methods for the detection of fatigue damage is 

scattered through literature on metal fatigue and literature on NJX. 

Few publications deal with this problem specifically. Some publications 

are limited to laboratory investigations, and the NM' application is 

conducted under controlled laboratory conditions. Such investigations 

contribute considerably to available knowledge and provide new or 

improved NDT methods for field use. However, there is a great difference 

between what can be done in the laboratory and what can be used in 

practical applications under field service conditions. 

3. NDI METHOD IN FIELD AND SERVICE USE. Most of the NDT methods that are 

used under field and service conditions are those capable of detecting 

fatigue cracks of various sizes. These methods are described in books 

on the subject of nondestructive testing and other publications such 

as reports, technical papers, and magazine articles. The NDT methods 

most commonly used in the field and service for fatigue crack detection 

follow. 

4. VISUAL INSPECTION. Visual inspection is the oldest, simplest, cheapest, 

and most widely used of all NDT methods. The basic principal used in 

visual inspection is to illuminate the object and examine the surface 

with the eye. 

a. The surface should be adequately cleaned before being inspected. 

Visual inspection for detection of fatigue cracks can be improved 

by aids such as mirrors, lenses, microscopes, periscopes, and 

telescopes. These devices compensate for limitations of the human 

eye. BoroscoQes permit direct visual inspection of the interior of 

hollow tubes, chambers, and other internal surfaces,, 

Par 1 Page 1 
AC 20-83 

Appendix 3 

17 Jan 73 

b. The capability of visual inspection to detect a fatigue crack depends 

on many factors such as the size and location of the crack, the illumination used,, optical aids employed, and skill of the inspector. 

It is often difficult to detect even a relatively large fatigue crack 

that is located, for example, at the corner of a groove or that coincides with a machining mark. There are, of course, also.limitations 

on the size of cracks that can be detected by visual inspection, 

depending on optical aids employed. 

6. 

LIQUID PENETRAp\pT. The liquid penetrant is one of the oldest methods of 

nondestructive testing and is capable of detecting cracks that may be 

impossible to find with the most careful visual inspection because either 

they are too small or because they are difficult to detect due to their 

location. The principle involves applying to the part surface a liquid 

penetrant having a low-surface tension and low viscosity. When used on 

a clean surface that the liquid will wet, the liquid is drawn into the 

cracks by capillary action. The presentie of the liquid in the cracks is 

revealed when, after wiping the excess liquid from the surface, a 

developer is applied that acts like a blotter and draws the liquid out. 

a. There are two types of liquid penetrants in general use. One contains 

a dye which usually gives a good color contrast against the selected 

developer; the other contains dissolved flourescent material, which 

makes it readily visible when viewed under a "black" (ultraviolet) 

light. 

b. Liquid penetrant inspection is inexpensive and readily applicable to 

field use. The surface must ‘be cleaned before inspection and also 

afterwards to remove the developer. 

MAGNETIC METHODS. Magnetic inspections are used to detect surface or near 

surface discontinuities in ferromagnetic materials, and they are well 

suited for the detection of fatigue cracks. 

a. The principle employed here is that once a magnetic field is induced 

in a material, any cracks and flaws that are present, will perturb 

or distort that magnetic field. These methods are most sensitive 

when the crack orientation and the magnetic field direction are 

perpindicular to each other. When they are parallel, the crack will 

not be detected. 

b. The magnetic particle method is the most frequently used. It consists 

of three basic steps: 

(1) Establishment of the magnetic field in the part to be inspected. 

(2) Application of magnetic particles to the surfaces of the part. 

(3) Visual examination of the surfaces for indications of fatigue 

cracks. These indications are provided by tne particles being 

Page 2 Par 4 
17 Jan 73 AC 20-83 

Appendix,3 

(3) Visual examination of the surfaces for indications of fatigue 

cracks. These indications are provided by the particles being 

attracted to the locations of the cracks (cr other defects) due 

to local variations in the magnetic field that are produced. 

c. Two classes of magnetic particles are available. The wet method 

particles use a liquid vehicle; the dry method particles are borne 

by air. These particles are usually: 

(1) Colored to give contrast with the surface being inspected, or 

(2) Coated with fluorescent material to make them readily visible 

under black light. Parts inspected by magnetic particle 

methods must be cleaned. 

RADIOGRAPHY. Radiography is a method of nondestructive testing which 

uses X-ray, gamma, beta, or neutron radiation. It is based on the ability 

of these radioactive sources to penetrate materials. The intensity of 

the penetrating radioactivity is modified by passage through materials 

and by defects in the material. These intensity changes are recorded on 

film as areas of varying density (or darkness) which permits distinguishing flaws and cracks. Obviously, maximum sensitivity occurs when the 

cracks are oriented such that its lonu ,est dimension is parallel to the 

direction of radiation. 

a. 

b. 

C. 

d. 

?f5j%xmzty. 

radiograB has two main advantages; (1) versatility, and 

TheX-ray energy source can be easily adjusted for 

variations in thickness. It is also adaptable to fluoroscopy and 

television systems. 

The advantages of gamma radiography are; (1) portability, and (2) a 

relatively.low cost. Portability comes from the fact that the source 

is small. This permits its effective use in the field particularly 

in remote areas. One of the difficulties with the gamma radioactive 

source is that the source cannot be varied or turned off so that 

safety precautions must be observed at all times. (X-ray constitutes 

a health hazard only during operation of the X-ray equipment). 

Conventional radiography is firmly estabiished and reasonably easy to 

understand. One of theoriginal drawbacks was the long time involved 

in the developing and processing of film. This has been overcome by 

modern automatic film processing techniques. 

Interpretation of the processed film is the most important phase of 

radiography. Adequate tools such as a film illuminator lens and good 

working conditions should be available to assist the interpreter in 

detecting cracks in the part displayed on the film. 

Par 6 Page 3 
AC 20-83 17 Jan 73 

Appendix '3 

8. ULTRASONICS. Ultrasonic methods have received wide acceptance in the 

aviation industry and are particularly useful for determining the integrity of a member of a structure. Basically, sound energy above the audible range is transmitted into a part, and a signal is received and 

analyzed. The ultrasonic wave is transmitted and received through transducers, which are placed upon the part to be inspected. The transducer 

must be properly coupled to the part and is the most critical aspect of 

the inspection. Good coupling can be achieved by using liquids at the 

transducer part interface. 

a. The ultrasonic wave ( or beam) may be evaluated in terms of either 

through transmission or reflection. The receiving transducer may 

be a separate unit (through transmission) or it may be the same 

transducer that sent the signal (reflection and resonance). For crack 

detection, the reflection technique is most commonly used. It permits 

the determination of the location of the crack wherever it might be 

within the part and only one transducer on one side of the part is 

needed. When ultrasonic wave (pulse) is sent into the part, a discontinuity(e.g. a crack) in its path on which it impinges will both 

absorb and reflect energy. A defect can be recognized by the relative 

time for return of the reflected energy to the transducer. 

b. The ultraSonic methods now available are rapid, economical, sensitive, 

and can have good accuracy for determining crack extent and position. 

Equipment is light and portable so that on-site inspections are possible. There.are conditions which can limit the usefulness of ultrasonic inspections. These include unfavorable part geometry (such as 

complexity, contour, and size) orientation of the cracks, and misleading responses which may occasionally be obtained. Also, ultrasonic inspection, as presently practiced, depends upon experience, 

skill, and judgment of the inspector. He must interpret the crack 

size and location by the indirect evidence presented by the electronic 

equipment (oscilloscope). He must be able to distinguish between 

significant signals and spurious ones. 

EDDY CURm. The eddy current method is a comparatively recent nondes- -. 

tructive testing technique. It is being frequently used for nonmagnetic 

materials. (When used for magnetic materials, it requires more complex 

systems). The principle involved is simple. A coiltha6 is carrying a 

high-frequency alternating current is br.ought near an electrical conductor 

and eddy currents are generated in the conductor. The eddy (or induced) 

current creates a magnetic field. Flaws, cracks, etc., cause resistance 

changes within the part. This affects the induced currents, and thus, the 

magnetic field produced by them. Detection and measurement of the magnetic field form the basis of the nondestructive testing. 

a. Two types of coils are in common use. One is a circumferential coil 

through which a part passes. The second is called a probe coil which 

is placed on the surface of the part to be inspected. Each type coil 

can be made in a number of designs, depending on the application. 

Page 4 
17 Jan 73 AC 20-83 

Appendix 3 

b. Eddy current instruments have various degrees of versatility. Many 

are portable. Once proven for a specific application, the inspection 

process is very rapid. Eddy current methods have been used very 

successfully for fatigue crack detection. However, eddy current 

methods are sensitive to many variables that influence the results 

obtained. Also; signals obtained are sometimes of a comparative 

nature, and reference standards are needed for interpretation. 

Par 9 Fige 5 


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