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FAA Form 1320-S ~s-~osmu~~~~m~~~ousmrno~ 
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AC NO: 20-M 

1 DATE: 22 Jan 73 

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+ * z ADVISORY “z 3 d 6 %rrs 0% /+ CIRCULAR 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

SUBJECT: MAINTENANCE INSPECTION NOTES FOR BOEING B-727 SERIES AIRCRAFT 

1. 

2. 

1 

3. 

PURPOSE. This advisory circular provides inspection notes which can 

be used for the maintenance support program for certain structural 

parts of the B-727 series aircraft. 

:cDESCRIPTION. Maintenance inspection matters on the wing and fuselage 

are reviewed with a view toward supplementing information currently 

available. 

REFERENCES. 

a. Advisory Circular 20-61, Nondestructive Testing Techniques for 

Aircraft. 

b. Advisory Circular 65-9, Airframe and Powerplant Mechanics General 

Handbook. 

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

Practices - Aircraft Inspection and Repair. 

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

June 1, 1968, Current Problems and Prevention of Fatigue. 

e. Advisory Circular 20-9, Personal Aircraft Inspection Handbook. 

1 Initiated by:MS-302/fiC-230 
AC 20-84 22 Jan 73 

4. HOW TO GET THIS PUBLICATION. 

a. Order additional copies of this publication from: 

Department of Transportation 

Distribution Unit, TAD-484.3 

Washington, D. C. 20590 

b. Identify the publication as Advisory Circular No. 20-84, 

Maintenance Inspection Notes For Boeing B-727 Series Aircraft. 

(y%5?q* 

C. R. MRLUGIN, JR. 

Acting Director, Flight Standards Service 

Page ii Par 4 
. 

l/31/75 

TABLE OF CONTENTS 

AC 20-84 CHG 2 

1. Introduction. 

2. Description. 

3. Background. 

4. General Discussion of Structure Surveillance. 

5. Type of Construction. 

6. Aircraft Station Diagrams. 

7. Abbreviations Used In This Document. 

8. Maintenance Reports. 

APPENDIX 1. CORROSION (2 pages) 1 

APPENDIX 2. STATION CHARTS (6 pages) 

Figure 1. Principal Dimensions 1 

Figure 2. Forward Body Station Diagram 2 

Figure 3. Aft Body Station Diagram 5 

Figure 4. Wing .Station Diagram 4 

Page No. 

Figure 5. Stabilizer and Elevator Station Diagram 5 

Figure 6. Side Engines Station Diagram 6 

APPENDIX 3. NONDESTRUCTIVE TESTING (5 pages) 1 

Page iii 

AC 20-84 CHG 1 

1. 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-727 airplanes. The material is 

based, in part, upon information 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. Appendices 1, 2, 3 and 4 

provide a technical digest of material on corrosion, fatigue failures, 

nondestructive testing and station charts, respectively. 

2. DESCRIPTION. This circular 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 reported 

since 1966 by air carrier operators. 

3. BACKGROUND. 

9: a. 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 9: 

familiar with the scope of required maintenance and the means which 

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

b. Maintenance "Know How." Since maintenance 'know how" 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. 

4. 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 bulletin index is available from the 

manufacturer which is updated periodically. 

1 Par 1 Page 1 
AC 20-84 CHG 1 818174 

* 

b. Other Documentation. Further, the manufacturer has published the ( 

following documents to aid in maintaining the aircraft in serviceable 

condition: 

Maintenance Planning Document (D6-8766) 

Maintenance Manual, (D6-Series, different for each operator) 

Overhaul Manual, (~6-4420) 

Structural Repair Manual (~6-4062) 

NM' Document (D6-7170) * 

These 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-727 structure, 

every operator should 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 structural 

history of his aircraft, 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 followup 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 usage of the aircraft (e.g., 

shorter flights, intermittent use, etc.). 

d. Airworthiness Directives. It is emphasized that the material in this 

circular does not supersede any of the requirements of airworthiness 

directives issued under Part 39 of the Federal Aviation Regulations. 

e. Certification. Type certificate No. A3WE was issued to The Boeing 

Company, Renton, Washington, on 24 December 1963. The basis for 

certification was CAR 4b (dated December 1953). The type certificate 

data sheet No. A3WR prescribes conditions and limitations for which 

the type certificate was issued. The latest revision of the type 

certificate data sheet, at the time this document was published, was 

* revision No. 10 (dated 1 September 1972). 3; 

Page 2 Par 4 
8/8/74 AC 20-84 CHG 1 

5. TYPE OF CONSTRUCTION. The aircraft structure is comprised of three major 

sections: the fuselage, the wing, and the empennage. The structure is 

designed to provide maximum strength with minimum weight by providing 

alternate load paths so that a failure of one segment will not result in 

failure of the complete structural component. The materials most 

commonly 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 by 

circumferential frames and longitudinal stringers. It is composed 

of four sections of which the forward three sections contain passenger, crew, and cargo compartments and extend from body station 178 

to body station 1183. The fourth section (aft) provides support for 

the engines and empennage. Except for the nose gear wheel well and 

the cutout which accommodates the center wing box and main landing 

gear wells, the entire shell between body stations 178 and 1183 is 

pressurized. The frames at body stations 740 and 870 incorporate 

the points at tihich the fuselage is attached to the wing front and 

rear spars. 

b. The wing structure consists of a left and right wing box and a center 

wing box. The left and right wing boxes are cantilevered from the 

center wing box and the thickness and chord of each wing tapers 

towards the tip and sweeps back from the center wing box. The surfaces consist of upper and lower skin panels and front and rear 

spars. The skin is reinforced by spanwise stringers and the spars 

are reinforced by vertical stiffeners. 

(1) The left and right win, 0‘ boxes are reinforced by chordwise ribs 

with the greater portion of their enclosed volume sealed to 

serve as fuel tanks. They extend from the wing root ribs at 

WBL 70.5 to the tank end rib at station 760.5. The center wing 

box is reinforced by a pair of spanwise beams and contains a 

series of bladder-type fuel cells. 

(2) The leadin, 0‘ edge of each wing supports three flaps and four 

slats. The trailing edge of each wing supports an inboard and 

outboard flap, an inboard and outboard aileron, and seven 

spoilers. 

c. The empennage consists of a vertical fin, adjustable horizontal 

stabilizer, two elevators and two rudders. The vertical fin structure is a three-spar box with longitudinal stringers to reinforce 

the skin. The box is reinforced by chordwise ribs. 

(1) The horizontal stabilizer is installed across the top of the 

vertical fin and consists of a complete dual box structure from 

left to right tip. 

1 Par 5 Page 3 
AC 20-84 CHG 1 818174 

J: 

(2) The rudders and elevators are dual spar structures with ribs and i 

boned skin panels. Each rudder has a full length antibalance 

tab%onstructed Of aluminum or fiberglass honeycomb. Each J; 

elevator has a' control tab and five balance panels are hinged 

to the nose of each elevator. 

6. AIRCRAFT STATION DIAGRAMS. The wing, fuselage, and empennage station 

diagrams included in this document were developed for the B-727 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. 

7. ABBREVIATIONS USED IN THIS DOCUMENT. 

NLG 

O/B 

P/N 

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 

1. MAINTENANCE 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 

and covers only a portion of time in the history of the B-727 series 

aircraft. 

a. Fuselage. 

(1) A 4" crack was found in fuselage frame at station 783.95 on the 

R/H side below the cabin floor. TAT 14,255 hours. 

(2) The NLG drag strut support shelf was found to have cracks in the 

L & R tangs. TAT 14,255 hours. 

(3) Tine aft pressure bulkhead left vertical beam was found cracked 

at the upper lightening hole. TAT 14,255 hours. 

c 

(&) During main base check, the tail skid door link lower attach 

fitting was found to be cracked, fitting P/N 65-59727-6. 

TAT 14,882 hours. 

Page 4 Par 5 
8/8/74 

(5) 

1 9: 

(6) 

(7) 

(8) 

(9) 

(10) 

(W 

(12) 

(13) 

(14) 

(15) 

(16) 

07) 

08) 

A 1" crack was found in the aft 

brace support beam. TAT 14,056 

lower corner of the NLG drag 

hours. 

Corrosion was found at the skin 

TAT 14,056 hours. 

lap at station 665 stringer 28~. 

AC 20-84 CHG 1 

A 10" crack was found in the upper vertical "1" beam on the L/H 

side of the aft entry door. TAT 8,998 hours. 

An unscheduled landing was made due to 6 L/H eyebrow window 

inner panel cracking during cruise, window P/N 5-7176-13. 

TAT 13,480 hours. 

During eddy current inspections, cracks were detected on the 

L & R sides of the bulkhead flanges at station 740. The cracks 

were between stringers 18 & 19 and 19 & 20. TAT on the several 

aircraft concerned ranged from 8,641 hours to 15,444 hours. 

During main base inspections, stringers 20 & 21 L & R and 22R 

at station 294.5 and stringer 2OR at station 360 were found 

cracked. TAT 14,786 hours to 15,610 hours. 

Routine inspection disclosed a 2" crack in station 930 L/H 

fuselage frame and stringer 18A, also cracked skin at station 

940. TAT 17,270 hours. 

The upper left fuselage skin lap was found bulged at station 900 

and the skin split at stations 990 & 1030 due to corrosion, ;t 

TAT 17,482 hours. 

Routine scheduled inspection disclosed cracks in the cabin floor 

beam at station 910. TAT 15,256 hours. 

During scheduled inspection, the forward hinge mount bracket on 

the forward cargo door was found broken and pulled out of the 

structure. TAT 6,150 hours. 

Extreme corrosion was found on main cabin seat track assemblies 

#3, 5, 6, and 7 between stations 4X, and 720. TAT 8,026 hours. 

The fuselage frames at both hinge cutouts for the aft cargo door 

were found cracked. TAT 13,OgO hours. 

3uring modification, the following damaged fuselage frames were 

replaced: stations 480, 500, 520, 540, 560, 580, 600, 600F, 

600G, 600H, and 6005. Also replaced damaged skin panel station 

420 to F. S. Bulkhead. TAT 30,380 hours, 

Fuselage frame station 880 found cracked at stringers #15 L & R. 

TAT 14,786 hours. 

1 

Par 8 Page 5 
AC 20-84 CHG 1 8/8/74 

(19) The bulkhead at station u-83 was found to have a l$' crack on 

the L/H side and a 2" and 4" crack on the R/H side. TAT 15,852 

hours. 

(20) During main base check, a crack was found in fuselage frame at 

station 1203 upper R/H torque box. TAT 14,639 hours. 

(21) A 20" tear in the ram air inlet between stations 660 and 680 was 

found and determined to be a result of excessive corrosion 

.between the skin and strap. TAT 15,450 hours. 

(22) Extensive corrosion was found below the floor of the forward 

cargo compartment from station 480 to 7~). 

(23) Ultrasonic inspection disclosed severe corrosion along the sides 

of the ram air splitter vein and around the ram air door actuator rod holes and a 16” crack underneath the splitter vein from 

station 642 to 658. TAT 15,678 hours. 

(24) The NIX collar forward segment was found cracked and the reporting operator attributed the crack to corrosion between the 

faying surfaces of the collar and bushing. TSR 6,234 hours. 

* (25) A 12" crack was found in #l pylon fitting, P/N 65-18722-3. 

TAT 4,259 hours. 

b. Wing; 

0) 

* (2) 

(3) 

(4) 

(5) 

* l 

During a main base check, the upper #lO left wing stringer at 

station 656 was found cracked. TAT 14,536 hours. 

The R/H MLG trunnion support beam, P/N 65-16230-4 (7079-T6 MTL) * 

has been found cracked. TAT 14877 hours. 

During scheduled inspection, the L/H flap O/B carriage support 

was found broken, P/N 69-39601-l. TAT 14,876 hours. 

Routine inspections disclosed six broken lobes on the R/H I/B 

aileron balance panel forward hinge at a TAT of 14,056 hours and 

ten broken lobes at a TAT of 15,400 hours. Nineteen forward 

hinge lobes were found broken on the L/H O/B aileron O/B panel 

at a TAT of 16,748 hours. 

A main base inspection disclosed the R/H wing rib at station 

686.5 cracked at the top at stringers #12 and 13 and at the 

bottom at stringer #6. The rib at station 629 was also cracked 

at the top at stringer #lb. The top stringer ##c station 601 

and top stringer #9c at station 686.5 were also cracked. TAT 

15,270 hours. 

Page 6 Par 8 
818174 

AC 20-84 CHG 1 

(6) The O/B aileron lockout support assembly was found broken at 

station 661.25 on the R/H wing. TAT 6,577 hours. 

(7) During inspection, corrosion was found at the R/H wing station 

422 front spar lower cap. TSO 10,659 hours. 

(8) A cracked R/H MU trunnion forward support wing attach fitting 

was found during scheduled maintenance check, fitting P/N 

69-19296-2. TAT 14,794 hours. 

(9) Routine service check disclosed the L/H wing O/B leading edge 

mid-flap interspar rib cracked in two places at WBL 319. 

TAT 11,647 hours. 

(10) Routine inspection disclosed two cracks in the R/H aileron O/B 

control rod, P/N 65-23544-l. TAT 6,804 hours. Similar cracks 

have also been found in the L/H aileron control rod. 

(11) A 2 3/4” crack was found in support P/N 65-21640-1 at station 

388, L/H I/B flap jackscrew forward attachment. TAT 6,849 hours. 

(12) Scheduled inspection disclosed heavy corrosion on R/H wing rear 

spar lower chord at station 601.5. TAT 20,030 hours. 

(13) Ultrasonic inspection disclosed a cracked MLG beam lower left 

support link pin, P/N 69-14908 at body station 940. The crack 

was three-fourths way around the circumference of the link pin. 

TAT 16,604 hours. 

(14) On scheduled inspection, found #2, 1113 and #14 outboard spoiler 

actuator support fitting cracked. TAT 13,151 hours. 

(15) Routine inspection revealed an 8-inch crack in radius of 

upper spar cap at station 720 right wing. TAT 14,452 hours. 

(16) During inspection found L/H inboard flap drive gear box 

cracked at three mounting lugs. TAT 15,517 hours. 

(17) Found right inboard flap inboard track cracked at mount bolt 

hole for landing gear support beam. 

(18) During inspection found corrosion at R/H W. S. 422 front 

spar lower cap, vertical tang. TSO 10,659 hours. 

(19) No. 7 spoiler actuator rod end failed and rod penetrated 

flight spoiler. TAT 20,069 hours. 

1 

Par 8 Page 7 
AC 20-84 CHG 1 8/8/74 

c. Empennage. 

(1) Forward hinge lobes on the R/H elevator 81 balance panel were 

broken. TAT 14,255 hours. 

* (2) During overhaul, a crack was found in the center section rear 

spar forging of the R/H horizontal stabilizer. TAT 14,113 dr 

hours. 

(3) The L/H inner flange of the vertical fin stiffener #9 at 

station 133.8 was found cracked. TAT 10,658 hours. 

(4) The #3 and #5 L/H elevator balance panels forward hinge lobes 

were found broken. Number 3 panel had 29 broken and #5 panel 

had six broken. TAT 14,056 hours. 

(5) A 2%" crack was found in the L/H elevator hinge support at the 

O/B end of #3 balance panel. TAT 14,056 hours. 

(6) A 4" crack in the lower skin panel and a crack in the stiffener 

were found at station 114 R/H elevator. TAT 14,056 hours. 

(7) During routine inspection, the trailing edge rib at station 

136.5 R/H horizontal stabilizer was found cracked. TAT 9,244 

hours. Cracked trailing edge ribs were also found in the 

L and R horizontal stabilizer at station 114.97 at a TAT of l 

5,274 hours. 

(8) A 10" crack was detected in the lower L/H elevator skin at the 

skin at the O/B end of the elevator tab. TAT 5,881 hours. A 

2" crack was also found in the R/H elevator lower skin at O/B 

end of the tab. TAT 5,563 hours. 

(9) A crack was found in the front spar web of the vertical fin aft 

of station 1183. TAT 15,256 hours. 

(10) Scheduled inspection disclosed a cracked and broken stringer 

in the vertical fin R/H side at approximately station 160. 

TAT 2,159 hours. 

(11) Routine inspection disclosed a 1" crack in the R/H horizontal 

stabilizer rear lower spar vertical leg at station 200. TAT 

15,800 hours. 

(12) A 1" crack was found in the vertical leg of the R/H horizontal 

stabilizer lower spar cap at station 135. TAT 15,828 hours. 

(13) During scheduled inspection, cracks were found in two elevator 

hinge support brackets, P/N 65-16931-1 at stations 99.79 and 

136.5. 

Page 8 Par 8 
818174 AC 20-84 CHG 1 

(14) 

(15) 

(16) 

(17) 

(18) 

Routine inspection disclosed a 2" crack in elevator hat section 

skin at station 173.21 and a 3" segment missing at station 

99.79. TAT 7,628 hours. 

The upper rudder hinge fitting was found cracked at the lower 

lug, fitting P/N 65-23337-l. TAT 11,647 hours. 

The upper rudder tab actuating fitting was found cracked, 

P/N 65-6244-2. TAT 16,145 hours, On another aircraft, the 

lower rudder tab rod attach bracket was cracked, P/N 65-17483-7. 

TAT 6,361. 

Three cracks in the O/B panel of the R/H horizontal stabilizer 

spar and one crack in the I/B panel of the L/H horizontal 

stabilizer spar were found during routine inspection. TAT 

6,706 hours. 

The L/H horizontal stabilizer spar flange was found cracked 

at station 208. TAT 8,816 hours. 

:: d. Landing Gear. 

(1) Inspection found #3 MLG wheel inner half broken. The inner 

bearing and hub were separated from the wheel half. TSN 3,925 

hours. 

(2) When lowering landing gear during approach, crew heard loud 

bang. Inspection after landing revealed the R/H landing gear 

retract cylinder and walking beam protruding through top of 

wing. Operator reported probable cause as failure of beam, 

P/N 65-17658-11. TAT 17,249 hours. Service Bulletin 32-167 

describes installation of new steel beam. 

(3) During scheduled inspection, the L/H MLG walking beam was found 

to be cracked at the O/B aft attach lug, P/N 65-17658-11. 

TAT 9,891 hours. Service Bulletin 32-167 describes installation 

of new steel beam. 9; 

Par 8 Page 9 

22 Jan 73 

APPENDIX 1. CORROSION 

1. CAUSES OF CORROSION. 

AC 20-84 

Appendix 1 

a. 

b. 

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. 

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 

forms of corrosion on aircraft structure. 

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

the metal involved, size, shape, atmospheric conditions and corrosion 

producing agents. 

a. 

b. 

Par 1 

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. 

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. 

Page 1 
AC 20-84 

Appendix 1 

22 Jan 73 

4. 

5* 

C. 

d. 

e. 

Intergranular corrosion. The grain boundaries of an alloy are 

attacked by this type of corrosion. Intergranular corrosion mav 

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

is a flaking of the metal at the surface caused by pressure of corrosion residual product buildup. 

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

combined effect of tensile stresses and corrosive environment. Stress 

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

steels. 

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. 

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. 

CORROSION PREVENT.ION. Corrosion-preventive maintenance includes: 

a. 

b. 

C. 

d. 

e. 

f. 

g. 

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 20-84~~~ 1 

Appendix 2 

ENDIX 2. STATION CHARTS 

-. 

, 

. \ 

I 

I i 

7 

b b 

-i 

Figure 1. Principal Dimensions Page 1 
AC 20-84 aG 1 

Appendix 2 

8/8/74 

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930- 

OlOm190 - MOa70 - 

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-- - -O- -- 

---I- 

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Page 2 

Figure 2, Forward Body Station Diagram 
. 

IN STA FIN STA 

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88 8 FIN s 

QrcR RUDDER 

---. A I 
CENTER SECTION ACCESS 

DETAIL A 

‘SPOILEC) 

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AC 20-84 CHG 2 

Appendix 2 

40.50 -- 

174.60 I l 

- 55.36 - 

Figure 5. Stabilizer and Elevator Station Diagram 

Page 5 
3Nl3N3 

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B/8/74 AC 20-84 CHG 1 

Appendix 3 

APPENDIX 3.NONDESTRUCTIVE TESTING 

1. MAINTENANCE TOOL. Simply stated, nondestructive testing (NDT) is a tool 

for performing maintenance inspection. This includes utilization of 

such maintenance tools as X-ray, ultrasonic, magnetic particles, eddy 

+: current, dye penetrant, and visual with or without magnification. 9: 

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 capable 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 ilidustrial 

applications. This is to minimize improper use of X-ray equipment 

with attendant health hazard. 

2. METHODS AND APPROACHES FOR DAMAGE DETECTION. Information on the application of NDT methods for the detection of fatigue damage is scattered 

through literature on metal fatigue and literature on NISI. Few publications deal with this problem specifically. Some publications are limited 

to laboratory investigations, and the NDT 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. NDT 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. Horoscopes permit direct visual inspection of the interior of 

hollow tubes, chambers, and other internal surfaces. 

Par 1 Page 1 
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Appendix 3 

22 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 cradks that can be detected by visual inspection, 

depending on optical aids employed. 

6. 

LIQUIDPENEIRANT. 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 presence 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 fluorescent 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 perpendicular 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 the particles being 

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22 Jan 73 AC 20-84 

Appendix 3 

attracted to the locations of the cracks (or 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. 

7. 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 crack is 

oriented such that its longest dimension is parallel to the direction of 

radiation. 

a. X-ray radiography has two main advantages; (1) versatility, and 

(2) sensitivity. The X-ray energy source can be easily adjusted for 

variations in thickness. It is also adaptable to fluoroscopy and 

television systems. 

b ,I 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 radio- 1 

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

that safety precautions must be observed at all times. (X-r-y constitutes a health hazard only during operation of the X-ray equipment.) 

c. Conventional radiography is firmly established and reasonably easy to 

understand. One of the original drawbacks was the long time involved 

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

modern automatic film processing techniques. 

d. 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. 

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. 

Par 6 Page 3 
AC 20-84 

Appendix 3 

22 Jan 73 

9. 

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 

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 CURRENT. The eddy current method is a comparatively recent nondestructive 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 coil that is carrying a 

high-frequency alternating current is brought 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. 

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 

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22 Jan 73 AC 20-84 

Appendix 3 

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. 

1 Par 9 Page 5 



AC NO: 20-84 

DATE: 22 Jan 73 

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