# AC 20-76: Maintenance Inspection Notes for Boeing B-707/720 Series Aircraft - Type: Advisory circular (guidance, not mandatory unless a rule or AD requires it) - Status: Historical - Approved: October 21, 1971 - Last reviewed: September 18, 2024 - This change: Canceled by signed memo, dated 06/01/2017. - Family: AC 20-76 - 14 CFR parts: 1 - Pages: 36 - PDF: https://openairdocs.org/files/sha256/3a/a3/3aa3f3368c97353e48d720634512a52a06e3d317c86c2e69fa0b13fad584b359 - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/826D6C96F75FB1B7862569AE006D383E.0001 - DRS record last changed: November 22, 2025 - Page: https://openairdocs.org/ac/20-76 ## Versions - [AC 20-76](https://openairdocs.org/ac/20-76) (Historical), approved Oct 21, 1971, 36 pages. Canceled by signed memo, dated 06/01/2017. ## Text Each page's text from the PDF; open the PDF at a page with #page=N. ### Page 1 AC NO: AC 20- 76 DATE: 21 Oct 71 **ADVISORY** **CIRCULAR MAINTENANCE INSPECTION NOTES FOR BOEING B-707/720 SE.RIES AIRCRAFT** **DEPARTMENT OF TRANSPORTATION** **FEDERAL AVIATION ADMINISTRATION** ### Page 2 AC NO: 20-76 DATE: 21 Oct 71 **ADVISORY** **CIRCULAR** **DEPARTMENT OF TRANSPORTATION** FEDERAL AVIATION ADMINISTRATION SUBJECT: MAINTENANCE INSPECTION NOTES FOR BOEING B 707 /720 SERIES AIRCRAFT 1. PURPOSE. This handbook provides maintenance inspection notes which can be used for the maintenance support program for certain structural parts of the B-707/720 series aircraft. 2. DESCRIPTION, Maintenance inspection matters on the wing and fuselage are reviewed with a view toward supplementing information currently available. **3. HOW TO GET THIS PUBLICATION.** a. Order additional copies of this publication from: Department of Transportation Federal Aviation Administration Distribution Unit, TAD-484.3 Washington, D.C. 20591 b. Identify this publication as: Advisory Ci:rcular 20- 76 Maintenance Inspection Notes for Boeing B-707/720 Series Aircraft I= Flight Standards Service Initiated by: FS-320 ### Page 3 21 Oct 71 **TABLE OF CONTENTS** 1. Introduction. 2. Description. 3. Background. 4. General Discussion of Structure Surveillance, 5. Cabin Windows. 6. Versions of the B-707/720 Series Aircraft. 7. Aircraft Station Diagrams. 8. Maintenance Information. LIST OF ILLUSTRATIONS Figure l, Control Cabin Window Locations. Figure 2. Passenger Cabin Window Locations. APPENDICES APPENDIX l, Fundamentals of Airframe Inspection. APPENDIX 2. Corrosion. APPENDIX 3. Fatigue Failures. APPENDIX 4. Nondestructive Testing. AC 20-76 Page No. l l 1 l 3 7 9 9 Page No. 4 6 (6 pages) (2 pages) (2 pages) ( 5 pages) Page i ### Page 4 21 Oct 71 AC 20-76 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, mak­ ing improvements in existing programs, and conducting inspection and repairs on certain structural parts of Boeing B-707/720 airplanes. The material is based, in part, upon information made available through dis­ cussions 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 so that it is not lost. 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, are diagrammatic sketches of the control cabin windows and passenger cab­ in windows. In addition, there is a listing of selected maintenance difficulties which have been reported since 1964. **3. BACKGROUND.** a. Older Aircraft. The administrator has realized that several dif­ ferent types of transport aircraft are being phased out of service by some airlines because of the availability of newer equipment. Such older aircraft are being purchased by other operators who may not be 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. 11 Since maintenance "know how" is not trans­ ferred 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, identifi­ cation of known areas where structural problems have been experi­ enced 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 knowl­ edge 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 in­ spection, repair, and modification of aircraft. Most of these bul­ letins 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 is available from the Page 1 ### Page 5 AC 20-76 21 Oct 71 manufacturer and is updated periodically. (Note:· Al though service bulletins may be referred to in Airworthiness Directives (ADs) issued by the FAA, the bulletins do not supersede any of the requirements of the ADs.) b. Other Documentation. ~urther, the manufacturer ha• published the fol­ lowing documents to aid in maintaining the aircraft in serviceable condition: Maintenance Planning Data Document, Maintenance Manual, Overhaul Manual, Structural Repair Manual, Non-Destructive Test Document. This documentation is 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 fleet­ wide significance and may or may not be followed by service bulletins. c. Maintenance Action. For adequate maintenance of the 707 and 720 structure, every operator should have in his possession and be conver­ sant with the above documentation, including service bulletins appli­ cable to his particular aircraft. He should also obtain complete service records from previous ­V % l&I I,) ar: w ~ w >­ Ill ~ .., < "... II. < FIGURE 2 . PASSENGER CABIN WINDOW LOCATIO?S ) Page 6 ### Page 10 21 Oct 71 AC 20-76 c. Criteria for Window Inspections. (1) Optical and visibility criterion should be established by the operator. (2) A crack is a fissure perpendicular to the surface which extends completely through the pane. Airplane should not be pressur­ ized with a cracked pane. (3) Crazing is a series of small fissures perpendicular to the sur­ face but not extending through pane. Crazing can be caused by improper pane installation or exposure to certain liquids. (a) Surface crazing is permissible within certain limits if not a routed edge. (b) Routed radius crazing of a pane is more serious than in the overall surface. Since routed crazing requires pane removal to monitor the extent of crazing, the pane may as well be replaced. (4) Delamination is a separation of adjac·ent layers of laminate parallel to pane surface. Manufacturer's maintenance manuals provide recommended delamination limits and should be consulted. (5) Scratches in routed radius will be impossible to see, unless window pane is removed. (a) Scratches are JDOSt likely found on the exterior surface of the pane which is exposed to weather and washing and accompanying foreign bodies which inevitably get on that surface. (b) Scratches are easily detected by visual examination and are critical if they exceed certain limits. Manufacturer's maintenance manuals provide recommended limits on scratch, depth and length and should be consulted. (Figures 1 & 2) 6. VERSIONS OF THE B-707/720 SERIES AIRCRAFT. The basic versions of the B-707/720 series aircraft follow. For a detailed description of each version which includes weight, loadings, performance, intermixing of engines, etc. the applicable FAA aircraft type certificate data sheet as indicated should be consulted. Further, the data sheet describes the type design and sets forth the operating limitations,_and other limitations and information found necessary for type certification of a particular model aircraft as prescribed by the FARs. ### Page 11 21 Oct 71AC 20-76 ·· a.. B-707-100 long body. The FAA type certifit:ate appro·val was issued on September 18, 1958. This aircraft was the first production version and was intended for continental flights but also had capability of over-water.operation. Thia aircraft is powered by four Pratt and Whitney JT3C-6 turbojet engines. (See Type Certificate Data Sheet· No. 4A21.) b. B-707-lOOB long body. The FAA type certificate approval was issued on March 1, 1961. The aircraft was a further development of the basic B- 707· 100 with four Pratt & Whitney JT3D-l turbofan engine·s . with similar changes incorporated in the B-720. Included were a new inboard wing leading edge and additional segments (four) of the leading edge flaps. (See Type Certificate Data She~t No. 4A21.) c. Model B-707-lOOB short body. FAA type certificate approval issued July 25, 1961. Same as B-707-1008 long body, except for shorter fuselage, wing changes, and other associated changes and limitations. Aircraft is powered by four Pratt & Whitney JT3D-l, or JT3D-IMC6 or JT3D-IMC7 turbofan engines. (See Type Certificate Data Sheet No. 4A21.) d. B-707-200. FAA type certificate approval was issued on November 5, 1959. This aircraft is identical to the B-707-100 long body but is powered by four Pratt & Whitney JT4A-3 engines. (See ) Type Certificate Data Sheet No. 4A21.) e. B-707-300 Intercontinental. FAA type ceritificate approval was issued on July 15, 1959. This aircraft is an over-water long range version featuring an increase in wing span and longer fuselage. This air­ craft is powr-red by four Pratt & Whitney JT4A-5 engines,JT4A-3 engines JT4A-9 engines or JT-4A-ll engines. (See Type Certificate Data Sheet No. 4A26.) f. B-707-300B·!ntercontinental. FAA type certificate approval issued on May 31, 1962. This aircraft was a further de··.,iopment of the B-707-300 and powered by four Pratt & Whitney JT3D-$ or JT3D-3B turbofan engines. The design changes included new leading and trailing edge flaps plus low-drag wing tips. (See Type Certificate Data Sheet No. 4A26.) g. B-707-300C Intercontinental. FAA type certificate approval was issued on April 30~ 1963. This aircraft features a cargo or mixed cargo passenger version of the B-707-300B with a forward cargo door (94 inches z-134 inches) and a cargo loading system using pallets and containers. The cargo apace includes the full upper deck and the two lower deck holds. This aircraft is powered by four Pratt & Whitney JT3D-7 en~ines. (See Type Certificate Data Sheet 4A26,) ) ### Page 12 21 Oct 71 AC 20-76 t h. B-707-400 Intercontinental. FAA type certificate approval was issued February 12, 1960. This aircraft is identical to the B-707-300 but powered by Rolls Royce Conway MK508 turbofan engines. (See Type Certificate Data Sheet No. 4A26.) i. B-720 Series Aircraft. The B-720 is essentially identical to the B-707-100 in external outline, main dimension, aerodynamic design and control systems although the B-720 is completely different from a weight and structural strength design standpoint. However, most B-720 aircraft parts and repairs are interchangeable with the B-707/720 aircraft. The B-720 aircraft features a wing leading edge change resulting in an aerodynamic variance which improved the air­ craft performance. (These aforementioned changes were incorporated in the B-707-lOOB version.) The B-720-720B series aircraft have several different models due to different installations of equip­ ment, interiors, airplane flight manuals, etc. (1) B-720. FAA type certificate approval was issued on June 30,1960. The basic aircraft is powered by four Pratt & Whitney JT3C-7 turbojet engines or JT3C-12 engines. (See Type Certificate Data Sheet No. 4A28.) (2) B-720B. FAA type certificate approval was issued on March 3, 1961. This aircraft is powered by four Pratt & Whitney JT3D-l or .IT-3D-3 turbofan engines. (See Type Certificate Data Sheet No. 4A28.) 7. AIRCRAFT STATION DIAGRAMS. The Manufacturers Structural Repair Manual should be consulted for aircraft station diagrams. 8. MAINTENANCE INFORMATION. The requirements for structural surveillance of older airplanes vary greatly. a, Factors that should be considered include: (1) Basic structural differences of the various models. (2) Production changes made to correct structural problems. (3) Production changes made for weight reduction. (4) Preventive modifications made per service bulletins. (5) Repairs made per service bulletins. (6) Repairs made per Structural Repair Manual. (7) Incorporation of special repairs, correcting structural problems. ### Page 13 AC 20-76 ,, 21 Oct 71 (8) Minor repairs made to cover external"damage. (9) Major repairs made as a result of accident. (10) Airplane usage (past, present and future), e.g., duration of· flights. (11) Environmental factors. (12) Intended service life. b. Operators may find that their several aircraft require a variety of· maintenance procedures because of differences in OTiginal delivery date, prior usage, service bulletin applicability and incorporation, and prior maintenance. c. Following is a summary of potential or actual ·problem areas. This information should be useful in identifying types of details, as well as typical locations, for structural inspections. Because of structural differences some of these items do not apply to all airplanes. d. Corrosion Control. For corrosion control, certain parts of the air­ craft require additional attention as the airplane gets older. If utilization is reduced or spasmodic, there may be greater accumulation of moisture inside the aircraft and more atmospheric corrosion elements on the exterior of the aircraft. A critical area review has been made to define possible corrosion sensitive locations: (1) The interior moisture sensitive areas are the crown and bottom of the fuselage. (2) The structure below the cargo deck and areas below galley and toilet facilities may require more attention than identified in maintenance planning for newer airplanes. (3) A wet insulation blanket is a clear alert signal that the blanket should he removed to inspect for corrosion. (4) The moisture•trap low areas in tbe wing may require attention if the fuel replenishing is irregular. (5) Inspect the above areas and the exterior of the aircraft for evidence of corrosion after washing. (Early identification of fatigue damage may also be revealed during these inspections.) ### Page 14 21 Oct 71 AC 20-76 e. Potentially Fatigue - Critical Areas. The following areas with difficult access merit special attention: (l) Fuselage frames and stringers behind rarely disturbed electrical, navigational and air-conditioning equipment. The greatest con­ centration of these components is in the sidewalls and below the floor of the cockpit. Close exterior inspection, and in­ spection of the interior when and where accessible, can be used to evaluate the condition of the structure. (Note: Several service bulletins have been directed to this area.) (2) Wing hidden or blind areas are as follows: (a) Upper and lower splice plates at the Wing Station 360 joint. {b) Upper and lower surface skins under the external rib chords and form part of the landing gear support. Cc) Lower surface skin in the center section, particularly between the BBL 12.78 lower beams and under the outboard wing to center section splice plates. (d) Portions of the production break forgings at Wing Station 733 that are sandwiched between the skin and stiffeners. (e) Doublers under the rib chords at the dry bays at Wing Station 360 and 501. (f) Front and rear spar chords under various fittings. f. In addition to the inspection recommended by the Manufacturer's Maintenance Planning Data (D6-7552), some of these areas may receive eddy current inspection during incorporation of a number of service bulletins. g. Sampling inspections, utilizing eddy current with selective fastener removal~ general X-ray and rigorous visual techniques can be used in selected areas to supplement the documented inspection requirements. h. General Inspection of Known Problem Areas. Cl) Fuselage. (a) Stringer and frame flanges - check for cracks at inter­ sections, upper and lower lobes. (b) Skin cracks at spotwelds attaching doublers, stiffeners, and fail safe straps. ### Page 15 AC 20-76.. 21 Oct 71 (c) Skin cracks at upper.row of countersunk rivets in .040 exterior skin at longitudinal lap joints. · Cd) Frames and skins around cutouts. Ce) Stress corrosion of forged fittings at floor beams below sill of upper deck cargo door, doorstops at entry, service and lower cargo doars, bulkhead at attachment of wing r~ar spar to body, floor beams supporting main landing _gear side struts, and supports of stabilizer jackscrew·and hinges. Cf) Fin front and rear spar attachment fittings and the local bulkhead structure near these fittings. (2) Wing. (a) Skins - Around cutouts, joints, stiffener terminations, and areas of attachment of other components such as spars, nacelle, flaps, and spoilers. Cb) Stiffeners - At joints (including lower surface spanwise skin splices a,t S-4 and S-8) and in the wing center section upper surface at intersection with fuselage floor beams. (c) Inspar rib webs from Wing Station 387 to Wing Station 752 and at the WBL 315 rib aft of the inboard nacelle. (d) The ribs, spar, fittings, and structure adjacent main landing gear support. to the (3) Empennage. (a) Stress corrosion in stabilizer center section lugs that attach outboard stabilizer and in rib chord adjacent to those lugs. Cb) Stress corrosion-of aluminum lug around stabilizer hinge fittings. Cc) Fatigue in center section skins by fasteners attaching jackscrew nut support fittings. ### Page 16 21 Oct 71 AC 20-76 (4) Nacelle Struts. (a} Nacelle strut skins. (b) Nacelle front spar chord at attachment of spar fitting and the spar fitting. i. Maintenance Reports. The following is a listing of selected main­ tenance difficulties experienced, representing examples of reports submitted by air carrier operators. It should be noted that this is a partial list only, and only a portion of time in the history of 707 and 720 aircraft is covered. (1) Fuselage. (a) During inspection found station 360 bulkhead (aft end nose­ wheel well) lower chord cracked at left outboard air lock fastener. Total aircraft time (TAT) 29,515 hours. (b) During scheduled inspection found support fitting P/N 65-27187-1 under sill of upper main cargo door cracked. Support fitting directly beneath second fitting from for­ ward end of sill. Replaced fitting. TAT 9,955 hours (B-707-300 series). (c) Found two cracks in floorbeam BS880 in lower flange from bolthole at LBL and RBL 7.29 inches. ·crack progressed outboard from LH and RH bolthole approximately 1.4 inches. Crack on LH side of airplane progressed inboard approxi­ mately 1.5 inches to the next fastener hole. Crack on RH side of airplane progressed inboard approximately 2.6 inches to second fastener hole. Repaired per manufacturer's instructions. TAT 11,832 hours. (d) Visual inspection revealed a crack in the right fuselage terminal fitting. Boeing PIN 65-1903-2. Crack appeared to start at the forward lower fastener hole common to stringer 18A and progressed up the heavy flange, approximately 6.5 inches and down around fillet radius 1.5 inches. TAT 24,244 hours. (e) During visual inspection found 15-inch long crack forward side of aft cargo door frame in r'adii. Repair made per manufacturer's instructions. ### Page 17 AC 20-76 21 Oct 71 Cf) During service inspection, found several fuselage top skin cracks between stringers lOL and lOR. Cracks at spot welds between tear strips and stringers. Replaced affected fuselag~ panels per Boeing instructions. TAT 30,944 hours. Cg) During scheduled inspection, found nosegear door torque tube support P/N 65-22475-2 RH corner broken off at station .294.5. Replaced support assembly, TAT 10,933 hours. Ch) Several skin and stringer cracks at fuselage station 960 to 1240 from stringers.lOL to lOR. Skin and stringers replaced. TAT 31,627 hours. (i) Found 3/4 inch crack in radius of chord angle fuselage bulkhead, station 1440 between stringers JR and 3L. Repaired with external strap with Boeing approval. (j) On the lower nose walkway pressure web found 3•inch crack on the lefthand side at station BS 320. TAT 27,263 hours. (k) X-ray inspection revealed 1/2-inch crack across outer flange of FS 1241 fuselage beltframe between stringers 4 and 5. Repaired per repair manual. TAT 17l54 hours. ( 1) Found a one-inch longitudinal crack on fuselage exterior skin at skin lap at forward righthand carrier of nosewheel well cutout at stringer 28, BS 259.5. Subsequent inspection of area revealed additional cracks as follows: 1 Crack across the forward flange of fuselage former Tangle between stringers 27 and 28 at BS 259.5. · 1 A crack on forward side former station 259.5 nonpressure web progressing up from former Tangle between stringer 27 and 28. 1 Cracks across the vertical and horizontal flange of attaching forward side from station 259.5 between stringers 27 and 28. Repairs accomplished per repair manual. TAT 22,537 hours. Cm) Found bottom fuselage skin cracked at aft edge of ~adio altimeter antenna cutoff. Crack extended aft l•l/2 inches. Repaired by doubler installation. TAT 24,289 hours. (n) During HBC found 9•inch crack in web fuselage station 360 rightside bet~een nosewheel well and vertical member at RBL 131 web also buckled badly from wheel-well outboard 18 inches. TAT 24,405 hours. ### Page 18 21 Oct 71 JC 20-76 Cc) Found cracks in upper cargo door latch support fitting P/N 65-27187-1, one at fuselage (body) station 500 and one at station 520. Fittings located just below bottom door sill. TAT 12,914 hours. (B-707-300 series). {p) In cruise at 35,000 feet, Captain's outer windshield cracked and small piece blew out. P/N 5-7176-1-3015. TAT 24,981 hours, TSLl 48 hours. (q) Outer pane of number 3 left cockpit window cracked. A short time later, inner pane cracked. Pressure differ­ ential reduced to 2PSI and landed without further incident. Changed window PIN 121-71763•13. (r) Captain's apostrophe windshield shattered in cruise. SUspect arcing cause of shattering. Changed windshield Boeing P/N 5-71761-3019. Operation then nomal. (2) ~. (a) During roµtine service, found right wing top skin at station 226 cracked from trailing edge forward to fastener hole No. 139 in rear spar. Repaired per structural repair manual. TAT 16,742 hours. (b) Evidence of fuel leak revealed chordwise crack extending between the splice plate tab fasteners in right lower wing skin inboard of No. 3 pylon WS 392, 2-1/2 inches a.ft of front spar. Crack repaired in accordance with Boeing Service Bulletin 1995 R6. Aircraft total time 24,875 hours. (c) 1>1..~ring scheduled ultrasonic inspection of right upper wing skin at rear spar, fasteners found cracked at fasteners Nos. 5, 7, and 48. The crack emanating from No. 5 fastener extends forward 1/8 inch. Crack from No. 7 fastener extends aft 1/8 inch. Repaired in accordance with Service Bulletin 2427. TAT 9,951 hours. (d) During check, found approximately an 8-inch spanwise crack at upper front spar cap on right wing at WS727• Repaired per Boeing drawing 65-70144. TAT 33,967 hours. (e) During routine X-ray inspection of rear spar area, a crack was found in fastener hole NR 102 as shown on Boeing Drawing 65-68301 at WS 208 left wing. The crack extended forward at 10 o'clock position to edge of upper spar cap and aft ap­ proximately .25 ·inches. Intemal repair was made per Boeing Drawing 65-40140. TAT 16,494 hours. ### Page 19 JC 20• 76 . _ 21 Oct 71 ( f) During routine service inspection, found 2•1/ i inch spanwise crack in upper skin of left wing. Crack approdaately 8 inches aft of front spar at WS S29. TAT 30,944 hours. <1> During scheduled inspection, found 1-1/~inch long crack in No. l nacelle f1:'0d: spar fitting aft outboard bole, assembly PIN 65•133475. ieplaced fitting assembly. (h) Ultrasonic inspection fOWld 1/8-inch crack on top of ~ght wing at No. 11 fastener at rear spar. Crack emanates fr011 fastener bole and eztends forward. Repaired in accordance with. S/B 2427. TAT 21,979 hours. ( i) Ultrasonic inspection of left upper wing skin at rear spar· fasteners, found crack at fastener No. 49. Crack approximately 1/8-inch long and emanates from hole in ~t direction. Repaired in accordance with Service Bulletin 2427. TAT 22.458 hours. Cj) Ultrasonic inspection of left and right upper sldn at rear spar fasteners revealed one crack in left wing and one in right wing. c~ack in. left wing was at rear spar fastener No. 56. The crack was 1/4-inch long and emanated from fastener hole in the forward direction. Crack in right wing was at fastener No. 9. The crack was 1/16-inch long and emanted from fastener hole in the aft direction. Repaired in accordance with SIB 2427. TAT 21,656 hours. (k) Inspection of upper skin in accordance with Boeing S/B 2378 found a crack in the upper sk:l.n right wing at innermost fastener of stringer 6A. Repaired in accordance with Boeiag Drawing WR 65•58379. TAT 221 919 hours. Cl) Crack 3/32-inch long on upper left wing rear .spar at aft row of fasteners, inboard of beaver tail fastener No. 8. Repaired per manufacturer's instructions. TAT 111975 hours. Cm) At scheduled inspection found crack in top wing skin of left wing at interspar skin just aft of front spar station 475. Crack extends fore-and-aft, 1-1/4-incb total length of fastener hole at teminus of No. 19 stringer. Repaired per Boeing instructions. TAT 241 299 boura. (n) In leadins edge skin, lower surface at station 3001 left wing• 4•1nch long chordwise crack. (o) Crack 3/32-inch long found on upper right wing rear spar at aft row of fasteners. Fastener No. 8 inboari of beaver tail. TAT 11,975 hours. ### Page 20 21 Oct 71 N:. 20•76 (p) Repaired 2-inch crack in left wing skin top at rear spar station 226. Cracked from hole No. 136. Reference Boeing Drawing 65•58301. TAT 12,249 hours. (q) Out of service due to fuel leak left wing Ho. 2 fuel tank, found 2-1/4-inch crack in skin at No. 1 aft main fuel boost pump. TAT 30,442 hours. (r) Crack indication in rear spar chord at fastener hole Nos. 5 and 6 in left wing, and at No. 19 in right wing. Repaired per SB 2427, Part v. TAT 15,810 hours. Cs) Ultrasonic inspection of left upper wing skin at rear spar fasteners found crack at fastener No. 7. Crack· approximately 1/8-inch long and emanated from hole in aft direction. Repaired in accordance with Service Bulle"-11 No. 2427. TAT 21.044 hours. (t) Found upper wing skin center section with intermittent crack over a 12-inch span at stringer 27 approximately 6 inches to left of centerline. Span contained a 3-inch visible crack, plus a 2-1/2-inch eddy current indication separated by 2-inch and 6-inch areas of wing skin. On opening, found several cracks in radius of stringer 26A. Repaired per Boeing recommendations. TAT 251 905 hours. (u) Nacelle rib station 212. Found 2-inch crack in web of No. 1 nacelle inboard and outboard sides at rear engine mount fit­ tings above horizontal firewall. Repaired per manufacturer's instructions. TAT 231 051 hours. (vl No. 1 nacelle. Found both end lugs lower spar fitting diagonal brace attach cracked through diagonal brace boltholes. Replaced fitting PIN 5-84468-3005. TAT 23,051 hours. (w) During service inspection found No. 4 pylon upper cap skin and side panel cracked. Outboard chord angle PIN 64-1071 cracked in two. Crack ~hrough fastener hole to forward fasteners through the outboard tang of the front spar fitting PIN 65-13347-5. Spar fitting changed due to crack indications in fastener holes. Repairing chord angle and affected skin. (x) Found No. 1 engine pylon over wing fitting cracked, completely through. 8 inches aft of forward end. Replaced over wing fitting assembly. ### Page 21 21 Oct 71AC 20-76 · · Cy> Found crack 1-1/8-in. ·1ong at hydraulic line support fitting hole No. 2 strut torque bulkhead web. Also found 6- and 7- inch long crack No. 3 strut torque bulkhe~d web. same location. TAT 29,825 hours. (3) Empennage. Ca) During routine service found upper clevis lower lug cracked in right horizontal stabilizer front ,par terminal fitting. Replaced fitting P/N 65-3409. TAT 15,810 hours. Cb) Replaced cracked fitting P/N 65-3409-8 horizontal stabilizer right hand forward spar ten1inal. Fitting was cracked in upper clevis hole inboard to edge. Inspection per Service Bulletin 2330. _TAT 12,457 hours~ Cc) Found heavy exfoliated corrosion inside of center section of horizontal stabilizer on left fore-and-aft stiffener angle bottom surface. TAT 23,050 hours. Cd) Found heavy exfoliated corrosion on center section of hori­ zontal stabilizer inside on center fore-and-aft stiffener angle bottom surface. Repaired per manufacturer instruc­ . )tions. TAT 23,098 hours. ### Page 22 21 Oct 71 AC 20·76 Appendix 1 **FUNDAMENTALS OF AIRFRAME INSPECTION** 1. MAINl'ENANCE INSPECTIONS. Basically, visual examinations of the structural members of the aircraft and/or components to determine its airworthiness are maintenance inspections, An inspection program might include walk-around inspections and detailed inspections, in­ volving airframe disassembly and use of complex inspection aids. a. The Federal Aviation Regulations (FARs) require maintenance and inspection programs.for all United States certificated aircraft. b. Certificated aircraft used in general aviation by private and corporate owners must be inspected completely at least once each year. c. Annual inspections of general aviation aircraft are conducted by FAA certificated mechanics holding, in addition, an inspection authorization issued by the FAA and by certain certificated repair stations and aircraft manufacturers. It is not practi­ cable for FAA inspectors to conduct such inspections of approxi­ mately 190,000 general aviation aircraft. d. Air carrier aircraft used in air transportation are subject to FAR provisions requiring that the aircraft be maintained under continuous maintenance programs tailored to the aircraft make/ model involved. These programs are sophisticated and comprehensive and provide for frequent inspections of the aircraft. e. FAA inspectors perform surveillance and monitor the condition of aircraft to assess the effectiveness of maintenance programs. Moreover, knowledge gained from aircraft accident and incident investigations is used to improve maintenance programs and aircraft design. Cl) An aircraft inspection program may include a regularly scheduled inspection, post-flight, or preflight inspection by either the mechanic or the crewmember and/or pilot of the aircraft. An inspection program is designed to main­ tain the aircraft in a continuous airworthy condition. However, if the inspection is conducted in a haphazard or irregular manner, there will invariably be a gradual deter­ ioration of the aircraft condition. As a result, time which must be spent in repairing the aircraft will exceed any time gained in earlier inadequate inspections. ### Page 23 AC 20.75· Appendix 1 21 Oct 71 Ca) Regularly acheduled inspections and ·preventive maintenance assure the continuous airworthiness of the aircraft. Fail­ ures and malfunctions of equipment are appreciably reduced, if exce11ive wear or minor defects are detected and reme· died early. The importance of in1pections and the proper use of records concerning these inspections cannot be overemphasized. Cb) Airframe inspections range from preflight or walk·around inspections to detailed inspections. The time intervals for the inspection periods vary with the models _of air·. craft involved and type of operation being conducted. When developing maintenance inspection program, the FAA District Office should be contacted. · (c) Airframe inspection program may b~ based on flight hours, calendar or cycles of usage. The latter usually involves fuselage pressurization cycles or landing gear retractions. Under the calendar system, the inspection is accomplished on the expiration of a specified number of calendar weeks, ~nths, etc. This is an efficient system from a mainte­ nance man,gement standpoint. Scheduled component replace­ ments, which have a stated hour life, are normally accomplished during the calendar inspection which occurs nearest th~ hour limitation. l In some instances, a flight-hour limitation is established to limit the number of hours that may be flown during the calendar interval. i Aircraft operating under the flight•hour system are inspec. ted when a specified number of flight hours are accumulated. Components with stated hour operating limitations are nor­ mally replaced during the inspection that falls nearest the hour limitation. (d) Required inspections. All civil aircraft must be inspected at certain intervals as provided jn the Federal Aviation Regulations (FARs) to determine the aircraft condition. 1 Generally, the inspection interval Will depend upon the type of operation in which the aircraft is engaged. l Some aircraft must be inspected at least once each 12 calendar months, while inspection is required for others after each 100 hours of flight. An aircraft may also be inspected in accordance with an inspection system set q, to provide for total inspection of the aircraft over a calendar or flight~time period. ) Page 2 ### Page 24 21 Oct 71 AC 20-76 Appendix 1 4 In order to determine the specific inspection requirement and rules regarding inspections, reference should be made to the Federal Aviation Regulations which prescribe the requirements for the inspection of aircraft in various categortes of operation. Ce) Examples of Airframe Inspections. Before starting an airframe inspection it is recommended that: 1 Be certain that all plates, access doori;;_, fairings, and cowlings have been opened or removed, and the structure should be cleaned. 2 When opening inspection plates, doors, and cowlings, and before cleaning, note for indications of oil or other fluid leakage and/or stains. 1 Always use a check list when performing the inspection. A checklist may be provided by the manufacturer or of local design. The airframe checklist should include the following areas: (aa) Fuselage group. ! Skin - for deterioration, distortion, other ev­ idence of failure, defective or insecure attach­ ment of fittings and fasteners, and corrosion. ii Components - for proper installation, apparent defects and satisfactory operation. (bb) Cabin and Cockpit group. i. Generally - for cleanliness and/or loose equip­ ment which should be secured. ii Seat and safety belts security. - for condition and iii Windows and windshields - for deterioration, cracks, delaminations, scratches, chipping (see Chap 5, windows). iv Flight (and engine) controls - for proper installation and mounting. v Batteries - for proper installation, corrosion, and leaking fluid. vi All systems - for proper installation, general condition, apparent defects and security of attachment. ### Page 25 AC-20-76 , , 21 Oct 71 Appendix 1 (cc) Landing gear group. i All units - for condition and security of attachments. ii Linkage, trusses, and members - for undue or excessive wear, fatigue and distortio~. iii Retracting and locking mechanism for proper operation. j_y Wheels - for cracks, defects, and condition of· bearings. v Tires - for wear and·cuts. (dd) Wing and center section. ! All components - for condition and security • .!! Skin - for deterioration, distortion, other evi­ dence of failure and security of attachment, fasteners, corrosion • .ill Internal structure - spars, ribs, compression members for corrosion and security • .!Y Movable surfaces - for damage or obvious defects, skin attachment and proper travel. v Control mechanism - for freedom of movement, alignment and security. vi Control cables - for proper.tension, fraying, wear and routing through pulleys and fairleads. (ee) Empennage ·group. ! Fixed surfaces· for damage or obvious defects, loose fasteners,' and security of attachment. .!! Movable control surfaces - for damage or obvious defects, loose fasteners or skin distortion. ill Skin - for abrasion, tears, cuts, defects, dis­ tortion, corrosion or deterioration. {f) Aircraft logbook - this applies to the aircraft log and all supplemental records concerned with the aircraft. The log and records are a history of the aircraft maintenance ) and operation, control of mainten$nce schedulest and data for time or cycle replacement of components or accessories. ### Page 26 21 Oct 71 AC 20-76 Appendix 1 l The aircraft logbook is the record in which all data con­ cerning the particular aircraft is recorded. Information accumulated in this log can be used to determine the aircraft condition, date of inspections, and time on aircraft (and engines). 2 It is a history of all significant events which have occurred to the airframe, its components and accessories and it provides a means of indicating compliance with the FAA Airworthiness Directives and manufacturers' service bulletins. {g) Special Inspections. During the service life of an air­ craft, occasions may arise when a landing ·is made in an 'overweight condition or during a portion of the flight severe turbulence is encountered. Also, for a variety of reasons, rough landings are experienced. l Special inspections procedures should be followed when one or more cf the aforementioned situations are en­ countered to detE!llline if the aircraft structure has experienced damage. 2 When performing these special inspections, the detailed procedures in the aircraft maintenance manual should be followed. (h) Hard or Overweight Landing Inspections. The stress induced in a structure by a hard or overweight landing depends both on the gross weight at touchdown and the severity of impact (rate of sink). It is difficult to estimate vertical veloc­ ity at the time of impact, and whether a landing has been sufficiently severe to result in structural damage. With this in view, a special inspection should be performed after: l A landing is made at a weight known landing weight, or to exceed the design A rough landing regardless of the landing weight. 3 Wrinkled wing skin is a sign of an excessive load which may have been imposed during a landing. Another indication easily detected is fuel leaks and/or fuel stains along riveted seams. 4 Other possible damage locations are spar webs, bulkheads, nacelle skin and attachments, wing and fuselage stringers. 1 If these areas do not indicate adverse effects, probably no serious damage has occurred. A more extensive inspec­ tion anq alignment is necessary if d~ge is noted. ### Page 27 AC 20- 76· Appendix 1 21 Oct 71 ( i) Severe Turbulence Inspection. If the combination of gust velocity and airspeed is too severe, the stress induced in structural members can cause damage. Inspection should be performed after a flight through severe turbulence as follows: ! Upper and lower wing surfaces - inspect for excessive buckles or permanent set wrinkles. 2 Spar webs - inspect for buckling, wrinkles~ sheared attachlnents. Nacelles - Inspect for buckling around nacelle and nacelle skin at the wing leading edge •. 4 Fuel leaks - fuel leaks may indicate open rivet seams with broken sealant. Fuselage skin - inspect top and bottom areas for wrinkles which are probably diagonal. 6 Empennage - inspect surfaces for wrinkles, buckling and/or sheared attachments, and also areas of attachment to fuselage. A more extensive inspection and alignment is necessary if damage is noted. (j) Publications. Aeronautical publications provide informa­ tion on the maintenance of aircraft. Proper use of these publications will aid in the efficient maintenance of air aircraft. 1 Service Bulletins - This is one type of publication issued by the aircraft manufacturer (see Chap 4. paragraphs a and b) which may include maintenance guidance material. 2 Maintenance Manual - The aircraft maint_enance manual is provided by the aircraft manufacturer and contains mainte­ nance instructions for all systems installed in the aircraft. The work is noraally perf~rmed on installed aircraft systems. overhaul Manual - The aircraft overhaul manual is provided by the aircraft manufacturer and contains information and step-by-step instructions on work which is performed on a member (or unit)° usually removed from the aircraft. 4 Structural Repair Manual - This manual contains information and specific instructions as provided by the aircraft manu· facturers for repairing the primary and secondary structure. 5 Illustrated Parts Catalogue - This catalogue presents breakdown of structural components with exploded views and illustrations of specific areas. ### Page 28 21 Oct 71 AC 20-76 Appendix 2 **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 com.binedwith 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 par­ ticularly 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 corr9sive 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 str~cture. 3. FORMS OF CORROSION. There are many forms of corrosion which depends 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 xrom contact between dissimilar metal parts in the presence of a conductor. A galvanic action like electroplating occurs at pointsof contact when insulation has broken down or was omitted. ### Page 29 AC 20·76 .. 21 Oct 71 Appendix 2 . . c. Intergranular corrosion. The grain boundaries of an alloy are attacked by this type of corrosion. lntergranular corrosion may exist without visible surface evidence. Severe intergranular corrosion may sometimes cause the surface of a metal to "exfoliate. 11 This is a flaking of the metal at the surface cm.teed by pressure of corrosion residual product buildup. d. ~tress 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 charac­ teristic of aluminum, certain stainless steels and high strength steels. c. 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. PREVENTIVE MAINTENANCE. Corrosion-preventive maintenance includes: a. An adequate cleaning. b. Thorough periodic lubrication. c. Detailed inspection for corrosion and failure of protective systems. d. Prompt treatment of corrosi~n and touchup of damaged paint areas. e. Keeping drainholes free of obstruction. f. Daily wipedown of exposed critical areas. g. Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days. h. Making maxilllWII use of protective covers on parked aircraft. ) Page 2 ### Page 30 AC 20·7621 Oct 71 Appendix 3 **FATIGUE FAILURES** l. PREDICTION AND PREVENTION OF FATIGUE. FAILURES. Fatigue failure in metals is a very complex subject. The situation is further complicated by vari­ abilities encountered in all aspects of the real, physical world. It is well known that material properties are not the constant characteristics of a material, but they may vary within certain ranges. A basic material property such as mechanical tensile strength will vary among specimens made from supposedly the same material. The variability in material fatigue properties is considerably higher than that of material static properties. This variability or "scatter," in terms of fatigue life, may exceed two orders of magnitude. a. When parts or structures are fabricated from the materials, addi­ tional variabilities are introduced by the manufacturing processes 1 and no two "nominally identical" parts are alike in all respects. b. Service or field use of identical parts may also vary from part to part. That is, the exact use conditions of the aircraft are not known beforehand. c. Actual fatigue behavior of such parts in service is influenced by all of the above factors and associated variabilities. 2. HISTORY OF A FATIGUE FAILURE. The structural members of an aircraft carrying loads may get 11 tired 11 from the repetitive stresses inherent in the flight and taxiing operation of the aircraft. This condition is called metal fatigue. a. A metal specimen designed to carry a certain load will do so, in· definitely, if the load is steady. If the load is cycled (applied and released) many times, the specimen may break. This can happen even when the load is smaller than the original design load limita­ tion. Repeti~ive loadings can cause slight deformation or stretch­ ing of the metal which results in a small crack. With continued loading and d~formation, the crack will spread. b. The uncracked area of the part is not strong enough to carry a smaller load and, subsequently, the part breaks. This is briefly, in simple terms, the history of a fatigue failure. 3. FATIGUE FAILURE IN AIRCRAFT. Fatigue failure is the tendency of materi­ als to fracture under many repetitions of stress at levels considerably less than the ultimate static strength. Examples are cracks in wing skin: ribs, stringers, and spars, or in fuselage skin. stringers, longerons 1 bulkhead fr8Dles and webs. a. Fatigue is not limited to the win& and fuselage structure, but occurs also in heavy hollow members such as landing gear struts, engine shafts and torque tubes, solid members such as fittings, bell cranks, tie rods, turbine wheels, links, bolts, and other areas. ### Page 31 1£ 20-76 21 Oct 71 Appendix 3 b. Fatigue occurs when the endurance limit of the material is exceeded. Endurance limit is extent of life, or number of cycles of flight loads or stress levels which a structural member can endure before fracture occurs. 4. STRESS RAISERS. Although-the life of a part may be increased by reduc­ ing the tensile stress imposed, tJu,s would not eliminate premature fail­ ures if high localized stresses are present. High localized stresses result from irregularities of form, such as holes, surface ~otches or . nicks, sharp edges and abrupt changes in cross section. These are called stress raisers; the phenomenon is called stress concentration. 5. FATIGUE FAILURE CONTROL. Fatigue failure problems may be controlled by employing adequate maintenance inspection programs and techniques. Ob­ viously, the objective is to detect an incipient fatigue failure before it occurs so that the structure can be repaired or .replaced. Thus, the inspection approach to the problem has several advantages: a. It can decrease the incidence of fatigue failures. b. It increases ~he operational reliability of the structure and equipment. c. It allows a better utilization of the structure by extending actual service life until shortly before an incipient failure instead of retiring a structure at some earlier predetermined time, which~ in many cases, would represent only a fraction of the structure's available useful life. d. On the other hand, any inspection program consumes time, costs IBOney, depending on the method used, and, may involve rather complex and expensive equipment. There are also some limitations and some com­ plicating factors associated with each inspection method. '6• MAINTENANCE INSPF.CTIONS REQUIREMENTS. It is the maintenance people, once the aircraft is in operation, who detect and repair any such fatigue damage. Further, maintenance has the added responsibility of not creating any points of stress concentration during any repair or modification. a. The prevention of fatigue failure in the structure by inspection depends on the ability of the inspection method(s) to detect an incipient fatigue failure. b. The inspection of a structure should yield a positive identification of the fatigue damage that has already incurred and that would eventually lead t~ failure. c. These inspection methods should be of a nondestructive type so that they would not do any damage to the items being inspected and would not affect their fatigue life. Various available nondestructive testin2 {NIYI') methods for detecting fatigue damage are covered inAppenaxx 4. ### Page 32 21 Oct 71 JC. 20-76 Appendix 4 **NONDESTRUCTIVE TESTING** 1. PRl!.'"VENTIVE MAINTENANCE. Simply stated nondestructive testing (NDT) is generally preventive maintenance. This includes utilization of such maintenance tools as Xray, ultrasonic, magnetic particles, eddy current, and dye penetrant. a. Maintenance Inspection. NDT permits maintenance inspection without removing CODlponents 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 inter­ preting the results. Also. many states require that an Xray techni­ cian have an approved certificate for use of Xray in industrial applications. This is to minimize improper use with attendant health hazard of Xray equipment. 2. METHODS AND APPROACHES FOR DAMAGE DETECTION. Information on the appli­ cation of NDT methods for the detection of fatigue damage is scattered through literature on metal fatigue and literature on NDT. Few publica­ tions 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 con­ siderably 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. Boroscopes permit direct visual inspection of the interior of hollow tubes• chambers, and other internal surfaces. ### Page 33 N; 20-76 · · 21 Oct 71 Appendix 4 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 il­ lumination 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 co~ incides with a machining mark. The~e are, of course, also limita­ tions on the size of cracks tpat can be detected by visual·inspection, depending on optical aids employed. · 5. LIQUID PENETRANT. 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 11,,lack 11 (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. 6. 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 ~s 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 con­ sists 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 )attracted to the locations of the cracks (or other defects) due to local variations in the magnetic field that are produced. ### Page 34 21 Oct 71 AC 20-76 Appendix 4 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 Xray, gaJDma•. beta. or neutron radiation. It is based on the ability of these radioactive sources to penetrate materials. The in­ tensity 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.~ sensitivity occurs when the cracks are oriented such that its longest dimension is parallel to the direction of radiation. ae Xray radiography has tWQ m~n advantages; (l) versatility. and (2) sensitivity. The Xray energy source can be easily adjusted for variations in thickness. It is also adaptable to fluoro­ scopy and television system~. · · b. The advantages of gamma radiography are; (1) portability, and (2) a relative 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 ~he difficulties with the gamma radioactive source is that ~he source cannot be varied or turned off so that safety precau~ions must be observed at all times. (Xray constitutes a health hazard only during operation of the Xray equipment). c. Conventional radiography is firmly established and reasonably easy to understand. One of the original drawbacks was the long time involved in the deve~oping and processing of film. This has been overcome by modern automatic film processiQg techni~ues. d. Interpretation of the processed film is the most uiportant phase of radiography. Adequate tools such as a film illuminator lens a..,d good working conditions shoulabe available to a~sist the interpreter in detecting cracks in the part displayed on the film. Page3 ### Page 35 21 Oct 71 s. ULrlASONICS. Ultrasonc methods have received wide acceptance in the aviation't.ndustry and are particularly useful for determining the in­ tegrity of a member of a structure. Basically. sound energy above the audible range is transmitted into a.part, and a signal is received and and analyzed. The ultrasonic wave is transmitted and received through transducers, which are ·placed upon the part to be inspected. The transducei- must be properly coupled to the part and is the most cri _ti­ cal 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 ~ransducer that sent the signal (reflection and resonance). For crack detection, the reflection technique is most c0111DOnly 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 abaorb and reflect energy. A defect can be recognized by the relative time for return Gf the reflected energy to the transducer. b. The ultrasonic methods now available are rapid, economical, sensitive 1 and can have good accuracy for determining crack extent and position. Eq1.1ipment is light and portable so that on-site inspections are pos­ sible. There 4re conditions which can limit the usefulness of ultra­ sonic inspections. These include unfavorable part geometry (such as complexity, contour 1 and size) orientation of the cracks, and mis­ leading responses w~ch may occasionally be obtained. Also, ultra­ sonic inspection, as presently practiced, dependsL·upon experience, skill, and judgment of the inspector. Be must interpret the crack size anq location by the indirect evidence presented by the electronic equipment (oscilloscope). Be must be able to distinguish between significant signals and. spurious ones. 9. EDDY CURRENT. The eddy current method is a comparatively recent non­ destructive testing technique. It is being frequently used for non­ magnetic materials. ('ih.en used for magnetic aaterials, it requires more complex systems). The principle involved is simple. A coil that is carrying a high-frequency alternating current is brought near an elec­ trical conduc~or and eddy currents are generated in the conductor. The eddy (or induced) current create 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. Page 4 ) ### Page 36 21 Oct 71 AC 20-76 Appendix 4 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. F.ddy current instruments have various degrees of versatility. M.~~y 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. lo. REFERENCE PUBLICATIONS. a. Advisory Circular 20-61, Nondestructive Testing Techniques for Aircraft. b. Advisory Circular 65-9 1 Airframe and Powerplant Mechanics, General Handbook. c. Advisory Circular 43.13-1, Acceptable Methods, Techniques, or Practices - Aircraft Inspection and Repair. d. Defense Metals Information Center (DMIC) Report, S-25, dated 1 June 1968, Current Problems and Prevention of Fatigue. e. Advisory Circular 20-9, Personal Aircraft Inspection Handbook. f. Advisory Circular 20-50, Ultrasonic Nondestructive Testing. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.