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 original 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.
Corrosion Control. For corrosion control, certain parts of the sircraft 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 1s a clear alert signal that the blanket should be removed to inspect for corrosion.
(4) The moisture-trap low areas in the 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.)
21 Oct 71 AC 20-76
e. Potentially Fatigue - Critical Areas. The following areas with difficult access merit special attention:
(1) Fuselage frames and stringers behind rarely disturbed electrical, navigational and air-conditioning equipment. The greatest concentration of these components is in the sidewalls and below the floor of the cockpit. Close exterior inspection, and inspection 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.
(c) 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.
(1) Fuselage.
(a) Stringer and frame flanges - check for cracks at intersections, upper and lower lobes.
(Б) Skin cracks at spotwelds attaching doublers, stiffeners, and fail safe straps.
AC 20-76.. 21 Oct 71
(c) Skin cracks at upper row of countersunk rivets in .040 exterior skin at longitudinal lap joints.
(d) Frames and skins around cutouts.
(e) Stress corrosion of forged fittings at floor beams below sill of upper deck cargo door, doorstops at entry, service and lower cargo doors, bulkhead at attachment of wing rear spar to body, floor beams supporting main landing gear side struts, and supports of stabilizer jackscrew and hinges.
(f) 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.
(b) Stiffeners - At joints (including lower surface spanwise skin splices at 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 to the main landing gear support.
(3) Empennage.
(a) Stress corrosion in stabilizer center section lugs that attach outboard stabilizer and in rib chord adjacent to those lugs.
(b) Stress corrosion of aluminum lug around stabilizer hinge fittings.
(c) Fatigue in center section skins by fasteners attaching jackscrew nut support fittings.
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.
Maintenance Reports. The following is a listing of selected maintenance 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 nosewheel 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 forward end of sill. Replaced fitting. TAT 9,955 hours (B-707-300 series).
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 IH side of airplane progressed inboard approximately 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 P/N 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 radii. Repair made per manufacturer's instructions.
AC 20-76 21 Oct 71
(f) During service inspection, found several fuselage top skin cracks between stringers 1OL and 10R. Cracks at spot welds between tear strips and stringers. Replaced affected fuselage panels per Boeing instructions. TAI 30,944 hours.
(g) 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.
(h) Several skin and stringer cracks at fuselage station 960 to 1240 from stringers. 101 to 10R. Skin and stringers replaced.
TAT 31,627 hours.
(1) Found 3/4 inch crack in radius of chord angle fuselage bulkhead, station 1440 between stringers 3R 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.
X-ray inspection revealed 1/2-inch crack across outer flange of FS 1241 fuselage beltframe between stringers 4 and 5. Repaired 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:
Crack across the forward flange of fuselage former I angle between stringers 27 and 28 at BS 259.5.
A crack on forward side former station 259.5 nonpressure web progressing up from former T angle between stringer 27 and 28.
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.
TAI 22,537 hours.
(m) Found bottom fuselage skin cracked at aft edge of radio altimeter antenna cutoff. Crack extended aft 1-1/2 inches.
Repaired by doubler installation. IAT 24,289 hours.
(n) During MBC found 9-inch crack in web fuselage station 360 rightside between nosewheel well and vertical member at RBL 13, web also buckled badly from wheel-well outboard 18 inches.
TAT 24,405 hours.
21 Oct 71 AC 20-76
(o) 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. IAT 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, ISL1 48 hours.
Outer pane of number 3 left cockpit window cracked. A short time later, inner pane cracked. Pressure differential reduced to 2PSI and landed without further incident. Changed window P/N 121-71763-13.
Captain's apostrophe windshield shattered in cruise.
Suspect arcing cause of shattering. Changed windshield Boeing P/N 5-71761-3019. Operation then normal.
(2) Wing.
(a) During routine 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 aft ož front spar. Crack repaired in accordance with Boeing Service Bulletin 1995 R6. Aircraft total time 24,875 hours.
(c) During 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. IAT 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 approximately •25 inches. Internal repair was made per Boeing Drawing 65-40140. TAT 16,494 hours.
AC 20-76.. 21 Oct 71
(f) During routine service inspection, found 2-1/2 inch spanwise crack in upper skin of left wing. Crack approximately 8 inches aft of front spar at WS 529. TAT 30,944 bours.
(g) During scheduled inspection, found 1-1/4-inch long crack in No. 1 nacelle from spar fitting aft outboard hole, assembly P/N 65-133475. Replaced fitting assembly.
(h) Ultrasonic inspection found 1/8-inch crack on top of right wing at No. 1l fastener at rear spar. Crack emanates from fastener hole and extends forward. Repaired in accordance with S/B 2427. TAT 21,979 hours.
(1) 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 aft direction. Repaired in accordance with Service Bulletin 2427. IAI 22,458 hours.
(3) Ultrasonic inspection of left and right upper skin at rear spar fasteners revealed one crack in left wing and one in right wing.
Crack in left wing was at rear spar fastener No. 56. The crack 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 S/B 2427.
(k) Inspection of upper skin in accordance with Boeing S/B 2378 found a crack in the upper skin right wing at innermost fastener of stringer 64. Repaired in accordance with Boeing Drawing WR 65-58379. TAT 22,919 hours.
(1) 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. TAI 11,975 hours.
(m) 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-inch total length of fastener hole at terminus of No. 19 stringer. Repaired per Boeing instructions. TAT 24,299 hours.
(п) In leading edge skin, lower surface at station 300, left wing, 4-1nch long chordwise crack, Crack 3/32-1nch long found on upper right wing rear spar at aft row of fasteners. Fastener No. 8 inboard of beaver tail.
TAT 11,975 hours.
21 Oct 71 AC 20-76 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 No. 2 fuel tank, found 2-1/4-inch crack in skin at No. 1 aft main fuel boost pump. IAT 30,442 hours.
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. TAI 15,810 hours.
(s) Ultrasonic inspection of left upper wing skin at rear spar fasteners found crack at fastener No. 7. Crack approximately 1/8-1nch long and emanated from hole in aft direction.
Repaired in accordance with Service Bulletin 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 2-1nch and 6-inch areas of wing skin. On opening, found several cracks in radius of stringer 26A. Repaired per Boeing recommendations. TAI 25,905 hours.
Nacelle rib station 212. Found 2-inch crack in web of No. 1 nacelle inboard and outboard sides at rear engine mount fittings above horizontal firewall. Repaired per manufacturer's instructions. TAT 23,051 hours.
(v) No. 1 nacelle. Found both end lugs lower spar fitting diagonal brace attach cracked through diagonal brace boltholes. Replaced fitting P/N 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 P/N 64-1071 cracked in two. Crack through fastener hole to forward fasteners through the outboard tang of the front spar fitting P/N 65-13347-5.
Spar fitting changed due to crack indications in fastener holes.
Repairing chord angle and affected skin.
Found No. 1 engine pylon over wing fitting cracked, completely through, 8 inches aft of forward end. Replaced over wing fitting assembly.
AC 20-76 21 Oct 71
(y) Found crack 1-1/8-in. long at hydraulic line support fitting hole No. 2 strut torque bulkhead web. Also found 6- and 7- inch long crack No. 3 strut torque bulkhead web, same Location. TAI 29,825 hours.
(3) Empennage •
(a) During routine service found upper clevis lower lug cracked in right horizontal stabilizer front spar terminal fitting.
Replaced fitting P/N 65-3409. TAT 15,810 hours.
Replaced cracked fitting P/N 65-3409-8 horizontal stabilizer right hand forward spar terminal. Fitting was cracked in upper clevis hole inboard to edge. Inspection per Service Bulletin 2330. TAT 12,457 hours:
(c) Found heavy exfoliated corrosion inside of center section of horizontal stabilizer on left fore-and-aft stiffener angle bottom surface. TAT 23,050 hours.
(d) Found heavy exfoliated corrosion on center section of horizontal stabilizer inside on center fore-and-aft stiffener angle bottom surface. Repaired per manufacturer instructions. TAT 23,098 hours.
21 Oct 71 AC 20-76 Appendix 1
FUNDAMENTALS OF AIRFRAME INSPECTION MAINTENANCE 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, involving 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 practicable for FAA inspectors to conduct such inspections of approximately 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.
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 maintain 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 deterioration 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.
AC 20-76 21 Oct 71 Appendix 1
(a) Regularly scheduled inspections and preventive maintenance assure the continuous airworthiness of the aircraft. Failures and malfunctions of equipment are appreciably reduced, if excessive wear or minor defects are detected and remedied early. The importance of inspections and the proper use of records concerning these inspections cannot be overemphasized.
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.
developing maintenance inspection program, the FAA District Office should be contacted.
(c) Airframe inspection program may be 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, months, etc. This is an efficient system from a maintenance management standpoint. Scheduled component replacements, which have a stated hour life, are normally accomplished during the calendar inspection which occurs nearest the hour limitation.
1 In some instances, a flight-hour limitation is established to limit the number of hours that may be flown during the calendar interval.
2 Aircraft operating under the flight-hour system are inspected when a specified number of flight hours are accumulated.
Components with stated hour operating limitations are normally replaced during the inspection that falls nearest the hour limitation.
(d) Required inspections. All civil aircraft must be inspected at certain intervals as provided in 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.
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 up to provide for total inspection of the aircraft over a calendar or flight-time period.
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 categories of operation.
(e) Examples of Airframe Inspections. Before starting an airframe inspection it is recommended that:
Be certain that all plates, access doors, fairings, and cowlings have been opened or removed, and the structure should be cleaned.
When opening inspection plates, doors, and cowlings, and before cleaning, note for indications of oil or other fluid leakage and/or stains.
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 evidence of failure, defective or insecure attachment 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 equipment which should be secured.
ii Seat and safety belts - for condition and security.
iii Windows and windshields - for deterioration, cracks, delaminations, scratches, chipping (see Chap 5, windows).
İv Flight (and engine) controls - for proper installation and mounting.
Batteries - for proper installation, corrosion, and leaking fluid.
vi All systems - for proper installation, general condition, apparent defects and security of attachment.