11 July 72 AC 20-78 CHAPTER 9. CORROSION PRONE AREAS
18. GENERAL• Corrosion is most likely to strike at the joints or attachment points in metallic structure because they provide entrapment areas for corrosive agents and moisture to accumulate. However, the problem can be compounded if metallic surfaces are exposed to an extremely corrosive environment, or if the surfaces are difficult to inspect and clean. The most important corrosion preventive measure is to keep them clean and dry.
19. EXHAUST TRAIL AREAS. Both jet and reciprocating engine exhaust deposits contain compounds that are very corrosive, and the structure in the path of exhaust gas is more likely to suffer from corrosion than is any other structure. It is customary to coat structure exposed to exhaust gases with protective finishes to prevent these gases from coming in direct contact with the metal. However, the deposits which collect on top of the protective finishes must be removed before they permeate the film and attack the metal. Most troublesome are those areas where exhaust gas deposits may become entrapped and cannot be reached by normal cleaning methods. Typical of these areas are seams, gaps, hinges, or fairings located in the exhaust gas path.
20. ENGINE FRONTAL AREAS AND COOLING AIR VENTS. These areas are subject to erosion by airborne contaminants, rain, and from foreign objects on the runways. Erosion will remove the protective finishes or oxide film from the metal surfaces, leaving them vulnerable to corrosive attack. In addition, much of the equipment installed within these recesses is also vulnerable to corrosion. when an aircraft is operated in a marine environment, salt deposits may accumulate in these areas and the ensuing corrosive attack can be rapid and destructive. It is imperative that these areas be frequently inspected and cleaned, and protective finishes be maintained.
21. BATTERY AREAS AND VENTS. Due to the highly corrosive nature of battery acid and its fumes, battery areas and their vents are protected by special acid-resistant paints. Generally, this is enougn to stem the tide of corrosion if scrupulous attention is devoted to keeping these areas clean.
22. LAVATORIES, GALLEYS, AND CABIN FLOORS. The usual spillage, condensation, and other contamination of these areas are extremely corrosive to aircraft metals. The most corrosive agents are acidic foods and beverages, and human excreta. Also, most chlorinated disinfectants are acidic and corrosive, and cannot be recommended for use in aircraft. It is impractical to assume that these areas can be kept clean and dry at all times, but it is important to inspect the structure carefully at suitable intervals, cleaning and renewing the finish as necessary. Areas behind lavatories, sinks, or ranges where waste may collect are potential trouble spots, as are personnel reliei and waste disposal vents or openings on the exterior of the aircraft.
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AC 20- 78 11 July 72
23. WHEEL WELLS AND LANDING GEARS.
Equipment installations in the wheel-well areas probably absorb more punishment than any other portion of an aircraft because they are exposed to moisture and flying debris during takeoffs and landings, and when the aircraft is perked, they are exposed to atmospheric moisture. After an airplane has entered service, it is difficult to maintain protective paint film on landing gears, equipment installations, and wheel well surfaces because the many complicated shapes, assemblies, and fittings in these areas obscure other surfaces. Items that should receive special attention during wheel-well inspections are:
(1) Magnesium wheels (especially the areas around the bolt heads, lugs, and wheel webs).
(2) Portions of rigid tubing obscured by clamps and identification tapes.
(3) Exposed electrical equipment.
(4) Crevices between ribs, stiffeners, and lower skin surfaces which can serve as water or debris entrapment areas.
b. Corrosion control in wheel well areas may best be attained by frequent cleaning, lubrication, paint touch-up and judicious use of wheel covers.
CONTROL SURFACE RECESSES. Control surface recesses are potential problem areas because normally they are difficult to inspect. If corrosive agents gain entry and accumulate in these areas, they may go unnoticed for some time. Frequent inspection and cleaning of the surfaces and installations located in these recesses will prevent corrosion from gaining a foothold.
SPOT-WELDED SKINS. Corrosive agents may become entrapped between the layers of metal adjacent to the spot-weld beads. If moisture enters the area, an electrolytic cell can be set up between the dissimilar metal phases in the spot-weld area, and one or more of these phases will be subject to preferential corrosive attack. Whenever practical, structural areas with spot-welded assemblies should be sealed to prevent the ingress of contaminants and moisture.
HINGES. Hinges are natural traps for corrosive agents. Often they are susceptible to galvanic corrosion when the hinges and pins are made of dissimilar metals. The most practical means of corrosion control is to inspect and lubricate hinges frequently. After lubrication, actuation of the door through several cycles is necessary to ensure complete penetration of the lubricant.
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27. LAP JOINTS BETWEEN ALUMINUM ALLOY EXTRUDED SECTIONS.
Extruded sections of high-strength materials such as 7075 and 7178 aluminum alloys are more susceptible to intergranular corrosion than are other aluminum alloys. Application and maintenance of chemical processes, sealants, paints, or combinations of these protective measures have proven most effective in controlling corrosive attack on these materials.
-I intergranular corrosion occurs, it is usually founa around fasteners or in lap joints. It is evidenced by raised areas or lumps under the paint film, and there have been instances where the attack has progressed to such an extent that there were actually bulges in the faying surfaces. These raised areas, lumps, and bulges are due to the accumulation of corrosion products, which take up more volume than did the affected material before it corroded.
28. FLUID ENTRA MEVI AREAS. Design specifications require that aircraft have drains installed in areas where water may collect. However, if drains are rendered ineffective because they are clogged by debris, sealant, etc., or because the level of the aircraft is changed from that of a normal ground attitude, corrosive agents can collect in a localized area. Low-point areas and drains should be inspected frequently to prevent the inception of corrosive attack.
Low points of integral fuel tanks are areas where water condensate can collect. This water and condensate is of doubtful purity, and if permitted to stand, it can permeate the protective coating of the tank and bring many corrosive agents held in suspension in contact with the vulnerable metallic surfaces. Water condensate should be drained regularly from fuel tanks and the integrity of the tank sealant maintained to prevent corrosion of integral tank surfaces.
b. When considering fluid entrapment areas, one inevitably thinks of aircraft drinking water and wash water systems. Corrosion in these systems ia rare because they are fabricated from nonmetallic materials and/or stainless steel. Water stagnation should pose no problems so long as sanitation regulations are heeded.
29. ELECTRONIC PACKAGE COMPARIMENTS. Often the safety of those on board an aircraft rests on the proper function of a little black box. The environment of electronic package compartments is carefully controlled to provide the most ideal conditions that can be achieved. The degree to which such sensitive equipment is exposed to corrosive agents is very small, but even small quantities of moisture and contaminants can adversely affect equipment reliability. Components in these areas should be inspected for corrosive attack as thoroughly as possible during routine checks, and advantage should also be taken of nonscheduled component removals for further inspection.
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AC 20- 78 11 July 72
30. CONTROL CABLES. Control cables have preservative coatings which, when intact, prevent corrosive attack. Due to their vital function, it is necessary to inspect these cables frequently to ensure that they are adequately protected. During these inspections, incipient failures due to other causes also may be detected. It is recommended that control cables be inspected periodically, then cleaned and treated as necessary before reapplication of the preservative.
31. MICROBIOLOGICAL FUEL CONTAMINATTON AND CORROSION. Microbiological contamination in turbine fuel is caused by bacteria and fungi which feed on the constituents of the turbine fuel. The result is a sludge, or mat deposit, which has been found in some aircraft fuel tanks and is often loosely referred to as green slime. However, some deposits have been found in various shades of grey, brown, red, and white. If allowed to develop and grow in the aircraft fuel tanks, microbiological contamination can cause a myriad of problems not the least of which is severe aluminum alloy corrosion in the aircraft integral fuel tanks.
Since the subject is large and complex, we recommend Douglas Service Magazine, Vol. KAlll, Issue #2, 1905, which contains information on microbiological contamination and other types of corrosion, their causes, and their control. Vol. XXIV, 1966 issue, also contains information on fuel tank microbiological contamination.
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11 July 72 AC 20-78 CHAPTER 10. MAINTENANCE INFORMATION DC-8 AIRCRAFT MAINTENANCE INFORMATION. The following is & listing of selected significant maintenance difficulties that have been reported by air carriers as mechanical reliability reports from 1970 and 1971 records. This information may be useful in identifying additional structural inspection problem areas. In all cases, check the corresponding trouble area, left right, top bottom, forward aft,etc.
Wing:
During routine inspection a l2" crack was found in the upper R/W skin at W/S Xrs 367. The total time on the aircraft was 30,358 hours. The operators report referred to this as being a corrosion type crack.
During overhaul a 6-inch crack was found in the L/W bulkhead upper cap fitting P/N 5615314-1 at W/S Xrs 508.
The crack was adjacent to the center spar along the bulkhead cap fitting radius. The aircraft total time 38,513 hours.
(3) Routine inspection disclosed a 2-inch crack in the L/W leading edge lower skin P/N 5754700-3 above #2 pylon and a one-inch crack above #3 pylon. A 4-inch crack was also found in the R/W leading edge skin P/N 5754701-3 above #4 pylon. (Reference marufacturer's All Operator Letter (AOL) 8-490 dated 10/28/70.)
(4) Routine inspection disclosed a 2-inch crack in L/W front spar upper splice angle at W/S Xps 63, and an 11-inch crack in the R/w front spar forward cap progressing 1/B from W/S Afs 67.5. Aircrait total time 31,992 hours.
(5) During overhaul found a 4-inch crack in L/W stringer #8 The crack was located along the radius formed by the vertical and 'Y" legs of the splice. Aircraft total time was 34,888 hours.
During overhaul a 2 3/4-inch crack was detected in the L/W lower front spar cap at W/S Xrs 67.5. The crack originated at an attach hole and progressed through two other attach holes. Aircraft total time 17,902 hours.
During approach the aircraft had a tendency to roll with wing flaps extended beyond 25° accompanied by a split flap indication. Inspection after landing disclosed a broken compensator link and shaft, P/N 2647101. Aircraft time since overhaul 1,048 hours.
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AC 20-78 11 July 72
(8) During inspection an 8-inch crack was found in the R/W flap support fitting, P/N 5614374-2, at W/S Xys 219. The aircraft total time was 34,261 hours.
During overhaul a 3-inch crack was found in the L/W support fitting, P/N 5614374-1 at W/S Xrs 219. Total aircraft time was 17,700 hours.
(10) During overhaul a 5-inch crack was found in the radius of attach bracket, P/N 3653812-3 located between the I/B support bearing and the R/H 0/B aileron. Total aircraft time 34,345 hours, time since overhaul 10,613 hours.
(11) Routine inspection disclosed broken mount lugs on the I/H I/B flap actuating cylinder, P/N 5642074-5015. Time since overhaul was 10,915 hours. Operators report attributed breakage to stress corrosion.
(12) One aircraft was taken out of scheduled service and ferried to a maintenance base due to finding broken I/H O/B flap hinge base fittings P/N's 5653754-1 and 5653755-1 at station 219.
Total aircraft time 37,788 hours, time since last service check was 329 hours. Another operator reported a 4-inch crack in the R/H 0/B flap hinge base at station 219. Aircrait total time was 37,640 hours.
Routine inspection revealed partial separation of the L/W flap follow-up cable at W/S Xps 22.750. Cable P/N 5652981-529, total aircraft time l1,346 hours. Operators report attributed separation to be a result of oxidation.
(14) Routine inspection disclosed a cracked R/H O/B flap hinge base fitting at station 339. Total aircraft time was 37,867 hours.
(15) During scheduled inspection an Il-inch crack was found in the crank arm of the R/H wing I/B slot torque tube.
(16) During routine inspection the L/H wing flap drive link, P/N 5655655-501, was found cracked at station 219. Total time since new was 23,145 hours.
During scheduled inspection the R/H flap support bracket, P/N 5614376-2, was found to have a crack approximately 10 inches in length. Total aircraft time was 35,170 hours.
(18) A flight returned due to loss of hydraulic fluid. Inspection after landing disclosed a crack in the R/H aileron actuating cylinder, P/N 5613697-5. The crack was along the circumference of the cylinder barrel.
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11 July 72 AC 20-78
b. Lylon.
(1) Routine inspection disclosed a 12-inch crack in #4 pylon upper 1/B skin at station Yop 230-240. Total aircraft time was 10,419 hours.
During overhaul, angle P/N 5769515-36 and doubler P/N 5769515-155 were found cracked on the I/B side of #3 pylon.
Total aircraft time 10,524 hours.
During routine inspection a chordwise crack was found in # pylon intercostal P/N 5769523-46. The crack was in the horizontal leg and located 1-inch 0/B of the pylon upper 0/B spar plane. Total aircraft time was 8,091 hours.
Routine inspection disclosed the lower spar I/B cap broken in two in #l pylon. The break occurred 12 inches from the found inh crane or spar web at top 248 and 248 and those creast ofted silar ascerpentes 41 pou two skin cracks at I 32,156 hours.
at Yop 243 with a total aircraft time of 30,141 hours.
(5) Inspection found a crack in the vertical leg of the o/B spar cap at station Yop 220 of #1 pylon.
(6) Routine inspection disclosed a cracked top 0/B spar rail at shit in 44 Bylon. Total aircraft time was 34,510 hours. and a 2-inch crack in the web at station Yor A 5-inch crack was found in the 0/B skin of #1 pylon at station Yop 235. Total aircraft time was 8,505 hours.
C. Landing Gear.
(1) During taxi after landing #3 wheel, P/N 152059, failed at the I/B rim, operator reported a total of 2,297 landings. Another operator reported #3 wheel failure during takeoff, total landings since overhaul was 63. Failure was attributed to cracks which originated at corrosion pits.
A flight was interrupted due to inability to retract the MIG after takeoff. Inspection after landing disclosed the L/H MIG down lock bungee cylinder piston rod broken, P/N 4641900.
During walkaround inspection found R/H MG bogie beam broken at the lover swivel lug, P/N 5774700. Operator attributed failure to corrosion due to lack of grease penetration and initiated a program of inspection and modification to reduce possibility of additional failures. A similar condition was also found on the L/H bogie beam of another aircraft.
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(4) A flight returned due to NIG not retracting, inspection found the NIG side brace broken at the retract strut attach lugs, side brace P/N 5771405-5. Total aircraft time 21,030 hours.
Several operators have reported flight interruptions due to left and right MIG retract cylinder attach pins being broken.
These reports showed that the first evidence of malfunction usually an unsafe gear position indication.
(6) Routine inspection disclosed a cracked L/H MIG bungee cylinder attach casting, P/N 5719176-5511.
Flight crew reported a severe jolt when the landing gear was extended and did not get a dom and locked indication. The aircraft was stopped on the runway and inspection showed the L/H MIG was not locked due to broken retract and bungee cylinders.
A flight was interrupted due to a loud roar and vibration when gear was retracted after takeoff, fuel was jettisoned and the aircraft returned. Inspection revealed a broken NIG door link, ₽/N 5646507-501. Total aircraft time was 11,830
(9) One operator reported finding a broken O/B brake link rod on the R/H MG, rod P/N 576390. Total time on rod 10,359 hours.
During routine brake change, #7 brake carrier lug was found broken off. A total of 225 landings had been made since last brake overhaul.
(11) After takeoff,. the left MIG could not be retracted.
Inspection after landing revealed a broken I/B bungee unlock bracket, P/N 6F-8574-501, which prevented the gear from being inlocked from the down position. Total aircrait time was 38,107 hours.
(12) The #4 wheel broke on landing on one aircrait, wheel f/N 1>42>1-1. There had been 100 landings accumulated since the last overhaul.
A flight returned due to the landing-gear warning light remaining on after gear was retracted. Inspection found a broken pivot lock arm, P/N 5769871-1, on the left MIG. Total aircraft time was 21,220 hours.
(14) During scheduled inspection, the R/H MIG forward bogie beam was found cracked at the upper attach lug for the pivot unlock arm. Total aircraft time was 28,269 hours.
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11 July 72 AC 20-78
d. Fuselage.
During inspection found severe corrosion on the inner surface of the aft center section lower skin panel P/N 5615372-3, F/S 1150-1290. Also found four cracks 1/2" to 4" in length and light corrosion on longerons and doublers.
Total aircraft time was 30,963 hours, time since overhaul 1,801 hours.
Routine inspection disclosed a 4-inch crack in the upper aft corner of the rear passenger door jamb skin. Total aircraft time 37,221 hours.
(3) During scheduled inspection cracks were found in the fittings at the top of the pressure bulkhead at F/S 857 in the left and right wheel wells, fitting P/N's 3646874-1 and -2. Total aircraft time 7,771 hours.
Several operators reporting finding cracks in fuselage former #24, L/H side at F/S 920 and 980, cracks ranged from 2 to 4 inches in length. Total aircraft time ranged from 7,771 hours to 33,091 hours.
During overhaul, a 22-inch crack in the 0/B flange radius of the L/H MIG support fitting web, support P/N 5750092-501.
Repairs were made in accordance with SB 63-61.
(6) Several occurrences of excessive corrosion have been reported on metal floorboards in the forward gally, center aisle opposite forward galley, and in the mid-lavatory areas.
Total aircraft time ranged from 8,027 hours to 11,075 hours.
(7) Moderate corrosion was found during routine inspection at the following locations on the fuselage skin between stringers 34L and 34R; F/S 640 to 650, F/S 1075 to 1260, F/S 1300 to 1300, and F/S 1430 to 1480. Total aircraft time was 32,135 hours.
Routine inspection revealed bulged skin and rivets popped at the following locations: F/S 1100, stringer #36 F/S 1300 at stringer #29, F/S 1420, 1440, 1460, and 1240 at 1400, and 1490 at stringers 27 through 31 right. Total aircraft time was 37,399 hours.
(9) Scheduled inspection disclosed corrosion of lower fuselage skin adjacent to fasteners and doublers at F/S 420 to 500 and F/S 1180 to 1300 between stringers 33R and 351. Total aircraft time was 34,510 hours.
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AC 20- 78 11 July 72
(10) During routine inspection corrosion was found in the lower aft corner of the forward service door jamb and skin laps.
Total aircraft time was 34,510 hours.
e. Miscellaneous.
(1) An attempt to dump fuel in flight failed. Inspection after landing disclosed a bent I/H dump chute link rod, P/N 3765140.
The first officer's windshield outer panel shattered during flight on one aircraft. The aircraft was at 28,000 feet with windshield heat in warm position.
(3) During routine inspection, two 5-inch cracks were found in the rudder fiberglas section at station Zy 253.0.
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11 July 72 AC 20-78 Appendix 1
STATIONS DESCRIPTION. The airplane is divided into reference points in inches, designated as station numbers. These station numbers provide a means for quickly identifying the location of components, the center of gravity, and the distribution of weight. There are three axes used in the plotting of station numbers: The lateral or X-axis, the longitudinal or I-axis, and the vertical or Z-axis.
a. Glossary of Terms.
(1) FRP The fuselage reference plane is used as a basic reference. It extends along the fuselage cusps which are formed by the intersection of the upper and lower sections of the fuselage. The reference plane has an angle of one degree nose down with reference to the ground and is the origin of Z-stations.
(2) WRP The wing reference plane contains the trailing edge of the wing and has an angle of dihedral of 6-1/2 degrees to the FRP.
(3) VRP The vertical reference plane (plane of symmetry) bisects the airplane through its centerline at an angle of 90 degrees to the FRP.
(4) X-axis Divided into segments in inches along the lateral plane.
(5) Y-axis Divided into segments in inches along the longitudinal plane. There is a I-axis subsystem designation for all subsystems.
(6) Z-axis Divided into segments in inches along the vertical plane.
b. Wing Subsystem Designations.
MOX (I) B62: This subsystem designation refers to the center, or constant, section of the wing. It consists of stations in inches, parallel to the centerline of the airplane and extending outboard 69.5 inches from its origin at the airplane centerline to its 7x (8) This wing subsystem designation denotes stations in inches, parallel to the centerline of the airplane and perpendicular to the WRP, beginning at the dihedral break and extending outboard indefinitely.