11 July 72 AC 20-78 CHAPTER 6. PYLONS/NACELLES DC-8 AIRCRAFT MAINTENANCE INFORMATION. The following is a listing of significant maintenance difficulties that have been reported by air carriers. This information may be useful in identifying structural inspection areas. Except where noted, pylon/nacelle structural references apply to left and right sides of the aircraft.
Access doors have been found cracked on SB 54-7 the R/H side of nacelles in the area of 5/18/60 cooling air exit ducts.
Cracks have been found in the tunnel and SB 54-17 junction box of each pylon. The cracks Rev. #1 occurred due to the welded seam which 9/26/61 joins the tunnel to the junction box.
Cracks have been found in the zee angles SB 54-20 located on the left side nacelle door in Rev. #1 the area of the upper aft pylon drain 8/7/61 fitting support.
d. Lower 0/B pylon-to-wing attach angles and SB 54-24 flexible ribs below pylon spars have been Reissued T/25/62 found cracked. and Revised 8/20/62
e. The vertical leg of the 0/B spar cap on SB 54-27 the O/B pylon upper spar was found cracked 1/26/62 at station Yop 246.000.
1. Failure of the upper I/B spar cap structure SB A54-33 has occurred in the O/B pylons in the area Rev. #2 1/24/64 oi station I 230 at the edges of support and AD 62-27-4 fitting P/N 3647306-501.
g. Failure of the upper 0/B spar cap structure SB 54-34 has occurred in the I/B pylons in the area of Reissue #l stations Yip 225 to Yip 233. 7/6/64 and AD 65-5-2
h. Cracks have been found in the I/B and 0/B Daco Service pylon stub wing structure. The cracks have Engineering occurred in skins, bulkheads, and spar caps. Letter C1-78-2016/DBA 10/18/66 SB 54-57 Rev. #1 12/9/69 and AD 67-5-3 Chap 6 Par 15 Page 17
AC 20- 78 11 July 72 Skin and doubler cracks have been found on SB 54-41 the 0/B side of the O/B pylons between Rev. #1 aircraft vith four percent extended wing <30.000 and Top 240.000 on 9/27/68 leading edge. Aircraft fuselage #148 and and subsequent were modified prior to delivery.
Cracks have been found on 0/B pylon stub SB 54-42 wings on aircraft with more than 6,000 1/6/67 flight hours. The cracks occurred at station Xw 454.000 and the I/B intersection adjacent to the leading edge lower slot door.
Operators have reported cracks in the SB 54-43 Pylon cant bulkhead fitting on aircraft Rev. #1 with 17,000 and 18,000 flight hours. 5/12/67
1. Cracks have been found in the D-duct spars, SB 54-44 adjacent structure, and pylon skin of the Rev. #1 I/B pylons. The reported cracks were on 2/10/67 aircraft with 14,000 to 21,000 flight hours between station Lip 204.000 and Iip 220.000.
Cracks have been found in the I/B pylon stub SB 54-49 wing skin and internal doubler at the cutout 8/25/67 for the wing leading edge lower slot door on The cracks were detected in the area aft of Thrust reverser stow latch pivot bolt failures SB 54-52 have occurred. This applies to series 60 3/28/68 aircraft; fuselage #332 and subsequent were modified prior to delivery.
Five failures of forward thrust reverser stow SB 54-53 latch on series 60 aircraft at approximately 8/20/68 2,000 flight hours have occurred.
An alert bulletin was issued due to reported SB A54-58 cracks in P/N 5753016-1 and -2 wing skin Rev #1 pylon support doubler. The cracks originated 12/16/68 at the forward edge of the doubler in the intersection of the I/B side of the I/B pylons and the wing front spar lower cap at W/S Xw
234.000. Aircraft fuselage #451 and subsequent were modified prior to delivery. Although cracks were not reported in 0/B pylons, the structure is similar and should also be inspected.
Chap 6 Page 18 Par 15
11 July 72 AC 20-78 CHAPTER 7. STABILIZERS
16. DC-8 AIRCRAFT MAINTENANCE INFORMATION. The following is a listing of significant maintenance difficulties that have been reported by air carriers. This information may be useful in identifying structural inspection areas. ixcept where noted, stabilizer structural references apply to leit and right sides.
Cracks have been reported in the formers SB 55-7 of the vertical stabilizer leading edge at 10/18/61 stations Zes 117.250 to Zfs 179.750 and Zfs SB 55-11
186.000 to Zfs 350.500. 5/2/63
b. Cracks have been detected in the radius of SB 55-8 the vertical leg of the horizontal stabilizer Rev. 1 closing bulkhead caps, P/N 5654549-1, -2 8/13/62 -501, and -502. The cracks have been up to 4" in length in the aft end of the caps.
Operators have reported cracked formers in SB 55-10 the horizontal stabilizer leading edges 5/2/63 originating at the aft end of the formers.
Flying time on aircraft with reported cracks was in excess of 5,000 hours.
Fatigue cracking has been reported in the SB 53-45 vertical stabilizer front spar to fuselage Rev. #1 attach fitting at F/S 1500.000 on aircraft 3/12/68 with 15,000 to 20,000 fight hours.
Chap ar 16 Page 19 (and 20)
11 July 72 AC 20-78 CHAPTER 8. MISCELLANEOUS
17. DC-8 AIRCRAFT MAINTENANCE INFORMATION. The following is a listing of significant maintenance difficulties that have been reported by air carriers. This information may be useful in identifying structural inspection areas.
a. Cracks have been found in the vertical SB 51-2 flange of the keel beam fittings between Reissue #1 the center and rear spar. 1/3/61 Operators have reported cracks in fuselage SB 51-7 frame at refrigerant condenser forward 9/3/63 exhaust flap hinges.
C. Forward and aft passenger entrance door SB 52-14 mechanism housings have been found cracked. 9/26/60 The cracks were a result of maladjustment of the latching mechanism.
d. The forward upper cargo door mechanism apex SB 52-47 pin assembly has been found cracked on aircraft rev. #1 with approximately 8,000 hours. 8/16/68 Chap 8 Par 17 Page 21 (and 22)
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.
Chap 9 Par 18 Page 23
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.
Chap 9 Page 24 Par 23
1l July 72 AC 20-78
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.
Chap 9 Par 27 Page 25
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.
Chap 9 Page 26 Par 30
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.
Chap 10 Par 32 Page 27
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.
Chap 10 Page 28 Par 32