AC 20-64 Page 5 8/1/69 CHAPTER 2. WING
11. L-188 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, wing structural references apply to both left and right wings.
Lockheed Reference
a. Lower Wing Skin Planks. For Details
(1) Lower wing skin plank damage was caused SIL-61 by integral engine start system compressor 10/17/62 line rubbing in #2 and #3 nacelles.
(2) Cracks occurred in the lower #3 wing skin SB 601 plank forward of the MG fulcrum fitting. 6/24/63 (Over 900 hours TSN.)
(3) SB 587 alert wire was issued to inspect #4 SB 587A lower wing plank drain hole for cracks at 4/10/64 station 159. Plus AD 65-15-4
(4) Cracks have been found at the fastener holes SB 631 around the fuel sight gage and fuel overflow 9/6/65 openings on the left- and right-hand lower wing planks. The cracks occur at wing station
128.0 and in the wing reinforcing doubler at station 266.0.
(5) Cracks have been found in lower wing plank #1 SB 653 between WS 205 and WS 275. 7/3/68
b. Upper Wing Skin Planks.
(1) Cracks have been found which appear to be SB 565 static fractures originating on the #l 4/27/62 upper wing plank inner surface.
(2) Cracks have been found in the upper surface SB 619A in the area of the main landing gear rib 5/21/65 forgings. These areas were previously rein- Plus AD forced by doublers in accordance with SB 65-9-4 306 and 337 except aircraft, LAC serial #1145 thru 1148 and 2020-2022, which were delivered with internal reinforcements.
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(3) Cracks have been found at screw holes 1n the SB 632 wing upper plank #2, at the plank #1 and #2 6/29/65 splice between WS 212.0 and 230.0.
Cracks have been found in the trailing edge of SB 634A wing planks #6, #7, and #8 at wing station 143 10/8/65 LEFT and RIGHT upper surface. In two cases, the backup fitting, P/N 803900, for the flap track attachment was also cracked.
(5) This reference applies to the inspection of SB 6250 wing plank splices aft of the main landing 5/16/66 gear fulcrum fitting and provides additional Plus AD repair 1nstructions for the WS 209 area (see 66-11-2 d(2) page 7).
This revision provides instructions to reduce SB 600A the possibility of cracks developing in the 2/21/68 upper wing planks under the nacelle skate Plus AD The original bulletin pertained only 64-11-3 to the inboard nacelle skate angles, but this revision supplies reinforcement for ALL
(7) A considerable number of cracks have been found SB 649 in the #l upper wing plank at the nacelle 2/29/68 fillet attachments. Cracks have been found at SB 665 the inboard side of all four nacelles at the Plus AD's attachment of the nacelle fillet with the 68-11-2 upper #1 plank. 68-23-5
(8) Cracks have been found in the #3 upper wing SB 652 plank along the first row of fasteners 4/30/68 adjacent to the skate angle of the outboard nacelle at WS 346.0.
c. Outer Wing to Center Wing Attach Fittings.
This SB revisionwas issued to identify SB 633A/633B requirements for the outer wing front cap fittings, 9/3/65 8 P/N 807352 and P/N 807354; specifically, corrosion 9/24/65 of fittings, attachment bolts, and boltholes. Plus AD 66-5-3
d. Wing - MG Ribs. These service bulletins instruct operators as follows:
(1) To inspect the MG ribs in each wing for SB 630 fatigue cracks in the web fillets at the 7/20/65 junctures of the rib truss members with the rib upper chord member. Similar cracks Par 11 Chap 2
AC 20-64 Page 7 (and 8) 8/1/69 have also been found in the web fillet between the forward truss member and the upper chord member beneath the #? wing plank.
(2) To inspect the vertical legs of the upper and SB 639 lower caps of the wing ribs for cracks at 11/7/66 wing stations 167 and 209.
Wing-to-Nacelle Attachment Plate. Cracks have been SB 638 found in the wings-to-nacelle attachment plate, 10/17/66 P/N 810054-1, which is located inside the inboard nacelle on the lower front beam of the LEFT WS 209.
(Propeller torque loads on the #2 nacelle augment the normal stresses on this plate.)
f. Wing Rear Beam Clips. Cracks have been found in P/N SB 654 805483-1 (WS 179), and P/N 805491-1 (WS 197) rib 4/8/68 attach clips location (dry bay area), at the rear beam forward face upper attachment. Cracks occur in the 0.16 radius of the flange relief cutout and extend toward the nearest fastener hole.
& - Wing Upper and Lower Access Door Lands. Corrosion SB 606 found on the lower wing plank access door lands 11/18/63 appears to be caused by condensation. (Door lands are the area surrounding the access opening in the wing plank.)
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AC 20-64 Page 9 8/1/69 CHAPTER 3. FUSELAGE
12. I-188 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, fuselage structural references apply to both left and right sides.
Lockheed Reference
a. Fuselage Stringers Station #1117. This For Details service bulletin provides for inspection SB 574 of the underfloor stringers at the aft 10/3/62 pressure bulkhead for cracks. The Revised 12/26/62 bulletin was later revised to cover all underfloor stringers, 1 thru 72.
b. Flight Station Pressure Deck. Cracks have SB 609A been found in the flight station pressure 11/2/63 deck structure, butt line 40 right beam, Revised 3/30/65 lower beam cap flange. Cracks in this location could result in a loss of cabin pressure.
Reported cracks have occurred on aircraft with over 13,000 total flight hours.
Forward and Aft Cargo Door Frame Corners. SB 608A Cracks have been found in the lower forward 2/5/65 and aft cargo door frames. Revised SB 608B 2/18/65
d. Fuselage Main Frame Fittings. A considerable SB 644 number of cracked main frame fittings have 2/10/67 been found at FS 570 and FS 695 Revised SB 644A 12/5/67 Plus AD " 67-11-4 Fuselage Bulkhead Ring Intersection with Center SB 646 Wing. Cracks have been found at the intersection 4/12/68 between the wing center section lower surface and the fuselage rings at fuselage stations 571 and 694.
f. Cabin Window Support Structure. Cracks have SB 647 been found in fuselage station 853.5, (cabin 5/6/68 window support structure.)
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g. Canted Fuselage Frame Upper Web. Cracks have been SB 579 round in the canted frame upper web, station 1167.7. 1/31/63 (See figure D-4 in appendix 4, page 4.) Revised SB 579A 6/17/68 Plus AD 63-17-3 Par 12 Chap 3
AC 20-64 Page 11 8/1/69 CHAPTER 4. FLIGHI CONTROLS
13. I-188 AIRCRAFT MAINTENANCE INFORMATION. The following 18 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, flight control structural references apply to both left and right sides.
Lockheed Reference For Details a . impennage.
(1) Elevator Counterweight Installation. SB 457 Excessive cumulative free play has been 7/15/60 found in the joints of the elevator counterweight balance arm linkages.
(2) Empennage Drain Hole. This service bulletin SB 573 was issued to prevent moisture accumulation 9/26/62 in the empennage by providing improved drainage.
(3) Canted Fuselage Station 1167.7 Frame Upper SIL-62 Web. Cracks have been found on both sides 11/17/62 of horizontal stabilizer station 29 at the lower cap in the 0.62-1nch radius.
(4) Rudder Trailing Edge. Cracks have been found SB 602 in the outer skin and beaded inner skin of the 7/3/63 rudder trailing edge. Cracks starting from holes at the aft end of the beads weaken the structure and cause additional cracks to develop in the outer skin at the spotwelds.
Spotwelds that may crack are located 1½ to 2 inches forward of the rudder trailing edge.
(5) Vertical Stabilizer to Fuselage Attach Channel. SB 616 Cracks have been found in the vertical 7/24/64 stabilizer to fuselage attach channel, P/N Revised SB 616A 803179-5, located at fuselage station 1117.6. 4/5/65 Plus AD 65-21-5
(6) Elevator Balance Weight Arms and Ribs. This SB 567 modification will eliminate repeated inspec- Revised 5/9/62 tion of the elevator balance weight arms SB 567C (for cracks), required by the original SB 7/30/65 567, revision 1, dated 12/3/62.
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b. Ailerons.
Aileron Skin Doublers. Cracks have been SB 444 found in the lower aileron skin between 5/27/60 the trim tab actuating rod access door and the rear beam, notably in the area where the forward and aft flanges of the internal beaded doublers mate with the skin.
(2) Aileron Counterbalance Attaching Angles. SB 451 Cracks have been found in the aileron 5/10/60 counterweight attaching angles. Revised 7/25/60 Plus AD 60-25-2
(3) Aileron Push-Pull Tubes. Cracks have been SB 607 found in the aileron push-pull tubes, a.so 10/17/63 wear, and misalignment of the support Plus AD assemblies and rollers. 64-16-4
c. Flaps.
(1) Wing Flap Torque Tubes. Operators have SB 570 reported flap torque tube support bearings 6/20/62 binding and/or rubber sleeves deteriorated or missing. This results in torque tube wear caused by this tube turning in the inner race of the P/N KP16BS bearing.
(2) Wing Flap Track Attaching Bolts. Operators SB 591 have reported extensive damage to a flap 4/3/63 due to the failure of the outboard lower Revised SB 591A flap track attach bolts. 1/7/64 Par 13 Chap 4
AC 20-64 Page 13 (and 14) 8/1/69 CHAPTER 5. LANDING GEAR
14. I-188 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, structural references on the main landing gear (MIG) refer to both left and right gears.
Lockheed Reference For Details
a. Main Landing Gear Actuator Support Fittings, SB 599A Cracks have been found in the MIG actuator 10/22/63 support fittings, P/N 800618.
b. Main Landing Gear Actuating Cylinder Attach SB 593A Fitting Intercostal. During accomplishment of 7/12/63 SB 538, a crack was found in the wing plank SB 593B surface of one aircraft and a crack in the wing 1/24/64 plank riser of another aircraft. Both cracks Plus AD were located under the MG actuator cylinder 64-14-5 attach fitting.
Nose Landing Gear Steering Housing. Cracks have SB 576 been found in the nose landing gear steering 10/29/62 housing. These housing cracks originate at the Revised 4/8/63 inboard end of the 4-1/8" - 12 UNS-3A screw thread SB 576B that receives the retaining nut of the steering 3/17/64 cylinder. The cause is attributed to fatigue Plus AD failure of the thread root. 66-28-4
d. Nose Landing Gear Assembly Trunnion Locking SB 604 Slots. Cracks have been found in the fulcrum 10/18/63 bearing spacer locking slots in the NIG LEFT Revised SB 604B trunnion arm. 6/29/65
e. MIG High Heat Treat Steel Plating and Stripping SIL-88 Practices. Landing gear shock strut components 7/25/66 have failed in service after passing through major overhaul and replating procedures.
f. Main Landing Gear Lubrication. This SIL outlines SIL-32 revised maintenance procedures and improved 5/23/62 lubricants for landing gear static and dynamic joint corrosion protection.
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AC 20-64 Page 15 8/1/69 CHAPTER 6. CORROSION PRONE AREAS 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.
16. 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.
17. 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.
BATTERY AREAS AND VENIS. Due to the highly corrosive nature of battery acid (sulfuric acid) and its fumes, battery areas and their vents are protected by special acid-resistant paints. Generally this is enough to stem the tide of corrosion if scrupulous attention is devoted to keeping LAVATORIES, GALLEYS, AND CABIN FLOORS. The usual spillage, condensation, and other contamination of these areas are extremely corrosive to aircraft metals. The most common 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 relief and waste disposal vents or openings on the exterior of the aircraft.
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20. 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 of flying debris during takeoffs and landings, and when the aircraft is parked 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:
a. Magnesium wheels (especially the areas around the bolt heads, lugs, and wheel webs).
b. Portions or rigid tubing obscured by clamps and identification tapes.
c. Exposed electrical equipment.
d. Crevices between ribs, stiffeners, and lower skin surfaces which can serve as water or debris entrapment areas.
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.
22. SPOI-WELDED SKINS. c'orrosive 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.
23. 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.
LAP JOINTS BEIWEEN ALUMINUM ALLOY EXTRUDED SECTIONS. Extruded sections of high-strength materials such as 7075 or 7178 aluminum alloys are more susceptible to intergranular corrosion than are other aluminum alloys.
Application and maintenance of chemical processes, sealants, paints, or Par 20 Chap 6
AC 20-64 Page 17 8/1/69 combinations of these protective measures have proven most effective in controlling corrosive attack on these materials.
If intergranular corrosion occurs, it is usually found around fastener:
r 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 suriaces. lhese 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.
25. FLUID ENTRAPMENT AREAS. Design specifications require that aircraft nave drains installed in areas where water may collect. However, li rains 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.
When considering fluid entrapment areas, one inevitably thinks of aircraft drinking water and wash water systems. Corrosion in these systems is 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.
26. ELECTRONIC PACKAGE COMPARTMENTS. 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 ar as should be inspected for corrosive attack as thoroughly as possib?
during routine checks, and advantage should also be taken of nonscheduled component removals for further inspection.
27. 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 ma be detected. It is recommended that control cables be inspected periodically, then cieaned and treated as necessary before reapplication of the preservatives.
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28. MICROBIOLOGICAL FUEL CONTAMINATION 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 Lockheed's "Field Service Digest issue of March-April 1961, Volume 7, Number 5, which is devoted to a full explanation of microbiological contamination, corrosion, treatment and prevention. Likewise, we recommend Lockheed's "Field Service Digest" issue Number 49, dated December 1965, which 1s devoted to general corrosion detection, removal, and control.
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