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.
Chap 6 Par 15
Page 16 AC 20-64 8/1/69
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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Page 18 AC 20-64 8/1/69
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.
Par 28 Chap 6
AC 20-64 Page 19 8/1/69 CHAPTER 7. MAINTENANCE INFORMATION
29. I-188 AIRCRAFT MAINTENANCE INFORMATION. The following 18 a listing of significant maintenance difficulties that have been reported by air carriers as mechanical reliabil1ty reports from 1967 and 1968 ADP 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
(1) During a routine visual inspection, two cracks were found in the lower right forward front beam cap wing stub fitting, P/N 819229. Each crack was 1 inch long at butt 11ne 65. Another crack 1 inch long was found at the top vertical support I- angle at station 49 below No. 6 wing plank. The aircraft total time was 19,451 flight hours.
(2) During a visual inspection, the lower left inboard forward wing-to-box-beam fitting, P/N 819229-1, was found cracked.
(3) An 18-1nch spanwise crack progressing outboard from WS 155 at the splice between #3 and #4 upper right wing planks was found during a visual inspection.
(4) During a fleet campaign using visual, magnifying glass, dye penetrant, and boroscope inspection methods, the forward top beam cap inboard of No. 2 nacelle at station 165 was found cracked. The cracks were one-eighth inch long and emanated from a screw hole towards the aft end of the beam cap. The aircraft total time was 21,953 flight hours.
(5) During maintenance, the left wing truss rib was found cracked at station 167. The total airframe time was 20,378 flight hours.
(6) During a visual inspection a 4-inch crack was found in the left rear beam wing stiffener at station 204.7. The aircraft total time was 20,254 flight hours.
(7) A routine maintenance inspection revealed a 3/4-inch crack in the left wing forward lower spar cap tang at station 331.
The airframe total time was 22,311 flight hours.
(8) During a scheduled ultrasonic inspection, the following cracks were found:
Chap 7 Par 29
Page 20 AC 20-64 8/1/69
(a) A 36-inch crack was detected at the forward end of No. 2 fuel tank access plate at the forward end of No. 4 wing plank. The crack extended from a point midway in the access plate to a point 36 inches inboard of the access plate. The crack depth varied from six thousandths of an inch to complete fracture over a 2-inch span.
(b) The same type crack (as (a) above) was found at No. 3 fuel tank access plate. The crack extended from a point midway in the access plate to a point outboard of the access plate.
During a fleet campaign inspection, 1/4-inch cracks were found emanating from the inboard No. 3 nacelle fillet fairing first and third attaching screw holes in No. 1 wing plank. The aircraft total time was 21,563 flight hours.
(9) On a routine inspection, the No. 4 sight gage aft mounting screw hole was found with a 1-inch crack in the leap doubler.
The airframe total time was 20,708 flight hours.
(10) During inspection wing skin cracks were found at two screw holes around fuel gages between No. 3 and No. 4 nacelles. The aircraft total time was 22,287 flight hours.
(11) Inspection disclosed a 3-inch crack in the inboard side of the No. 4 fuel tank surge box. The total airframe time was 21,528 flight hours.
(12) Maintenance inspection revealed a crack 5 inches aft of the hinge on the access panel at the trailing edge of the wing, 5 feet outboard of No. 4 engine. The aircraft total time was 24,074 flight hours.
(13) A 1/2-inch crack was found in the left wing tip upper aft former. Total airframe time was 21,520 flight hours.
(14) During a scheduled inspection a 3 1/2-inch crack was found in the left outboard wing-to-nacelle attach plate, P/N 818854-1.
The crack occurred 1 5/8 inches from the bottom edge of the plate. Total aircraft time was 21,686 flight hours.
(15) Overhaul inspection revealed random cracking of the wing and nacelle skin former webs and stiffeners at the following locations:
(a) No. 1 nacelle former flange outboard side station 0.0.
Par 29 Chap T
AC 20-64 Page 21 8/1/69 No. 2 nacelle former attach angle at lower outboard longeron at aft hinge for No. 1 main landing gear door.
(c). No. 3 nacelle attach angle at lower aft edge of No. 3 forward main landing gear door. Total airframe time was 17,700 flight hours.
b. Fuselage.
Inspection revealed fuselage skin crack on the left side above the aft power receptacle. The airframe total time was 22,603 flight hours.
(2) During a fleet campaign, three 1/2-inch skin cracks were found at the lower aft baggage door frame. Cracks occurred at the lower forward frame radius. Total aircraft time was 18,752 flight hours.
(3) Inspection disclosed cracked corners above and below the No. 9 left and right emergency exit windows. Crack lengths were 1/8 to 1 1/4 inches. The total airframe time was 17,797 flight hours.
During a routine visual inspection, a 2-inch crack was detected in the lower rear corner of the passenger door frame.
Total time on the aircraft was 19,733 flight hours.
(5) During a visual inspection, a complete fracture of a fuselage web former was found at fuselage station 107.4. The crack was on the right side below the rudder pedals. Total time on the During a walkaround visual inspection, confirmed by dye penetrant inspection, a 2-inch crack was found in a fuselage cap strip at station 118.8. The affected cap strip is on the right side of the nose gear wheel well, below the cockpit flooring. Total time on the aircraft was 22,547 flight hours.
(7) An overhaul inspection revealed two fuselage skin cracks. One was found on the left side of the fuselage at the nose wheel well cutout at fuselage station 186. The other crack was found below and to the right of the No. 2 hydraulic pump, between stringers 28 and 29 at fuselage station 738. Time on the aircraft was 17,700 flight hours.
(8) During a dye penetrant inspection, a 3/8-inch crack was found in the right rear fuselage-to-wing attaching ring, P/N 801031-102, located just above the floor line. Total time on the aircraft was 20,310 flight hours.
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Page 22 AC 20-64 8/1/69
(9) A crack was found in a pressure web at butt line 40, fuselage station 195. The pressure web affected is located on the right side. Total time on the aircraft was 22,781 flight hours.
(10) A 1 1/2-1nch crack was found in a former r1b on the right side at station 550. The crack was located 6 inches below the floor level. Total time on the aircraft was 20,568 flight hours.
(11) During overhaul inspection, several fuselage structure cracke were detected as follows:
(a) Former flange cracked on left and right sides of fuselage at stringers 30 and 31, station 570.97.
(b) Former flange cracked at fuselage station 1150 at stringers 7, 10, and 12 in the tail compartment.
(c) Former flange cracked at station 1133 at stringers 70 and 71 in the tail compartment.
(d) Former flange cracked at station 768 at stringer 23 located above floor level in the hydraulic compartment.
(e) Former flange cracked at upper aft end of forward cargo pit door.
(f) Right side cabin floor beam flange cracked at station 652.
(g) Right and left side cabin floor beam flange cracked at fuselage station 580 above the forward spar.
(12) During structural modification to a cargo configuration, the contractor found the left aft main fuselage frame fitting cracked at station 695. Total time on the aircraft was 19,276
(13) During routine visual inspection, a 3-inch crack was detected across the belt frame at fuselage station 715.50. The crack was on the left side at the floor line. Total time on aircraft was 14,945 flight hours.
(14) During inspection, the belt frame on the right side of the fuselage at stringer 19, station 730, was found cracked through.
(15) During a scheduled inspection, stringer No. 37 was found cracked at fuselage station 1113. The crack occurred at the forward clip fastener hole. Total time on aircrart was 19,778 flight hours.
Par 29 Chap 7
AC 20-64 Page 23 8/1/69 Fuselage stringer No. 49 was found cracked at fuselage station 1117. The crack occurred at the No. 5 fastener on the forward side at the rear pressure bulkhead. Total time on the aircraft was 20,842 flight hours.
(17) Fuselage formers were found cracked at the 12 o'clock position at fuselage station 1133, and at the 9 and 2 o'clock positions at fuselage station 1158. Total time on the aircraft was 21,898 flight hours.
(18) Fuselage formers were found cracked at 9, 10, 11, 11:30 and 2 o'clock positions at fuselage station 1150, and at the 9 o'clock position at fuselage station 1167. Total time on the aircraft was 21,932 flight hours.
(19) Right and left reinforcing angles, P/N 803185-47 and -48, were found cracked at station 1185. One crack was found in the former-to-skin attachment at fuselage station 1167, and another crack was found at station 1133. Total time on the aircraft was 21,151 flight hours.
(20) Fuselage former attach tab was found cracked at the 10 o'clock position at fuselage station 1193. Total time on the aircraft was 22,207 flight hours.
c. Flight Controls.
(1) A 1 1/2-foot fatigue, crack was found at the trailing edge of the wing flap. The crack was located aft of the Mylar strip at the rear of the No. 2 engine.
(2) A 9 1/2-inch crack was found on the right horizontal stabilizer front beam web near stabilizer station 29. Total time on the aircraft was 19,853 flight hours.
(3) During inspection, the No. 8 balance weight open type arm, P/N 816900-4, on the left elevator was found cracked. Total time on the aircraft was 19,452 flight hours.
(4) During main base check, a 1 1/2-inch crack was found in the vertical stabilizer right attach angle at station 1l17. The crack had progressed three-fourths inch forward and aft through the Hi-Lok fastener hole. Total time on the aircraft was 19,437 flight hours.
Chap T • Par 29
Page 24 AC 20-64 8/1/69 Pilot reported No. 3 engine vibration indication at and above 220 knots. The vibration was also noticeable by slight buzzing of the power lever on the ground. During cruise, a definite vibration was experienced in the ailerons, elevators, and rudder. The engine vibration was pronounced above 800 degrees engine tailpipe temperature. Inspection of the control surfaces revealed rudder rib damage requiring reinforcement.
The rudder was replaced. Total time on the airframe was 15,949 flight hours.
d. Main Landing Gears,
(1) During overhaul inspection two 3/4-inch cracks were found at the top of the left main landing gear inboard truss rib tabs at wing station 167. The cracks originated from the tab cutouts extending downward and aft toward the truss cap between risers Nos. 16, 17, 18, and 19 of the No. 3 upper wing plank. Total time on the aircraft was 21,734 flight hours.
During inspection, the left main landing gear door actuator fitting, P/N 802058-1, was found broken. The mounting bolt was holding the fitting in place. There were 857 landings since the last removal of the door actuator. Total time on the aircraft was 20,268 flight hours.
During a dye penetrant inspection, a 1/2-inch crack was detected on the left main landing gear actuator support fitting, P/N 841275-101. The crack was located aft of the outboard hole.
Total time on the aircraft was 14,356 flight hours.
e • Corrosion.
During inspection, exfoliation corrosion was found on the left fuselage longeron, P/N 897596-1, between fuselage stations
474.5 and 484.5. Total time on the aircraft was 20,740 flight hours.
During a main base check inspection, corrosion was found in several spots on the aft cargo door fuselage doubler. The bottom edge of the aft cargo door sill is covered by a stainless steel striker plate. Total time on the aircraft was 17,721 flight hours.
(3) Heavy corrosion was found on the floor beams at fuselage stations 333 and 371. Total time on the aircraft was 21,520 flight hours.
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AC 20-64 Page 25 (and 26) 8/1/69
(4) Three separately corroded areas were found on the left upper wing at station 65. The corroded areas were .030 inch deep and 1/2 inch in diameter around the fuselage-fillet-to-wing attach fasteners at wing planks Nos. 1, 6, and 7. Total time on the aircraft was 23,315 flight hours.
During scheduled inspection, corrosion was found on the lower wing surface. The corrosion occurred under the fuselage-towing fillet chafe strip at numbers 4, 5, and 6 wing planks.
Four spots of corrosion were found measuring .011 inch, .024 inch, .024 inch, and .013 inch. The areas were treated and and filled with epoxy filler before installing repaired doublers. Total time on the aircraft was 21,715 flight hours.
(6) Corrosion was found on the lower section of the captain's sliding window frame. Total time on the aircraft was 21,620 flight hours.
(7) During inspection, an area of approximately 12 inches by 6 inches on the outside skin at stringer No. 30 and fuselage stations 500 to 549 was found to be corroded beyond limits.
Also, two small areas approximately 1 inch square were found corroded at stringer 22, fuselage stations 685 and 690. Total time on the aircraft was 24,649 flight hours.
Miscellaneous.
(1) An unscheduled landing was made as a precautionary measure due to in-flight cracking of the first officer's windshield. The windshield heat was in the low position. The aircraft was ferried to Buffalo, where the windshield was replaced. Flight time since the last inspection was 17 hours. Total time on the aircraft was 19,644 flight hours.
(2) The No. 4 oil cooler flap door was found cracked at the door actuator attach bracket. The crack was approximately one-half inch in length and progressed out of a screw hole toward the door hinge. The oil cooler flap door, P/N 836505-9, was replaced. Total time on the aircraft was 14,286 flight hours.
(3) During ground inspection, the No. 4 propeller oil access door, /N 811567-11, was found excessively worn. The wear allowed the upper hinge end of the door to protrude sufficiently to be caught by the propeller blade cuff and be torn free. The aircraft was ferried to its main base, where the propeller cowl panel, P/N 813395-27, was replaced and the propeller cuff repaired.
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AC 20-64 Appendix 1 8/1/69 Page 1 FIGURE A-1. BASIC DIMENSIONS
ELEVATOR HINGE O% CHORI BUTT LINE
357.03 29 FT 1 IN.
SUIT FWD FACE FRONT LINE BEAM: 19.11% CHORD •
186.97 42 FT 10 IN.
29 FT 7 IN.
• 4 FT 7 5 IN AFT FACE FRONT BEAM: 15% CHORD 7 FT 7 IN.
FWD FACE REAR BEAM: 58,56% CHORD & RUDDER HINGE:
& AILERON HINGE: 72.5% CHORD 70% CHORD
WATER LINE FUS - 415.0 STA
571.19 17 FT 4 IN 32 FT .10.5 IN.
WATER LINE 150.00 .000 g000 000. 000 10° 5F5 IN 8FT 5 IN - FLOOR 8 FT 1 IN WATER LINE
- 5 FT 1 IN. 1 FT 9 IN. 123.94 37 FT O IN 104 FT 8 IN
42.00 STA FUS 177.00 FUS STA 621.00 FUS STA STATIC GROUND 195,000 LBS WITH C.G. @ 25% M. A.C.)
11 FT 4 IN. MAX DIA
- 3 FT 3.7 IN. CLEARANCE 13 FT 6 IN
• WING REFERENCE PLANE
- &7 IN.