Friday, December 21, 2012

Cessna 210 Centurion Spar Inspection


               Cessna published SEL-57-01 after receiving reports from the Australian CASA of cracks in the lower spar inboard of wing station 43. This service letter requires a repetitive inspection of the area at a 100 hour or annual interval, whichever occurs first on all cantilever wing Cessna 210’s. When this service letter is accomplished, a report form must be returned to Cessna reporting the airplane’s condition: cracked or un-cracked.

             The Federal Aviation Administration published AD2012-10-04 May 21st, with an effective date of June 5th, 2012, requiring a one-time inspection and report of findings to the FAA, Wichita ACO. Cessna encourages continued compliance with the service letter inspection interval and reporting in addition to compliance with the AD. 
                    Cessna is working with the FAA to provide terminating action to this issue while maintaining the safety and viability of the 210 fleet. Terminating action in this instance can be a life-limit and part replacement program, or a damage tolerance-based inspection program.
                Cessna is moving forward with an engineering study to develop a damage tolerance inspection program which if successful will allow airplanes to remain in-service without spar replacement.
                  This program will include a flight strain survey, material tests of the spar cap and analysis of the data to complete crack growth models; and component testing to verify the residual strength capability of the spar cap with a crack.
             Since publication of this service letter, two additional airplanes have been found with a lower spar cap cracked. A summary of the incidents appears in the table below.

Thursday, September 20, 2012

Inspection Requirements -


 Two types of inspection requirements are available based on operating usage and two additional types of inspections are available based on operating environment.
(1) Operating Usage
     (a) Severe Usage Environment
          1 If the average flight length is less than 30 minutes, then you must use the SEVERE
              inspection time limits.
          2 If the airplane has been engaged in operations at low altitudes such as pipeline
             patrol, fish or game spotting, aerial applications, police patrol, sightseeing, livestock
              management, etc. more than 30% of its life you must use the SEVERE inspection
             time limits.
    (b) Typical Usage Environment
        1 If neither 2(A)(1)(a)(1) or 2(A)(1)(a)(2) above applies, the TYPICAL usage
           environment applies.
                                                                           

(2) Operating Environment
    (a) Severe Corrosion Environment
         1 If the airplane is operating more than 30% of the time in a zone shown as severe on
            the corrosion severity maps in Section 2A-30-01, then the SEVERE CORROSION
            environment time limits apply.
    (b) Mild or Moderate Corrosion Environment
         1 If 2(A)(2)(a)(1) does not apply, then the MILD/MODERATE CORROSION
           environment time limits apply.
 After the operating usage and the operating environment are determined, make a logbook   entry that states which inspection schedules (TYPICAL or SEVERE operating usage and MILD/MODERATE or SEVERE operating environment) are being used.

Monday, September 3, 2012

Corrosion Characteristics


 Corrosion is the deterioration of a metal by reaction to its environment. Corrosion occurs because most metals have a tendency to return to their natural state.
Corrosion Characteristics
A. Metals corrode by direct chemical or electrochemical (galvanic) reaction to their environment. The following describes electrochemical reaction:
(1) Electrochemical corrosion can best be compared to a battery cell. Three conditions must exist before electrochemical corrosion can occur:
(a) There must be a metal that corrodes and acts as the anode (+ positive).
(b) There must be a less corrodible metal that acts as the cathode (- negative).
(c) There must be a continuous liquid path between the two metals, which acts as the electrolyte. This liquid path may be condensation or, in some cases, only the humidity in the air.
(2) Elimination of any one of the three conditions will stop the corrosion reaction process.
(3) A simple method of minimizing corrosion is adding a layer of pure Aluminum to the surface. The pure Aluminum is less susceptible to corrosion and also has a very low electro-potential voltage relative to the remainder of the alloyed sheet. This process is conducted at the fabricating mill and the product is called Alclad. Model 152 airplanes had sheet metal parts constructed of Al-clad sheet.
(4) One of the best ways to eliminate one of the conditions is to apply an organic film (such as paint, grease or plastic) to the surface of the metal affected. This will prevent electrolyte from connecting the cathode to the anode so current cannot flow and therefore, prevent corrosive reaction and was not available for production Model 152 airplanes.
(5) Other means employed to prevent electrochemical corrosion include anodizing and electroplating. Anodizing and other passivating treatments produce a tightly adhering chemical film which is much less electrochemically reactive than the base metal. Because the electrolyte cannot reach the base metal, corrosion is prevented. Electroplating deposits a metal layer on the surface of the base material, which is either less electrochemically reactive (Example:chrome on steel) or is more compatible with the metal to which it is coupled (Example: cadmium plated steel fasteners used in aluminum).
(6) At normal atmospheric temperatures, metals do not corrode appreciably without moisture. However, the moisture in the air is usually enough to start corrosive action.
(7) The initial rate of corrosion is usually much greater than the rate after a short period of time. This slowing down occurs because of the oxide film that forms on the metal surfaces. This film tends to protect the metal underneath.
(8) When components and systems constructed of many different types of metals must perform under various climatic conditions, corrosion becomes a complex problem. The presence of salts on metal surfaces (sea or coastal operations) greatly increases the electrical conductivity of any moisture present and accelerates corrosion.
(9) Other environmental conditions that contribute to corrosion are:
(a) Moisture collecting on dirt particles.
(b) Moisture collecting in crevices between lap joints, around rivets, bolts and screws.

Types of Corrosion



                          Common types of corrosion that are encountered in airplane maintenance are described here. In many instances more than one form of corrosion may exist at the same time. While this makes it difficult to determine the exact type of corrosion, it should still be possible to determine that a corrosive process is taking place. If it is impractical to replace an assembly or component, contact an authorized repair shop.
Direct Chemical Attack : Direct chemical attack may take place when corrosive chemicals, such as battery electrolyte, caustic cleaning solutions or residual flux deposits are allowed to remain on the surface or become entrapped in cracks or joints. Welding or soldering flux residues are hydroscopic and will tend to cause severe pitting. Any potentially corrosive substance should be carefully and completely removed whenever such spillage occurs.
 Pitting Corrosion : The most common effect of corrosion on polished aluminum parts is called pitting. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface . 
             When the deposit is cleaned away, tiny pits can be seen in the surface. Pitting may also occur in other types of metal alloys.
 Intergranular Corrosion. : Intergranular corrosion takes place because of the nature of the structure of metal alloys. As metals cool from the molten state, a granular structure is formed. The size and composition of the grains and the material in the grain boundaries depend on several factors including the type of alloy and rate of cooling from the molten state or cooling after heat-treating.
         The grains differ chemically and may differ electrochemically from the boundary material. If an electrolyte comes in contact with this type of structure, the grains and boundary material will act as anode and cathode and undergo galvanic corrosion. The corrosion proceeds rapidly along the grain boundaries and destroys the solidity of the metal.
Exfoliation gives the appearance of sheets of very thin metal separated by corrosion products. It is a form of intergranular corrosion. Since the corroded products are thicker than the uncorroded aluminum, exfoliation shows itself by “lifting up” the surface grains of a metal by the force of expanding corrosion. This type of corrosion is most often seen on extruded sections, where the grain thicknesses are usually less than in rolled alloy form.
 Dissimilar Metal Corrosion.  Dissimilar metal corrosion occurs when dissimilar metals are in contact in the presence of an electrolyte. A common example of dissimilar metal contact involves the attachment of aluminum parts by steel fasteners.
Concentration Cell Corrosion : Concentration cell corrosion occurs when two or more areas of the same metal surface are in contact with different concentrations of the same solution, such as moist air, water and chemicals.The general types of concentration cell corrosion are identified as metal ion cells and oxygen cells.
Filiform Corrosion.: Filiform corrosion is a “concentration cell” corrosion process. When a break in the protective coating over aluminum occurs, the oxygen concentration at the back or bottom of the corrosion cell is lower than that at its open surface. The oxygen concentration gradient thus established, causes an electric current flow and corrosion results. Filiform corrosion results when this happens along the interface between the metal and the protective coating and appears as small worm-like tracks. Filiform corrosion generally starts around fasteners, holes and countersinks and at the edge of sheet metal on the outer surface of the airplane. Filiform corrosion is more prevalent in areas with a warm, damp and salty environment.
 To help prevent filiform corrosion development, the airplane should be:
(a) Spray washed at least every two to three weeks (especially in a warm, damp environment).
(b) Waxed with a good grade of water repellent wax to help keep water from accumulating in skin joints and around countersinks.
(c) Keep the airplane hangared to protect it from the atmosphere.
(d) Fly the airplane to promote aeration of enclosed parts.
(e) Ensure all vent/drain holes are open to ventilate the interior of airplane.
(3) To remove filiform corrosion once it has been discovered:
(a) Remove paint from corroded area.
(b) Remove corrosion by sanding area to metal surface, using either a ScotchBrite pad or 320 grit sandpaper (aluminum oxide or silicone carbide grit).
(c) Clean and refinish surface.
I. Stress Corrosion Cracking.
(1) This corrosion is caused by the simultaneous effects of tensile stress and corrosion. The stress may be internal or applied. Internal stresses are produced by nonuniform shaping during cold working of the metal, press and shrink fitting general hardware and those induced when pieces, such as rivets and bolts, are formed. The amount of stress varies from point to point within the component. Stress corrosion is most likely to occur at points of highest stress, which are also subject to corrosion influence.
J. Fatigue Corrosion.
(1) Fatigue corrosion is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion.

Determination of the Corrosion Levels



A. Find the Corrosion Levels.
(1) Corrosion found on a structure when you use the Corrosion Program and Corrosion Prevention 7(CPCP) Baseline Program will help find the extent of the corrosion.
(2) The second and subsequent inspections will find how well the CPCP program has been prepared or if there is a need to make adjustments to the Baseline Program.
(3) A good quality CPCP is one that controls corrosion to Level 1 or better.
(4) If Level 2 corrosion is found during the second or subsequent inspection, you must do something to decrease the future corrosion to Level 1 or better.
(5) If Level 3 corrosion is found, you must also do something to decrease the future corrosion to Level 1. Also, a plan to find or prevent Level 3 corrosion in the same area on other airplanes must be added to the CPCP.
(6) All the corrosion that you can repair in the allowable damage limits, (less than 10 percent of the part thickness) is Level 1 corrosion.
(7) If all corrosion is Level 1, the CPCP is correctly prepared.
(8) If you must reinforce or replace the part because of corrosion, the corrosion is Level 2.
(9) If the part is not airworthy because of the corrosion, you must do an analysis to find out if the corrosion is Level 3.
(10) The chart found in this section will help find the level of the corrosion.
(11) The probability that the same problem will occur on another airplane is dependent on several factors such as: past maintenance history, operating environment, years in service, inspectability of the corroded area and the cause of the problem

Level 2 Corrosion Findings



A. All Level 2 corrosion that is more than the rework limits of the approved repair procedures must be reported to Cessna Aircraft Company. Cessna Aircraft Company engineering will do an analysis to make sure the corrosion is not an urgent airworthiness concern.
B. When doing the analysis, Cessna Aircraft Company will consider:
(1) Can the cause of the corrosion be identified, such as a chemical spill or protective finish breakdown?
(2) Has the same level of corrosion been found on other airplanes?
(3) Are the corrosion protection procedures applied during manufacture the same for earlier and later models?
(4) Age of the corroded airplane compared to others checked.
(5) Is the maintenance history different from the other airplanes in the fleet?

Actions Follow Corrosion


 A. If corrosion is found, find the corrosion level, then do the necessary steps for a specific inspection.
B. If Level 1 corrosion is found during the first CPCP inspection.
(1) Repair the structure. Contact Cessna  for an approved repair procedure.
(2) Continue with the Baseline Program.
(a) Optional: Document the results of the inspection for use in validating program compliance.
C. If Level 2 corrosion is found during the first CPCP inspection.
(1) Repair the structure. Contact Cessna  for an approved repair procedure.
(2) Report the details of the corrosion you see to Cessna Aircraft Company and the DGCA.
(3) Continue to use the Baseline Program but check the corroded area carefully when you do a subsequent CPCP inspection.
(4) It is recommended that you record the results of the inspection to show compliance with the program.
D. If Level 3 corrosion is found during the first CPCP inspection.
(1) Immediately contact Cessna Aircraft Company and the DGCA of the corrosion you found. 
(2) Give sufficient information to make sure that the condition is a possible urgent airworthiness concern for your fleet. Get assistance from Cessna Support to develop a plan of action.
(3) Apply the corrosion program inspection, which includes the repair of the structure. Contact Cessna Aircraft Company for an approved repair procedure.
(4) Do a report that has the information of the findings. Refer to Corrosion Prevention And Control Program Reporting System - Description And Operation.
(5) Continue with the Baseline Program and other steps of procedure required by the DGCA. Examine this area carefully during future inspections.
E. If no corrosion is found during the second or subsequent CPCP inspection:
(1) Continue with the current Corrosion Prevention and Control Program. No adjustment of the current program is required.
(2) It is recommended that you record the results of the inspection for a possible increase of the corrosion inspection interval.
.

Corrosion Found


 If Level 1 corrosion is found on the second or subsequent CPCP inspection:
(1) Do the corrosion program inspection, which includes the repair of the structure. Contact Cessna Aircraft Company for an approved repair procedure.
(2) Continue with the Baseline Program.
(3) No adjustment of the existing program is required.
(4) It is recommended that you record the corrosion inspection number and the results of the inspection to show that the program was complied with.
 If Level 2 corrosion is found on the second or subsequent CPCP inspection:
(1) Repair the structure. Contact Cessna Aircraft Company for an approved repair procedure.
(2) Do a report that shows the information about the corrosion and send it to Cessna Aircraft Company and the FAA (or applicable regulatory authority).
(3) If corrosion damage required the removal of material just beyond the allowable limits (within 10 percent), complete a check of the other airplanes in the fleet before you change your aircraft's maintenance program.
(a) If the corrosion is typical of Level 2, use the fleet data to find what changes are required to control corrosion to Level 1 or better.
(b) If fleet damage is typically Level 1, examine the corroded area during subsequent inspections on all affected airplanes.
(c) Make changes to your aircraft's maintenance program if the typical corrosion becomes Level 2.
(4) Further evaluation by Cessna Aircraft Company is recommended for Level 2 corrosion findings that are well beyond the allowable limits and there is an airworthiness concern in which prompt action is required.
NOTE: The airworthiness concern is because of the possibility to have similar but more severe corrosion on any other airplane in the operator's fleet prior to the next scheduled inspection of that area.
 (5) Find the action required to control the corrosion to a Level 1 or better, between future successive inspections. These can include the items that follow:
(a) A structural modification, such as additional drainage.
(b) Improvements to the corrosion prevention and control inspections, such as more care and attention to corrosion removal, reapplication of protective finish, drainage path clearance.
(c) Decrease the inspection interval for additional airplanes that go into the program.
(6) Send a plan of corrective action to the FAA (or applicable regulatory authority) for approval and to Cessna Aircraft Company as needed.
(7) Use the approved plan of action.
H. If Level 3 corrosion is found on the second or subsequent CPCP inspection:
(1) Contact Cessna Aircraft Company and the FAA (or applicable regulatory authority) about the corrosion that was found.
(2) Send a plan to examine the same area on other affected airplanes in the operator's fleet.
(3) Apply the corrosion program inspection, which includes the repair of the structure. Contact Cessna Aircraft Company for an approved repair procedure.
I. Find the action needed to control the corrosion finding to Level 1 or better, between future successive inspections. These can include any or all of the following:
(1) A structural modification, such as additional drainage.
(2) Improvements to the corrosion prevention and control inspections, such as more care and attention to corrosion removal, reapplication of protective finish, drainage path clearance.
(3) A decrease in the inspection interval for additional airplanes entering the program.
J. Send a plan of corrective action to the FAA (or applicable regulator authority) for approval and Cessna Aircraft Company as needed.
K. Use the approved plan of action.
L. It is recommended that you give the details of the findings to Cessna Aircraft Company

Factors Influencing Corrosion Occurrences


                  If you find Level 2 or Level 3 corrosion, when you think about how to change your CPCP, think about the list that follows.
(1) Is there a presence of LPS-3 Heavy-Duty Rust Inhibitor?
(2) Is there a presence or condition of protective finish?
(3) What was the length of time since the last inspection and/or application of corrosion inhibiting compound?
(4) Was there inadequate clean-up/removal of corrosion prior to application of corrosion inhibiting compound, during previous maintenance of the area?
(5) Are the moisture drains blocked or is there inadequate drainage?
(6) What was the environment, the time of exposure to the environment and the use of the airplane?
(7) Was there a variation in past maintenance history and or use of the airplanes in the operator's fleet?
(8) Were there variations in the production build standard in the operator's fleet?

CPCP Reporting & Implementation


 Reporting
A. The minimum requirements to prevent or control the corrosion in the Corrosion Prevention and Control Program (CPCP) were made on the best information, knowledge and experience available at the time. As this experience and knowledge increases, the CPCP's intervals will be changed as necessary.
Refer to CPCP Damage Report Form (Figure 2 in Section 2A-30-00).
(1) You must contact the Cessna Aircraft Company about all Level 2 or 3 corrosion of the structure that is on the list in the Baseline Program that is found during the second and subsequent corrosion program inspections. Refer to Reporting System.
NOTE: You do not have to contact the Cessna Aircraft Company about corrosion that is found on structure that is not on the list in the Baseline Program, for example the secondary structure.
 Program Implementation
A. When a CPCP is started it is important to do the items that follow:
(1) Start inspections at the recommended interval following the completion of the first SID inspection.
(2) Once the corrosion program inspection (CPI) is started, repeat the subsequent applications of the CPI at the recommended interval for each CPI.
(3) You can start a CPCP on the basis of individual CPIs or groups of CPIs.
(4) Cessna Aircraft Company highly recommends to start all of the CPIs as soon as possible. This is the most cost effective way to prevent or control corrosion.

General Corrosion Repair


                             
A. This section provides general guidance on the repair of corroded area. The procedure presented is:
(1) Gain access to the entire corroded area.
(2) Mechanically remove the corrosion products
(3) Determine the extent of the corrosion damage
(4) Repair or replace the damaged components
(5) Finish the new or repaired parts.
(6) Replace removed components
B. Gain access to the entire corroded area.
(1) Corrosion products typically retain moisture. If those products are not removed, corrosion will continue. Corrosion can take place within layered construction or under (behind) equipment fastened in place.
C. Mechanically remove the corrosion.
(1) Chemicals will not remove corrosion. The best chemicals can do is interrupt the corrosion cell by either displacing water or shielding corrosion products from oxygen. In either case, the effect is temporary and will need to be renewed.
(2) Sand mild corrosion.
(3) Use rotary files or sanding disks for heavier corrosion. Finish up with fine sand paper.
NOTE: Do not use metallic wool. Metal particles will be embedded in the surface, which will initiate additional corrosion.
D. Determine the extent of corrosion damage.
(1) Direct measurement is simplest.
(2) Indirect measurement may be necessary
(a) Eddy Current or ultrasound tools can be used for thickness measurement away from part edges.
E. Repair or replace corrosion damaged components
(1) Replace damaged or corroded steel or aluminum fasteners.
(2) If the material is sheet or plate, the thickness is allowed to be as little as 90% of the nominal thickness.
(3) This general allowance is not allowed if:
(a) The area of the part contains fasteners.
(b) The reduced thickness compromises the fit or function of a part.
F. Finish the new or repaired parts
(1) Apply Alodine or similar anticorrosion compounds to new or repaired parts or
(2) Apply zinc chromate or
(3) Apply epoxy fuel tank primer.
(4) Paint the exterior or visible interior parts
G. Replace Removed Components.

Corrosion - Battery Electrolyte.


          Aluminum appears high in the electrochemical series of elements and its position indicates that it should corrode very easily. However, the formation of a tightly adhering oxide film offers increased resistance under mild corrosive conditions. Most metals in contact with aluminum form couples,which undergo galvanic corrosion attack. The alloys of aluminum are subject to pitting, intergranular corrosion and intergranular stress corrosion cracking.
Battery Electrolyte.
(1) Battery electrolyte used in lead acid batteries is composed of 35% sulfuric acid and 65% water. When electrolyte is spilled, it should be cleaned up immediately. A weak boric acid solution may be applied to the spillage area followed by a thorough flushing with clean, cold running water. If boric acid is not available, flush the area with clean, cold water.
(2) If corrosion appears, use an approved repair method to repair the structure.
 (1) The construction of piano type hinges forms moisture traps as well as the dissimilar metal couple between the steel hinge pin and the aluminum hinge. Solid film lubricants are often applied to reduce corrosion problems.
(2) Care and replacement of solid film lubricants require special techniques peculiar to the particular
solid film being used. Good solid film lubricants are lubricants conforming to Specification MILPRF- 81322.
(a) Solid film lubricants prevent galvanic coupling on close tolerance fittings and reduce fretting corrosion. Surface preparation is extremely important to the service or wear life of solid film lubricants.
(b) Solid film lubricants are usually applied over surfaces coated with other films, such as anodize and phosphate. They have been successfully applied over organic coatings such as epoxy primers.
CAUTION: Solid film lubricants containing graphite, either alone or in
mixture with any other lubricants, should not be used since
graphite is cathodic to most metals and will cause galvanic corrosion in the presence of electrolytes.

Corrosion - Steel Control Cable.


(1) Checking for corrosion on a control cable is normally accomplished during the preventative maintenance check. During preventative maintenance, broken wire and wear of the control cable are also checked.
(2) If the surface of the cable is corroded, carefully force the cable open by reverse twisting and visually inspect the interior. Corrosion on the interior strands of the cable constitutes failure and the cable must be replaced. If no internal corrosion is detected, remove loose external rust and corrosion with a clean; dry, coarse weave rag or fiber brush.
CAUTION: Do not use metallic wools or solvents to clean installed cables.
Metallic wools will embed dissimilar metal particles in the cables
and create further corrosion. Solvents will remove internal cable
lubricant, allowing cable strands to abrade and further corrode.
(3) After thorough cleaning of exterior cable surfaces, if the cable appears dry, the lubrication originally supplied on the cable has probably oxidized and needs to be replaced with a light oil (5w motor oil, "3 in 1" oil, LPS-2, WD-40 or Diesel Fuel). Apply the oil with a cloth and then rub
the cable with the cloth to coat the cable with a thin layer of oil. Excessive oil will collect dust
and be as damaging to the cable as no lubrication.
D. Piano Type Hinges.

Corrosion - faying surfaces


 Requirements peculiar to faying surfaces of airframes, airframe parts and attaching surfaces of equipment, accessories and components.
(1) When repairs are made on equipment or when accessories and components are installed, the attaching surfaces of these items should be protected. The following requirements are peculiar to faying surfaces on airframes, airframe parts and attaching surfaces of equipment, accessories and components:
(2) Surfaces of similar or dissimilar metals.
(a) All faying surfaces, seams and lap joints protected by sealant must have the entire faying surface coated with sealant. Excess material squeezed out should be removed so that a fillet seal remains. Joint areas, which could hold water, should be filled or coated with sealant.
(3) Attaching Parts.
(a) Attaching parts, such as nuts, bushings, spacers, washers, screws, self-tapping screws, self-locking nuts and clamps, do not need to be painted in detail except when dissimilar metals or wood contact are involved in the materials being joined. Such parts should receive a wet or dry coat of primer.
NOTE: Corrosion inhibiting solid film lubricants, Specification MIL-PRF-46010 and/or MIL-L-46147, may be used to protect attaching parts from corrosion.
(b) All holes drilled or reworked in aluminum alloys to receive bolts, bushings, screws, rivets and studs should be treated before installation of fasteners or bushings.
(c) All rivets used to assemble dissimilar metals should be installed wet, with sealant, conforming to Specification MIL-PRF-81733 Corrosion inhibiting sealer (Type X).
(4) Close tolerance bolts passing through dissimilar metals should be coated before installation,with a corrosion inhibiting solid film lubricant conforming to Specification MIL-PRF-46010 and/or MIL-L-46147.
(5) Washers made of aluminum alloy of suitable design should be used under machine screws, countersunk fasteners, bolt heads and nuts.
(6) Adjustable parts threads such as tie rod ends, turnbuckles, etc., should be protected with solid film lubrication conforming to Specification MIL-PRF-46010 and/or MIL-L-46147.
(7) Slip fits should be assembled using wet primer conforming to Specification MIL-PRF-23377G or later, non-drying zinc chromate paste or solid film lubricant conforming to Specification MILPRF- 46010 and/or MIL-L-46147.
(8) Press fits should be accomplished with oil containing material conforming to Specification MILC-11796, Class 3 and/or MIL-C-16173, Class 1 or with other suitable material that will not induce corrosion

Corrosion - Electrical


Electrical.
(1) Bonding and ground connections should be as described by the installation procedure.
(2) Potting compounds are used to safeguard against moisture. Corrosion in electrical systems and resultant failure can often be attributed to moisture and climatic condition.
(3) Corrosion of metal can be accelerated because of the moisture absorbed by fungi. Fungi can create serious problems since it can act as an electrolyte, destroying the resistance of electrical insulating surfaces. Specification ASTM D3955 or ASTM D295-58 outlines moisture and fungus resistant varnish to be used.

CPCP Baseline Program Implementation


 The Baseline Program is divided into specific inspection areas and zone locations. The inspection areas and zone locations apply to all airplanes.
 Reporting System
A. CPCP Reporting System
(1) The CPCP includes a system to report to Cessna data that will show that the Baseline Program is sufficient and, if necessary, make changes.
(2) At the start of the second Corrosion Program Inspection of each area, report all Level 2 and Level 3 Corrosion results that are listed in the Baseline Program to Cessna.
Send the Control Prevention and Control Program Damage Reporting Form to: Cessna.
 Periodic Review
A. Use the Service Difficulty Reporting System to report all Level 2 and Level 3 Corrosion results to the FAA and to Cessna. All corrosion reports received by Cessna will be reviewed to determine if the Baseline Program is adequate.
Corrosion Related Airworthiness Directives
A. Safety-related corrosion conditions transmitted by a Service Bulletin can be mandated by an Airworthiness Directive (AD).
Appendix A - Development Of The Baseline Program
A. The CPCP Baseline Program
(1) The function of the CPCP  is to give the minimum procedures necessary to prevent and control corrosion so that continued airworthiness is not at risk. The PSE's are areas where the CPCP applies.
(2) The CPCP Baseline Program consists of a Corrosion Program Inspection (CPI) and an inspection time. Each inspection is to be done in an airplane zone.


CPCP – Base Line


 Introduction: Corrosion can cause damage to the airplane's structural integrity and if it is not controlled, the airframe will carry less load than what is necessary for continued airworthiness.
(1)  A CPCP is a system to control the corrosion in the airplane's primary structure. It is not the function of the CPCP to stop all of the corrosion conditions, but to control the corrosion to a level that the airplane's continued airworthiness is not put in risk.
CPCP Function
A. The function of this document is to give the minimum procedures necessary to control the corrosion so that the continued airworthiness is not put in risk. The CPCP consists of a Corrosion Program Inspection number, the area where the inspection will be done, specified corrosion levels and the compliance time. The CPCP also includes procedures to let Cessna  and the regulatory authorities know of the findings and the data associated with Level 2 and Level 3 corrosion.
This includes the actions that were done to decrease possible corrosion in the future to Level 1.
 Baseline Program
A. The Baseline Program is part of the CPCP. It is divided into Basic Task and Inspection Interval. In this manual the Basic Tasks are referred to as the Corrosion Program Inspection. This program is to be used on all airplanes without an approved CPCP. Those who currently have a CPCP that does not control corrosion to Level 1 or better must make adjustments to the areas given in the Baseline Program.
B. Typical Airplane Zone Corrosion Program Inspection Procedures.
(1) Remove all the equipment and airplane interior (for example the insulation, covers and, upholstery) as necessary to do the corrosion inspection.
(2) Clean the areas given in the corrosion inspection before you inspect them.
(3) Do a visual inspection of all of the Principal Structural Elements (PSEs) and other structure given in the corrosion inspection for corrosion, cracking and deformation.
(a) Carefully examine the areas that show that corrosion has occurred before.
NOTE: Areas that need a careful inspection are given in the corrosion inspection.
(b) Nondestructive testing inspections or visual inspections can be needed after some disassembly if the inspection shows a bulge in the skin, corrosion under the splices or corrosion under fittings. Hidden corrosion will almost always be worse when fully exposed.
(4) Remove all of the corrosion, examine the damage and repair or replace the damaged structure.
(a) Apply a protective finish where it is required.
(b) Clean or replace the ferrous metal fasteners with oxidation.
(5) Remove blockages of foreign object debris so that the holes and clearances between parts can drain.
(6) For bare metal on any surface of the airplane, apply corrosion prevention primer, refer to the Application of Corrosion Preventative Compounds.
(a) Apply a polyurethane topcoat paint to the exterior painted surface. Refer to the manufacturer's procedures.
(7) Install the dry insulation blankets.
(8) Install the equipment and airplane interior that was removed to do the corrosion inspection.

CPCP - APPLICATION


             Corrosion can cause damage to the airplane's structural integrity and if it is not controlled, the airframe will carry less load than what is necessary for continued airworthiness.
             A CPCP is a system to control the corrosion in the airplane's primary structure. It is not the function of the CPCP to stop all of the corrosion conditions, but to control the corrosion to a level that the airplane's continued airworthiness is not put in risk.Complete the initial CPCP inspection in conjunction with the first SID inspection.
 Control Prevention and Control Program Application
A. The CPCP gives the information required for each corrosion inspection. Maintenance personnel must fully know about corrosion control. The regulatory agency will give approval and monitor the CPCP for each airplane.
(1) The CPCP procedures apply to all airplanes that have exceeded the inspection interval for each location on the airplane.
(a) Cessna  recommends that the CPCP be done first on older airplanes and areas that need greater changes to the maintenance procedures to meet the necessary corrosion prevention and control requirements.
(2) Maintenance programs must include corrosion prevention and control procedures that limit corrosion to Level 1 or better on all Principal Structural Elements (PSEs) and other structure specified in the Baseline Program. If the current maintenance program includes corrosion control procedures in an inspection area and there is a report to show that corrosion is always controlled to Level 1 or better, the current inspection program can be used.
(a) The Baseline Program is not always sufficient if the airplane is operated in high humidity (severe) environments, has a corrosive cargo leakage or has had an unsatisfactory maintenance or repair. When this occurs, make adjustments to the Baseline Program until the corrosion is controlled to Level 1 or better. Refer to Section 2A-30-01, Corrosion Severity Maps, to determine the severity of potential corrosion.
(3) The CPCP consists of the corrosion inspection applied at a specified interval and, at times, a corrosion inspection interval can be listed in a Service Bulletin. For the CPCP to be applied, remove all systems, equipment and interior furnishings that prevent sufficient inspection of the structure. A nondestructive test (NDI) or a visual inspection can be necessary after some items are removed if there is an indication of hidden corrosion such as skin deformation, corrosion under splices or corrosion under fittings. Refer to the Baseline Program.
(4) The corrosion rate can change between different airplanes. This can be a result of different environments the airplane operates in, flight missions, payloads, maintenance practices (for example more than one owner), variation in rate of protective finish or coating wear.
(a) Some airplanes that operate under equivalent environments and maintenance practices can be able to extend the inspection intervals if a sufficient number of inspections do not show indications of corrosion in that area. Refer to the Glossary.
(5) Later design and/or production changes done as a result of corrosion conditions can delay the start of corrosion. Operators that have done corrosion-related Service Bulletins or the improved procedures listed in the Corrosion Program Inspection can use that specified inspection interval.
Unless the instructions tell you differently, the requirements given in this document apply to all airplanes.
(6) Another system has been added to report all Level 2 and Level 3 corrosion conditions identified during the second and each subsequent CPCP inspection. This information will be reviewed by Cessna to make sure the Baseline Program is sufficient and to change it as necessary

CPCP Basic


CORROSION PREVENTION AND CONTROL PROGRAM (CPCP)
1. Introduction
A. As the airplane ages, corrosion occurs more often, while, at the same time, other types of damage such as fatigue cracks occur. Corrosion can cause damage to the airplane's structural integrity and if it is not controlled, the airframe will carry less load than what is necessary for continued airworthiness.
(1) To help prevent this, we started a Corrosion Prevention and Control Program (CPCP). A CPCP is a system to control the corrosion in the airplane's primary structure. It is not the function of the CPCP to stop all of the corrosion conditions, but to control the corrosion to a level that the
airplane's continued airworthiness is not put in risk.
B. Complete the initial CPCP inspection in conjunction with the first SID inspection.
2. Corrosion Prevention and Control Program Objective
A. The objective of the CPCP is to help to prevent or control the corrosion so that it does not cause a risk to the continued airworthiness of the airplane.
3. Corrosion Prevention and Control Program Function
A. The function of this document is to give the minimum procedures necessary to control the corrosion so that the continued airworthiness is not put in risk. The CPCP consists of a Corrosion Program Inspection number, the area where the inspection will be done, specified corrosion levels and the compliance time. The CPCP also includes procedures to let Cessna Aircraft Company and the regulatory authorities know of the findings and the data associated with Level 2 and Level 3 corrosion.
This includes the actions that were done to decrease possible corrosion in the future to Level 1.
B. Maintenance or inspection programs need to include a good quality CPCP. The level of corrosion identified on the Principal Structural Elements (PSEs) and other structure listed in the Baseline Program will help make sure the CPCP provides good corrosion protection.
NOTE: A good quality program is one that will control all structural corrosion at Level 1 or better.
C. Corrosion Program Levels.
NOTE: In this manual the corrosion inspection tasks are referred to as the corrosion program inspection.
(1) Level 1 Corrosion.
(a) Corrosion damage occurring between successive inspection tasks, that is local and can be reworked or blended out with the allowable limit.
(b) Local corrosion damage that exceeds the allowable limit but can be attributed to an event not typical of the operator's usage or other airplanes in the same fleet (e.g., mercury spill).
(c) Operator experience has demonstrated only light corrosion between each successive corrosion task inspection; the latest corrosion inspection task results in rework or blend out that exceeds the allowable limit.
(2) Level 2 Corrosion.
(a) Level 2 corrosion occurs between two successive corrosion inspection tasks that requires a single rework or blend-out that exceeds the allowable limit. A finding of Level 2 corrosion requires repair, reinforcement or complete or partial replacement of the applicable structure.
(3) Level 3 Corrosion.
(a) Level 3 corrosion occurs during the first or subsequent accomplishments of a corrosion inspection task that the operator determines to be an urgent airworthiness concern.
4. References
A. This is a list of references for the Corrosion Prevention and Control Program.
(1) FAA Advisory Circular AC120-CPCP, Development and Implementation of Corrosion Prevention and Control Program
(2) FAA Advisory Circular AC43-4A, Corrosion Control for Aircraft
(3) Cessna Illustrated Parts Catalog - part numbers P692-12.
(4) Cessna Service Manual - part number D2064-1-13.