Piper Aircraft - Rear Spar Corrosion

 The FAA recommends that owners and operators incorporate the actions in Piper SBs 789A, 977, and 1006 based on the applicability of the individual SB. Due to similarity of design (steel plate 66762-00 adjacent to aluminum spar) and since improper maintenance or painting procedures have been a contributing factor in past service reports, all PA-28, PA-32, PA-34, and PA-44 airplanes are susceptible to this corrosion development. 
             Therefore, the inspections in the SBs should be implemented for all PA-28, PA-32, PA-34, and PA-44 models. We recommend that all the actions be taken within 100 hours. We further recommend inspection and incorporation as soon as feasible, but no more than 100 hours, for those airplanes at higher risk due to age, environment, lack of protective coatings, etc., as discussed above. We also recommend that owners and operators incorporate the actions in Piper SB 1244B following the compliance time in the SB.

SAIB: CE-11-12R1  Date: December 2, 2015

Severe corrosion

CESSNA 510           20/08/2015
Severe corrosion on emergency escape hatch door.
During a pre maintenance inspection a large isolated patch of corrosion was noted on the lower outer skin of the emergency escape hatch door. This area wasn't included in any of the scheduled inspections for this input but was visible during the pre-maintenance walkaround.

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?

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.