4. TRANSPARENCIES - Non-pressurized aircraft window panels other than windshields and canopies
a. For flat window panels, you may use the following substitutions:
(1) You may substitute any commercial grade acrylic sheet (commonly known as Plexiglas) with an identical thickness polycarbonate sheet (commonly known as Dupont Lexan), of equivalent tint (transmissivity), that meets specification SAE-AMS-P-83310 (formerly MIL- P-83310).
(2) You may substitute any commercial grade polycarbonate sheet with an identical thickness acrylic sheet, of equivalent tint (transmissivity), that meets specification MIL-PRF- 8184 Type I or Type II, Class 2. Type I is suitable for "as cast" and for "stretching" applications, while Type II is suitable only for "as cast" sheets. Class 2 calls for the desirable improved moisture absorption resistance.
b. Using material that meets the above specifications ensures consistency of the mechanical properties, resistance to crazing, and resistance to ultraviolet (UV) exposure, etc.
Note 1: If you substitute "Dupont Lexan" for "Plexiglas" or vice-versa, there may be different maintenance issues such as type of cleaners, polishing techniques, etc.
Manufacturing techniques (cutting, drilling, or forming) will also differ according to the material chosen. You should follow the specific manufacturer's recommendations for maintenance.
Approval: This is a minor alteration and you may document it by a logbook entry. The logbook entry must reference the manufacturer's identification and industry specification for the original and replacement transparency.
5. TUBING This part substitution does not apply to structural tubing, or systems with working pressures greater than 3,000 PSI (per-square-inch).
a. Identify installed tube and specification number if possible.
b. Metal tubes, particularly aluminum and stainless steel, will generally have an ASTM specification or material identification number printed on the outer surface.
c. Reference the specification or material number to determine physical characteristics of tube (copper, aluminum, stainless steel). Measure inside and outside diameters and determine the wall thickness.
d. Determine the operational pressure or vacuum of the system the tube is installed in. For example, if using the tube in a landing gear retraction system, refer to the aircraft maintenance manual and/or the pump manufacturer's data to determine the maximum oil pressure specified for operation.
(1) Determine the required proof and burst pressure test requirements for the systems (hydraulic, pneumatic, part 23 or CAR 3).
(2) Ensure that the replacement tubing test requirements have been met according to the applicable rule for the certification basis for the aircraft, whether that is part 23 or CAR 3.
e. If no identification marks are found, use a tube meeting or exceeding a U.S. material specification, such as ASTM or MIL, for the material being replaced. For replacement of unmarked tubes, you must research the specifications of the replacement tube to make sure it has the same I.D. and that the rating meets or exceeds the maximum rating for the amount of vacuum and or pressure the system will place on the tube.
f. You may replace copper and/or brass tubing with aluminum tubing in the "O" or fully annealed condition if the tubing has the same I.D. and has the appropriate pressure service rating for the system the tube is installed in.
Note 2: The "O" condition is important because applications that need to allow for vibration loads and manual bending of the lines often use copper and brass tubing.
Approval: This is a minor alteration and you may document it by a logbook entry. The logbook entry must reference the material specification, I.D. and wall thickness of the replacement tubing and the manufacturer's identification (if a standard part). Also reference the original tubing identification number if known. If the original tubing information is not known, then list the minimum/maximum operating environment specifications (material, fluid, service pressure, temperature, I.D., wall thickness, etc.).
6. ENGINE PARTS. You may install substitute/alternate engine parts for a part that is approved as original equipment or a PMA part for the engine installed on your aircraft in the following instance:
a. The substitute/alternate part is PMA approved for a certificated engine, even if that engine is not the model approved for use on your aircraft model, and
b. If the part is identical to (used on multiple engine models) the part for your engine model, or is listed as an approved substitute part by the engine manufacturer, then you may install the alternate part, and document the installation with only a logbook entry.
Approval: This is a minor alteration and you may document it by a logbook entry. The logbook entry must reference the manufacturer's identification and industry specification for the original and replacement part.
APPENDIX 2
MATERIAL SUBSTITUTIONS
1. AMERICAN IRON AND STEEL INSTITUTE (AISI) 1020 OR 1025 CARBON STEEL Metallurgical Considerations in the Substitution of AISI 4130 for AISI 1020 or 1025:
a. Assumptions and limitations.
(1) We limit this appendix to non-heat treated structures. Post weld heat treatments such as a quench and temper operation significantly improve the average strength of a structural member. In heat treatment of structures, section size and alloy content dramatically alter heat treat response; therefore, you should not use material substitutions of heat-treated components without appropriate metallurgical engineering analysis. The altered heat treat response will impact the static strength as well as the fatigue strength of the component. Examples of older aircraft with heat treated components include: landing gear struts in the Luscombe 8 series and Stinson 108 series engine mounts.
(2) For material substitution related to fatigue life, we assume that the structure is ferrous (steel). Aluminum or titanium structures are out of scope as they have different material responses. We limit our conclusions to AISI 1020/1025 and AISI 4130 types of steel alloys.
(3) We also assume that the structure is designed for and operated in stress ranges below the base material yield strength (elastic region). The static strength of the weldment base material influences the fatigue strength of the weldment as the maximum stress in the fatigue cycle approaches the static strength of the base material. In this case, a higher strength base material will increase the "low cycle" fatigue strength of the weldment. The base material type in the elastic region does not affect S-N curves for welds. In the elastic-plastic region, the S-N curves for the different weld classifications will vary depending on the yield strength of the base material. Any repetitive stresses above the yield strength will be affected by the material's alloy content. For this reason, you should consider only infinite life components. Any components designed with limited lives by the manufacturer should not have material substitutions without metallurgical engineering analysis.
(4) Even with tight controls in the manufacturing process of tubing, no one knows the thermal history of the structure. The welding operation results in various portions of the structure having an infinite potential for heating and cooling and the resultant strengths. Since the weld operation uses hot liquid metal in the weld pool and other areas on the part or structure are cold, we have a variety of starting microstructures. The cooling rate is a function of the welder's skill and practices, location, type of repair, etc. Because of this unknown, we assume that somewhere in the loaded structure the strength of the material is a very low value, which is taken into account by the original designer when selecting the tubing size and wall thickness.
(5) External factors affecting fatigue of components are well documented. We only present a cursory review to illustrate the relative importance of other factors governing the fatigue strength:
(a) Notch sensitivity of the joint can have stress concentration from 1 to 4 times the nominal applied loads.
(b) In a similar vein to notch sensitivity is surface finish of the part. A 25 percent reduction in fatigue life is associated with rough vs. smooth surfaces.
(c) Corrosion can cause a reduction in the fatigue strength in the range of 25-35 percent. Thus weldments in areas subject to corrosion should have additional precautions.
Normal variation in these factors controls the structural load carrying capability and fatigue life, rather than the difference in minimum strength of the base material as a function of alloy content.
b. Certification Considerations in the Substitution of AISI 4130 for AISI 1020 or 1025:
You may substitute AISI (or other industry standard) 4130 low alloy steel in place of original AISI (or other industry standard) 1020 and 1025 plain carbon (non-sulfurized) steel. This applies only to annealed or normalized materials. Neither the original AISI 1020/1025, nor the replacement material AISI 4130 should be heat treated (high strength).
For pre-1945 aircraft designs, the original material carbon content may not be known. You may use AISI 4130 material in place of the original equipment manufacturer's (OEM) material. AISI 1020 and 1025 are low carbon steels containing 0.20 percent and 0.25 percent carbon (nominal) respectively. AISI 4130 has a medium carbon content of 0.30 percent (nominal) with chromium and molybdenum as principal alloying elements.
For many years, the alloy AISI 4130 has been substituted for AISI 1020 and 1025, and it is used extensively for new designs. AISI 4130 is more readily available, and has more desirable material properties than AISI 1020 and 1025-carbon steel. Typical parts originally manufactured from AISI 1020 or 1025 include (but are not limited to) fuselage, wing, empennage, engine mounts, landing gear supports, including associated gussets, doublers, etc.
For aircraft certificated before 14 CFR part 23, Amendment 23-7 (August 13, 1969), the regulations did not require fatigue analysis (ref. 14 CFR part 23, § 23.572). So, we do not require a new fatigue strength analysis when making this material substitution for these aircraft.
For aircraft certificated after this date, you should determine whether a new fatigue strength analysis is necessary, by examining the Type Certificate Data Sheet (TCDS) for the certification basis of the aircraft, or by contacting your local ACO.
Sometimes, a damage tolerance analysis may exist as well, and you may have to re-analyze the part using the new material. The need for a new fatigue or damage tolerance analysis will depend on the part being replaced and whether any of the following exists for the current part:
(1) Stated life limit.
(2) Required inspection program by;
(a) Airworthiness Directive (AD)
(b) Instructions for Continued Airworthiness (ICA) 16
(c) OEM Maintenance or Operations Manual If an AD covers the part, you must obtain an Alternate Means of Compliance (AMOC), in addition to any other approvals (i.e., Form 337 field approval, an STC, etc.). If the part has an existing life limit, you must determine the new life limit using the substitute material. If the part is under any other existing inspections, you must evaluate the basis for the inspection and determine what type of analysis you must do and determine the new inspections required, if any.
The new analysis and inspections (if any) are required as part of the material substitution. The guidance in this AC solely addresses the material substitution. The suitability of the material substitution to a particular part must be evaluated on a case-by-case basis. This AC does not circumvent the approval process for alterations to an existing aircraft. A field approval, coordinated field approval, STC, etc., may still be required, depending on the repair/alteration.
c. Design Considerations in the Substitution of AISI 4130 for AISI 1020 or 1025:
The part fabricated from the substitute material should have the same gauge or wall thickness as the original approved part. AISI 4130 (in general) has higher tensile ultimate strength, yield strength, and fatigue strength than plain low carbon steel AISI 1020 and 1025. However, the fracture toughness of 4130 may be lower than that of plain low carbon steel, so you must investigate the damage tolerance on any part that has an existing inspection requirement. In addition, the elongation property of AISI 4130 is usually lower than that of AISI 1020 or 1025.
Therefore, evaluate any formed part to determine if the bend radius of the original part can be duplicated without cracking when using the new material. If a larger bend radius must be used to avoid cracking, the functionality of the part may be affected and fastener loads may increase if the fastener line is relocated to clear the larger radius. You must evaluate these alterations as part of the material substitution.
For welded structures, the quality of the weld is an important safety consideration regardless of the steel type. You should refer to and follow conventional welding practices specified for AISI 4130 material when substituting this material. Welding AISI 4130 requires specific pre-heat, interpass heat, post heat, and low hydrogen content welding rods. Disregard for these precautions may result in welds with unpredictable fatigue and ultimate strength due to hydrogen embrittlement.
You may want to take advantage of the increased tensile strength and fatigue properties of AISI 4130, by decreasing the material gauge, and thereby lowering the cost and weight. However, this will also decrease the stiffness and compression strength, and may have an overall affect (depending on the location and extent of the gauge reduction) on the aeroelastic properties of the airframe. A change to the aeroelastic properties will require a re-evaluation of the flutter characteristics of the aircraft as a minimum. Therefore, any change to the material gauge (along with the substitution) is beyond the scope of this AC. You should be aware that a complete reinvestigation of compliance, perhaps even a new type certificate could be required if you attempt this type of change.
Summary: The primary concern is allowing the substitution of SAE 4130 alloys in structures that originally were designed and fabricated from SAE 1020 grades. Thin walled tubing is more readily available in 4130 than in the lower alloy for the sizes typically needed.
(1) Heat treated components are not to be considered for material substitutions.
(2) Matching tubing diameter and wall thickness, when following repair or replacement procedures, are the only concerns when substituting 4130 alloys for 1020 in structural repairs.
(3) Surface finish, corrosion, geometry (stress concentrations), and base material strength are the prime determinates of fatigue strength in structures.
(4) Base material strength cannot be predicted in field-fabricated structures; therefore, design must accommodate all potential scenarios.
(5) The tensile strength of 4130 is marginally higher than 1020 in the conditions normally found in welded structures. As a result, the static and fatigue strength may increase by a minimal amount.
Conclusion: For structural tube on applicable aircraft, we permit the substitution of AISI 4130 steel (normalized) for either AISI 1020 or 1025 steel tube. We do not permit the substitution of AISI 1020 or 1025 steel for 4130. All work should be performed in accordance with the aircraft manufacturer's manuals or AC 43.13-1B, if these manuals do not exist.
Approval: This material substitution on secondary structure is a minor repair or alteration and should be documented by logbook entry. However, if the material is substituted on primary structure then it is a major repair or alteration, and, as such, you must comply with various requirements in 14 CFR to obtain approval. You may use the data in this AC as approved data for the material substitution in those instances.