The FAA has revised part 23 (the regulations for non-transport airplanes), and PMA applicants seeking approvals for parts that meet Part 23 requirements will need to pay attention to these changes (but remember that your certification basis might be a prior revision level of Part 23).
One change is the addition of a design metric related to ” the expected operating conditions of the airplane.”
The current rules (which are being revised) state that a designer must “take into account the effects of environmental conditions, such as temperature and humidity, expected in service.”
§ 23.603 Materials and workmanship.
(a) The suitability and durability of materials used for parts, the failure of which could adversely affect safety, must –
(1) Be established by experience or tests;
(2) Meet approved specifications that ensure their having the strength and other properties assumed in the design data; and
(3) Take into account the effects of environmental conditions, such as temperature and humidity, expected in service.
(b) Workmanship must be of a high standard.
The new Part 23 rule will require applicants for PMA for part 23 airplane parts to “design each part, article, and assembly for the expected operating conditions of the airplane.”
Sec. 23.2250 Design and construction principles.
(a) The applicant must design each part, article, and assembly for the expected operating conditions of the airplane.
(b) Design data must adequately define the part, article, or assembly configuration, its design features, and any materials and processes used.
(c) The applicant must determine the suitability of each design detail and part having an important bearing on safety in operations.
(d) The control system must be free from jamming, excessive friction, and excessive deflection when the airplane is subjected to expected limit airloads.
(e) Doors, canopies, and exits must be protected against inadvertent opening in flight, unless shown to create no hazard when opened in flight.
A related regulation continues to use the “likely environmental conditions” language of the prior regulations:
Sec. 23.2260 Materials and processes.
(a) The applicant must determine the suitability and durability of materials used for parts, articles, and assemblies, accounting for the effects of likely environmental conditions expected in service, the failure of which could prevent continued safe flight and landing.
(b) The methods and processes of fabrication and assembly used must produce consistently sound structures. If a fabrication process requires close control to reach this objective, the applicant must perform the process under an approved process specification.
(c) Except as provided in paragraphs (f) and (g) of this section, the applicant must select design values that ensure material strength with probabilities that account for the criticality of the structural element. Design values must account for the probability of structural failure due to material variability.
(d) If material strength properties are required, a determination of those properties must be based on sufficient tests of material meeting specifications to establish design values on a statistical basis.
(e) If thermal effects are significant on a critical component or structure under normal operating conditions, the applicant must determine those effects on allowable stresses used for design.
(f) Design values, greater than the minimums specified by this section, may be used, where only guaranteed minimum values are normally allowed, if a specimen of each individual item is tested before use to determine that the actual strength properties of that particular item will equal or exceed those used in the design.
(g) An applicant may use other material design values if approved by the Administrator.
What does this “expected operating conditions” language in section 23.2250 mean for test & computation applicants? This phrase is used in AC 25-25A in the context of returning a stall protection to non-icing setting. It is also used in AC 20-151 for approval of TCAS units. But neither of these resources describe what this phrase means in the context of a meeting regulatory requirements.
The FAA explains in the preamble to the rule what it means when it says “expected operating conditions.” The FAA’s explanation is not perfectly illuminating, but at least it provides some guidance about what sort of operating conditions should be anticipated and accounted-for in the design process:
“The FAA did not intend to limit this requirement only to the normal operational environment because, if the failure conditions are an expected environment, then an applicant should consider those conditions and protect the structure. Deterioration or loss of strength due to corrosion, weathering, and abrasion are all examples of failure conditions because capability has been degraded. For many years, the rule has expressly required consideration of these causes. It was an expected environment for items to be corroded, weathered, and abraded, but applicants had to consider any other causes too.”