Sofema Aviation Services (SAS) Considers essential differences in design philosophy between FAR/CS-23 (small aircraft – typically ≤ 19 passengers or ≤ 12,500 lbs MTOW) and FAR/CS-25 (transport category aircraft), with a focus on the most technically rigorous, regulatory-aligned, and safety-driven perspective
Introduction
- The design philosophy of CS/FAR-23 aircraft focuses on simplification, affordability, and accessibility, particularly for General Aviation, air taxi, and entry-level commercial roles. Safety objectives are contextualized to lower consequence-of-failure environments.
- In contrast, CS/FAR-25 aircraft demand deep system integration, comprehensive safety assurance, and complex compliance artifacts due to the high stakes of commercial airline operations. These aircraft must be tolerant to failures, robust to environmental conditions, and capable of meeting stringent performance and reliability criteria.
Key Drivers of Design Philosophy Differences
- Safety Margins & System Redundancy
- Complexity & Operational Envelope
- Passenger Capacity & Risk Exposure
- Failure Condition Classification
- Compliance Demonstration & Testing Philosophy
- Reliability, Maintainability & Dispatch Availability
- Certification Basis & Rulemaking Philosophy
Safety Objectives & System Safety Assessment CS/FAR-23:
- Applies simplified safety analysis methods.
- Typically accepts single-failure tolerance.
- Lower consequence of failure, often tolerable at minor or major level.
- Accepts lower system reliability, given reduced operational complexity.
Safety Objectives & System Safety Assessment CS/FAR-25 (Transport Category):
- Demands a formal System Safety Assessment (SSA) aligned with ARP 4761.
- Requires compliance with stringent failure condition classifications:
- Minor, Major, Hazardous, Catastrophic
- Systems must demonstrate:
- Probability of catastrophic failure ≤ 10⁻⁹/hr
- Full redundancy and segregation for critical functions.
- Incorporates probabilistic safety targets per system.
System Design Philosophy
Aspect: System Architecture
-
CS/FAR-23: Simple design, usually one channel
-
CS/FAR-25: Complex design, with multiple channels that can handle faults and keep working safely
Aspect: Redundancy
-
CS/FAR-23: Extra systems are generally not needed
-
CS/FAR-25: Extra systems are needed for critical functions to ensure safety
Aspect: Fault Detection
-
CS/FAR-23: Basic or manual alerts when something goes wrong
-
CS/FAR-25: Advanced systems that automatically check for problems
Aspect: Power Supply
-
CS/FAR-23: One power source or a backup battery is enough
-
CS/FAR-25: Two or three separate power sources to ensure everything keeps working
Flight Control Systems CS/FAR-23:
- Usually mechanical or cable-based.
- Control laws are simple.
- FBW systems are rare (but possible in new Part 23 reboot for “simplified” FBW under performance-based regs).
Flight Control Systems CS/FAR-25:
- Complex Fly-by-Wire (FBW) systems with multiple levels of redundancy.
- Integrated envelope protection.
- Full Flight Control Computer (FCC) logic.
- Compliant with DO-178, DO-254, and ARP 4754 for SW and HW development.
Human Factors & Crew Workload Management CS/FAR-23:
- Assumes single-pilot operation.
- Workload may be higher per pilot due to reduced automation.
- Design includes basic ergonomic and situational awareness provisions.
Human Factors & Crew Workload Management CS/FAR-25:
- Operated by multi-crew.
- Emphasis on crew coordination (CRM), HMI integration, and alert management systems.
- Extensive focus on automated systems to support low workload in all phases of flight.
Flight Envelope & Structural Design
Parameter
CS/FAR-23
CS/FAR-25
Lightning & HIRF Protection
Not always required
Fully mandated & verified per environmental categories
Fire Protection & Environmental Systems CS/FAR-23:
- Fire protection may be limited to engine compartments.
- Pressurization systems may be absent.
- Simplified ECS (Environmental Control System).
Fire Protection & Environmental Systems CS/FAR-25:
- Mandatory fire detection & suppression systems in engines, APU, cargo, lavatories.
- Full-scale pressurization, air conditioning, and smoke evacuation requirements.
- High-fidelity analysis of cabin environment survivability.
Performance-Based vs Prescriptive Approach – New CS-23 (post-2017 reorganization):
- Performance-Based Regulation (PBR) model.
- Allows for innovative designs (e.g. electric propulsion, simplified FBW).
- Accepts consensus standards (ASTM) in lieu of prescriptive compliance.
Performance-Based vs Prescriptive Approach – CS/FAR-25:
- Prescriptive & deterministic regulations.
- Rule-based framework grounded in legacy compliance philosophy.
- Emphasizes repeatability, conservatism, and mature technology.
Economic & Operational Considerations CS/FAR-23 designs prioritize:
-
- Cost-efficiency
- Lightweight
- Reduced certification burden
- Emerging technology integration (UAS, eVTOL, GA electrification)
Economic & Operational Considerations CS/FAR-25 designs prioritize:
-
- Dispatch reliability
- High utilization
- Commercial revenue protection
- Robustness in adverse environments
Next Steps
Follow this link to our Library to find & download related documents for Free.
See Sofema Aviation Services or Sofema Online for the following course: Electrical Wiring Interconnection System (EWIS) and FAA-EASA Part 23-25 Certification – 4 Days. Please see the websites or email [email protected].
Tags:
System Safety Assessment CS/FAR-23, Structural Design, Flight Envelope, Crew Workload Management, Power Supply, Fault Detection, Redundancy, System Architecture, Transport Category, Human Factors, Transport Category Aircraft, Small Aircraft, FAR-CS-23, Design Philosophy, FAR/CS 25, SAS blogs, Safety Objectives
