Aircraft System Design and Structural Safety Concepts

021-01-01Pablo Asensio Martínez2026-03-273 min

Aircraft system design incorporates rigorous operating procedures, design philosophies, and certification standards to ensure safety and reliability. From the detailed instructions in operations manuals to structural concepts like Safe-Life and Fail-Safe, every aspect is regulated and engineered to minimize risk and handle failures effectively.

Operating Procedures and Instructions

According to EASA AIR OPS AMC1 SPA.LVO.125, operators must specify detailed operating procedures in their operations or procedures manual. These instructions must be compatible with the Aircraft Flight Manual (AFM) and cover:

  • Checks for the satisfactory functioning of aircraft equipment, both before departure and in flight.
  • Procedures for the take-off, approach, flare, hover, landing, rollout, and missed approach.
  • Procedures to be followed in the event of failures, warnings (including HUD/HUDLS/EVS), and other non-normal situations.
  • The minimum visual reference required and actions necessary arising from a deterioration of visual reference.
  • The importance of correct seating and eye position.
  • Allocation of crew duties to allow the pilot-in-command to devote themselves mainly to supervision and decision making.
  • The rule for all height calls below 200 ft to be based on the radio altimeter.
  • The rule for the localiser sensitive area to be protected.
  • The use of information relating to wind velocity, wind shear, turbulence, and runway contamination.
  • Specific procedures for LTS CAT I, OTS CAT II, approach operations utilising EVS, and practice approaches.

Structural Design Concepts

Two primary design philosophies are developed to ensure the safe operation of aircraft components:

Safe-Life Design

Safe-Life defines the minimum life during which it is known that no catastrophic damage should occur. The life of such a structure can be defined in cycles, flight hours, pressurization events, or calendar time.

  • After this interval has been reached, the structure should be replaced, inspected, or overhauled per manufacturer recommendations, regardless of its condition.
  • It typically involves a single load path where the structural element is removed at the end of its lifetime.

Fail-Safe Design

Fail-Safe design ensures that if a structure fails, an alternative load path provided by an adjacent part is able to take over the loads for a limited amount of time. This redundancy allows the structure to withstand loads until the damage is identified during scheduled maintenance and the part is replaced.

  • It relies on multiple load paths.
  • It is not replaced after limited usage but repaired or replaced when damage is detected.
  • It features built-in redundancy.
  • A Fail-Safe design can withstand a certain amount of weakening without catastrophic failure.

Redundancy

Redundancy means the existence of additional or duplicate systems or load paths. It is an essential component of the Fail-Safe design concept.

  • For structures: This usually involves multiple load paths so that if one breaks, the others can take the required forces.
  • For systems: There are usually extra sources (e.g., electrical power, hydraulic pressure) to prevent a complete failure of essential systems, such as flight controls or landing gear extension.

Certification Specifications (CS)

All aircraft are certified according to specifications determined by the competent authority. Certification Specifications (CS) are non-binding technical standards adopted by EASA to meet the essential requirements of the Basic Regulation. CSs are used to establish the certification basis (CB) and lay down requirements for:

  • Design
  • Material quality
  • Build quality
  • Behavioural characteristics