CS 25 Introduction to Airworthiness – Safety Assessment – Consideration by Sofema Aviation Services (SAS) www.sassofia.com
Introduction Related to Failure Condition Classification
For large airplanes, the airworthiness standards (CS/FAR-25), require fail-safe structures, except when it is not possible to implement such a design due to geometrical restrictions. An example of such a case is the landing gear and its attachments.
Principle Loads
For CS/FAR-25 airplanes, the principal loads that should be considered in establishing a loading spectrum are
- Flight loads (gust and manoeuvre),
- Ground and pressurization loads.
In assessing the possibility of serious fatigue failures, the design is examined to determine possible points of failure in service.
In this examination, results for stress analysis, static and fatigue tests, strain gauge surveys, tests of similar configurations, and service experience are considered.
Failure Conditions are classified according to their severity as:
- Minor. Failure conditions that do not reduce airplane safety significantly. These involve crew actions well within their capability. Example: Malfunction in the rate of turn indication system. And
- Major. Failure conditions that would reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions. Significant reductions in safety margins or functional capabilities or a significant increase in crew workload may result. Discomfort to occupants, possible injuries.
- Hazardous. Failure conditions that would reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions such that large reductions in safety margins or functional capabilities occur. The physical distress or workload of the flight crew is so high that the flight crew cannot perform their tasks accurately or completely. Serious or fatal injury to a small number of occupants may result. Example: Malfunction in the display of altitude information.
- Catastrophic. Failure conditions that would prevent continued safe flight and landing. Example: Loss of all means of attitude information.
Airworthiness – Safety Assessment
- An inverse relationship exists between the severity of failure conditions and probability of occurrence:
o Minor failures are probable and could arise several times in the aircraft’s life.
o Major failures are remote (~10-5), might arise once in an aircraft’s life, and would arise several times in the whole fleet’s life.
o Hazardous conditions are extremely remote (~10-7), might arise once in the whole fleet’s life.
o Catastrophic conditions are extremely improbable (~10-9), are unlikely to arise in the whole fleet’s life.
Note: The safety assessment of equipment, systems, and installation is a very important part of aircraft design. The techniques of safety assessment are a specialist matter and it is very important to start the assessment from the very early phases of design. Late assessments will obviously result in expensive design changes and delays (which are also expensive).
Airworthiness – Fatigue Strength
The airworthiness standards consider two kinds of aircraft structure:
- Single load path structures.
- Multiple load path structures.
Single load path structures.
- Applied loads distributed through a single member. Failure results in the loss of the structural capability to support the applied loads. For example, a wing-fuselage attachment.
- The structure must result in safe life and must be able to sustain a number of cycles during which there is a low probability of the structure to degrade below its ultimate design load value due to fatigue.
Multiple Load Path Structures.
- Redundant structures in which the applied loads would be safely distributed to other load-carrying members in case of the failure of an individual element.
- The structure is damage-tolerant, it is able to retain its residual strength for a period without being repaired after a failure or partial failure occurs due to fatigue, corrosion, etc. This kind of structure is called fail-safe.
Next Steps
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Aircraft Airworthiness, aircraft design, Aircraft Design Changes, Airworthiness, Airworthiness Standards, aviation safety, Catastrophic Failure conditions, CS 25, CS/FAR-25, EASA, Failure Condition Classification, Fatigue, Hazardous Failure conditions, Major Failure conditions, Minor Failure conditions, Multiple load path structures, SAS blogs, Single load path structures, Stress Analysis
