Industrial practice on mixed criticality engineering and certification in the aerospace industry
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Industrial practice on mixed-criticality engineering and certification in the aerospace industry. 2013-03-22 DATE 2013 / WICERT 2013. Development process in Aerospace. Development process - Definition. Development process – Integration and V&V. Verification vs. Validation.

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Industrial practice on mixed-criticality engineering and certification in the aerospace industry

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Industrial practice on mixed-criticality engineering and certification in the aerospace industry


DATE 2013 / WICERT 2013

Development process in Aerospace

Development process - Definition

Development process – Integration and V&V

Verification vs. Validation

Who defines the requirements

  • Customer

    • Typically OEM – formal and … “less formal” definition

  • Civil Aviation Authority

    • FAA, EASA, etc., through Technical Standard Orders(TSO, ETSO)

  • Internal standards

    • Body performing the development

! Functional, non-functional, and process requirements

Who checks whether requirements are met

  • Customer

    • Typically OEM – Reviews, Validation, Integration testing

  • Civil Aviation Authority

    • FAA, EASA, etc. - Through Certification

  • Internal standards

    • Body performing the development – As defined by operating procedures. Typically aligned with aerospace development processes.

How is certification done

  • Civil Aviation Authority does not only check the requirements testing checklist.

  • CAA primarily controls the Development process and checks the evidence that it was followed

    • Typically, an agreement must be reached with the CAA on acceptable means of compliance/certification. Typically, this includes:

      • Meeting DO-254 / ED-80 for electronics

      • Meeting DO-178B / ED-12B for software (or newly DO-178C)

      • Meeting agreed CRI – Certification Review Item

    • The following items are agreed:

      • The means of compliance

      • The certification team involvement in the compliance determination process

      • The need for test witnessing by CAA

      • Significant decisions affecting the result of the certification process

Why not having one process?

  • CS-22 (Sailplanes and Powered Sailplanes)

  • CS-23 (Normal, Utility, Aerobatic and Commuter Aeroplanes) – <5.6t,<9pax or twin-prop <9t,<19pax

  • CS-25 (Large Aeroplanes)

  • CS-27 (Small Rotorcraft)

  • CS-29 (Large Rotorcraft)

  • CS-31GB (Gas Balloons)

  • CS-31HB (Hot Air Balloons)

  • CS-34 (Aircraft Engine Emissions and Fuel Venting)

  • CS-36 (Aircraft Noise)

  • CS-APU (Auxiliary Power Units)

  • CS-AWO (All Weather Operations)

  • CS-E (Engines)

  • CS-FSTD(A) (Aeroplane Flight Simulation Training Devices)

  • CS-FSTD(H) (Helicopter Flight Simulation Training Devices)

  • CS-LSA (Light Sport Aeroplanes)

  • CS-P (Propellers)

  • CS-VLA (Very Light Aeroplanes)

  • CS-VLR (Very Light Rotorcraft)

  • AMC-20 (General Acceptable Means of Compliance for Airworthiness of Products, Parts and Appliances)

Small vs. large aircraft (CS23 vs. CS25)

Table modified from FAA AC 23.1309-1D

Figure from FAA AC 25.1309-1A

>10-5 10-5-10-9 <10-9


Can happen to any aircraft


Happens to some aircraft in fleet

<10-9 Never happens

Software-related standards – DO-178B(C)

  • Development Assurance Level for software (A-D)

    • Similar definition also exists for Item, Function …

  • Specifies

    • Minimum set of design assurance steps

    • Requirements for the development process

    • Documents required for certification

    • Specific Objectives to be proven

More than one function

  • Two situations:

    • Single-purpose device (e.g. NAV system, FADEC – Full Authority Digital Engine Controller, etc.)

      • Typically developed and certified to a single assurance level

      • Exceptions exist

    • Multi-purpose device (e.g. IMA – Integrated Modular Avionics)


  • Same HW runs SW of mixed criticality

    • For SW, more or less equals to mixed DAL

  • Implicates additional requirements

    • Aircraft class specific

    • CS-25: Very strict

      • Time and space partitioning (e.g. ARINC 653)

      • Hard-real-time execution determinism (Problem with multicores)

      • Standards for inter-partition communication

    • CS-23: Less strict

      • Means to achieve safety are negotiable with certification body

Design assurance, verification, certification

  • Design assurance + qualification  does it work?

    • Functional verification

    • Task schedulability

    • Worst-case execution time analysis

    • … and includes any of the verification below

  • Verification  does it work as specified?

    • Verifies requirements

      • Depending on the aircraft class and agreed certification baseline, requirements might include response time or other definition of meeting the temporal requirements

        • On singlecore platforms, this is composable – isolation is guaranteed

        • On multicore platforms – currently in negotiations (FAA, EASA, Industry, Academia, …)

      • Requirements are typically verified by testing

  • Certification  is it approved as safe?

    • Showing the evidence of all the above

Software verification means (1)

  • Dynamic verification

    • Testing – based on test cases prepared during the development cycle. Integral part of the development phase.

      • Unit testing – Is the implementation correct?

      • Integration testing – Do the units work together as anticipated?

      • System testing – Does the system perform according to (functional and non-functional) requirements?

      • Acceptance testing – As black-box - does the system meet user requirements?

    • Runtime analysis – Memory usage, race detection, profiling, assertions, contract checking … and sometimes (pseudo)WCET

    • Domain verification – not with respect to requirements, but e.g. checking for contradictions in requirements, syntax consistency, memory leaks

Software verification means (2)

  • Static analysis – type checking, code coverage, range checking, etc.

  • Symbolic execution

  • Formal verification – formal definition of correct behavior, formal proof, computer-assisted theorem proving, model checking, equivalence checking

    • Not much used today … just yet

Verification for certification

  • Typical approach is to heavily rely on testing, supported by static analysis and informal verification

  • DO-178C (recent update of DO-178B) defines requirements for using

    • Qualified tools for Verification and Development – DO-330

    • Model-based development and verification – DO-331

    • Object-oriented technologies – DO-332 (that’s right, 2012)

    • Formal methods – DO-333

      • To complement (not replace) testing

  • Tool qualification

    • Very expensive

      • Tools that can insert error: basically follow the same process as SW development of same DAL. Usable only for small tools for specific purposes.

      • Tools that replace testing: for A,B: developed and tested using similar development process. For C,D: subset needed. Can use COTS tools.

Near future

  • Replacement of (some) testing by qualified verification tools

     Towards formal verification

  • Adoption of DO-178C instead of DO-178B

  • Definition of technical and process requirements for multicore platforms

  • Growth of model-based development and verification

  • Enablers for incremental certification (which assumes composability of safety)

... and that’s it!

Ondřej Kotaba

[email protected]

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