Key Characteristics, Critical Items & Special Requirements Knowledge App

Explore how Special Requirements (SR), Critical Items (CI) and Key Characteristics (KC) are identified, related, flowed down and controlled through design, industrialisation, production, the supply chain, special processes, change and in-service feedback.

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Worked examples
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Control and analysis tools
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Special-process relationships
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Standards and guidance sources

Understand the three concepts

Core relationship: Special Requirements identify heightened requirement risk. Critical Items identify where significant consequences may reside. Key Characteristics identify measurable product or process features whose variation must be managed to protect the required outcome.
SRRequirement risk

Special Requirement

A requirement identified by the customer or organisation as having high risk of not being achieved, and therefore needing inclusion in operational risk management.

  • May be technically difficult, novel or capability-limited.
  • Can arise from performance, technology, schedule, supplier or verification constraints.
  • Is not necessarily a physical feature.
  • Should have an owner, risk treatment and verification strategy.
CIConsequence focus

Critical Item

A function, part, material, software element, process or characteristic having a significant effect on product realisation, use, safety, performance, form, fit, function, producibility or service life.

  • Can originate from safety analysis, DFMEA, reliability or manufacturing risk.
  • May be an assembly, component, function, process or source.
  • Requires specific actions and controls.
  • Should remain traceable to its failure consequence.
KCVariation focus

Key Characteristic

A feature of a material, process or part whose variation has a significant influence on product fit, performance, service life, manufacturability, assembly or another important outcome.

  • Must be measurable or otherwise monitorable.
  • May be a product characteristic or process parameter.
  • Requires variation management, not just acceptance inspection.
  • Selection should be technically justified and kept meaningful.

How they relate

SPECIAL REQUIREMENTWhat is unusually difficult or risky to achieve?

Identified during contract review, requirements capture, feasibility and operational risk assessment.

→
CRITICAL ITEMWhere could a significant consequence originate?

Identified through design, safety, reliability, manufacturing and supplier-risk analysis.

→
KEY CHARACTERISTICWhich variation influences the outcome?

Controlled using capable processes, measurement, monitoring, reaction plans and feedback.

Not a mandatory one-to-one chain: an SR can be removed by design action, a CI may contain several KCs, and a KC can be designated internally even where the customer has not marked it. Every requirement still requires conformity, but not every requirement is an SR, CI or KC.

Classification logic

QuestionLikely classificationEngineering response
Is the requirement unusually difficult, novel, capability-limited or high risk?Candidate SRRecord in operational risk management; define mitigation, ownership, evidence and review points.
Could failure or nonconformity have a significant safety, performance, reliability, producibility, cost or schedule consequence?Candidate CITrace to the failure effect; establish prevention, detection, verification, configuration and change controls.
Does variation in a measurable feature or process parameter significantly affect the outcome?Candidate KCDefine measurement, process controls, capability/stability evidence, reaction plan and ongoing variation management.
Is the item simply contractually required but without elevated risk or significant variation sensitivity?Normal requirementApply normal planning, production and acceptance controls; do not over-designate.

Common mistakes

Lifecycle control of SRs, CIs and KCs

Select a stage

Worked Examples Library

Product / situationSRCIProduct KCProcess KC / control parameterControls and evidence

Engineering and control toolkit

Special processes and characteristic management

Select a process
Special-process relationship: where results cannot be fully verified by later inspection or test, confidence depends on validated methods, qualified personnel and equipment, controlled materials and parameters, environmental control, records and periodic verification. The process itself may be a CI, while its influential parameters may be KCs or similarly heightened control parameters.

Classify the scenario

Reveal the model answer after considering each case

Knowledge check

Select a question

Choose a question and then select an answer. The app will explain the reasoning.

Standards and guidance map

Use controlled requirements: this app provides educational summaries only. The applicable contract, drawing, customer specification, statutory or regulatory requirement and current controlled edition of each purchased standard take precedence.

Reference links