Start on the drawing, not in the spreadsheet
Special, key and critical characteristics are defined by the customer drawing, the specification, the contract, or AS9103 variation-management requirements. Internal symbols that do not map to those sources create a second language nobody else can audit.
List every designated characteristic before you open the PFMEA. Include notes that act like characteristics — “100% inspect,” “no nicks in sealing surface,” “serialize and report.”
The four-document chain
1. Drawing / specification. The requirement, tolerance, classification and any mandated method.
2. PFMEA. How that characteristic can go wrong at the operation that creates or protects it, how severe the effect is, what currently prevents or detects the failure, and whether the ranking justifies a tighter control.
3. Control Plan. The control that production will actually run: method, equipment, frequency, reaction. Classification on this row must match the drawing language your customer uses.
4. Inspection plan and records. The same characteristic, the same method, a place to write the result, and a reaction that matches the Control Plan. If the Control Plan says CMM every fifth piece and the inspector is doing a go/no-go check, the chain is broken.
Work instructions sit beside that chain. They do not replace it. The workstation document should tell the operator what to do; the Control Plan tells Quality what was agreed.
Where the chain breaks
- The drawing uses one symbol, the Control Plan uses another, and the PFMEA uses a highlight color.
- The characteristic is inspected at receiving or final only, while the PFMEA risk sits at the machining operation that created it.
- Capability studies exist for a key characteristic that is not on the Control Plan at all.
- A customer flowed down a new key characteristic on a change notice and only the ballooned drawing was updated.
AS9103 is about variation, not labels
Calling a feature “key” without a monitoring method, a reaction, and a way to see whether the process is holding is just labeling. Variation management expects you to know which characteristics can move the product out of requirements and to watch those characteristics on purpose.
If you need the chain rebuilt against a current drawing, that is the special characteristics work. Inspection planning and Control Plan revisions are quoted from the same scope review.