AS9102 Form 3 Characteristic Numbering Rules

When I built my first Form 3 from a multi-sheet aerospace bracket, I numbered every characteristic sequentially starting from 1. Simple enough. Then the customer asked me to trace characteristic 47 back to the drawing. It took me four minutes of flipping between sheets to find it.

That is when I learned that AS9102 Form 3 characteristic numbering rules matter more than you think.

Every row in Form 3 needs a characteristic number. It sounds trivial, but the numbering scheme you choose affects traceability, auditability, and whether the customer accepts or rejects your FAI without even looking at the measurements.

AS9102 Form 3 Characteristic Numbering Rules: What the Standard Says

AS9102 Rev C (the current revision) requires that each design characteristic be assigned a unique number. It does not mandate a specific numbering scheme. This is both a blessing (flexibility) and a curse (inconsistency). We cover all Rev C requirements in AS9102 Rev C requirements explained.

What the standard does require: every characteristic must have a unique number, the numbering must be traceable back to the drawing (the reviewer must be able to find each characteristic on the drawing), and if the drawing has balloon numbers, the Form 3 should reference them.

The traceability requirement is the key. Rev C strengthened this compared to Rev B. The reviewer must be able to locate any characteristic on the drawing within seconds.

If they cannot, the FAI fails the traceability requirement regardless of whether the measurements are correct.

The Three Common Numbering Approaches

Approach 1: Sequential by Sheet and View

Number characteristics in the order you encounter them as you walk through the drawing sheet by sheet, view by view.

Sheet 1 general notes get characteristics 1 through 6.

Sheet 2 top view gets 7 through 20.

Sheet 2 section A-A gets 21 through 35.

Sheet 3 detail views get 36 through 50. And so on.

This is the most common approach because it matches the natural workflow: the quality engineer reads the drawing front to back and numbers as they go. It is also the easiest to build because there is no need to pre-plan the numbering scheme.

The disadvantage is revision management. If the drawing is revised and a new feature is added to Sheet 2, you either renumber everything downstream (disrupting traceability to previous FAIs and making partial FAI documentation harder) or insert a 20a, 20b numbering scheme (functional but visually messy).

We cover revision management implications in AS9102 partial FAI vs full FAI requirements.

Approach 2: Matching Drawing Balloon Numbers

If the drawing has balloons (numbered callout markers pointing to each feature), use the balloon numbers as characteristic numbers. Balloon 10 on the drawing becomes characteristic 10 on the Form 3. Balloon 33 becomes characteristic 33.

This provides perfect traceability. The reviewer looks at balloon 33 on the drawing and immediately finds row 33 on the Form 3. No searching, no ambiguity. This is why most aerospace customers and auditors prefer this approach when balloons exist.

The disadvantage is that balloon numbers may not be sequential (they might go 10, 12, 15, 18, 19, 21). Some features may not have balloons. And if you are generating balloons from an extraction system, the balloon numbering depends on the extraction order and deduplication logic.

We cover the ballooning process in how to balloon a drawing for FAI.

Numbering is the first thing FAI software automates; the tools differ on everything after that.

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Approach 3: Grouped by Feature Type

Group characteristics by category: general notes first, then dimensions, then GD&T, then surface requirements. Characteristics 1 through 5 cover general notes and standards. 6 through 40 cover dimensional characteristics. 41 through 55 cover GD&T callouts. 56 through 58 cover surface finish and treatment.

This approach makes life easier for the inspector (all CMM measurements are grouped together, so they can program and run them as a batch) and easier for the reviewer to check completeness by category ("I count 15 GD&T callouts on the drawing and 15 GD&T rows in Form 3, looks complete").

The disadvantage is that the grouping does not map to the drawing layout. The reviewer has to hunt across sheets to find each characteristic. A dimension in row 12 might be on Sheet 5, while the next dimension in row 13 might be on Sheet 2. This makes the 10-second traceability test harder to pass.

What Auditors and Customers Actually Want

In practice, most aerospace customers and AS9100 auditors prefer Approach 2 (matching balloon numbers) when balloons exist, and Approach 1 (sequential by sheet) when they do not.

The key test is simple: can the reviewer find characteristic 33 on the drawing within 10 seconds? If yes, your numbering scheme works. If it takes them 2 minutes of searching across 7 sheets, your FAI will be sent back for revision regardless of whether the measurements are correct.

I have seen FAI packages with perfectly accurate measurements rejected purely because the reviewer could not efficiently trace characteristics back to the drawing. The measurements were fine. The traceability was not.

Practical Tips for Numbering

Tip 1: Include a "Reference Location" column. For every characteristic, note the sheet number and view name ("Sheet 3, Section A-A"). This eliminates ambiguity regardless of which numbering scheme you use. It is also required by AS9102 Rev C for traceability.

Tip 2: Do not number title block metadata. Part number, revision, material specification, and weight are Form 1 data, not Form 3 characteristics. Numbering them inflates the characteristic count and confuses reviewers who compare your count against their own. We cover what counts and does not count as a characteristic in what counts as a characteristic in AS9102 Form 3.

Tip 3: One row per GD&T callout. A positional tolerance referencing datums A|B|C is one characteristic, not three. Do not split multi-datum callouts into separate rows. This is one of the most common Form 3 mistakes and a frequent rejection reason.

Tip 4: Keep a numbering log for revisions. When the drawing gets revised and the FAI is updated, document which characteristics were added, removed, or renumbered. This supports partial FAI justification on Form 1 and maintains traceability between the full FAI and subsequent partial FAIs.

Tip 5: Start general notes at characteristic 1. The governing tolerance standard, blanket notes, and edge treatment requirements should be the first rows in Form 3. This establishes the context for every subsequent row. When the reviewer opens Form 3 and sees "Characteristic 1: General tolerances per ISO 2768mk" as the first row, they immediately know the framework for the entire report.

Tip 6: Handle "x places" features consistently. A through hole called out as "Dia 4.5 x 6 places" can be one balloon/characteristic with a note "6 places, all measured" or six individual characteristics (one per hole location). Either approach is acceptable, but be consistent throughout the Form 3. If you use "one balloon, six measured" for holes, use the same approach for other repeated features.

How Numbering Affects Downstream Processes

The characteristic numbering on Form 3 propagates to the CMM program. Many CMM operators name their measurement routines using the Form 3 characteristic numbers. Measurement point 33 in the CMM program corresponds to characteristic 33 in Form 3.

If you renumber the characteristics during a revision update, the CMM program may need to be updated as well.

This is another reason to use Approach 2 (balloon matching) when possible. The balloon numbers on the drawing remain stable across minor revisions (new features get new balloons, but existing balloons keep their numbers). This stability flows through to the CMM program and the Form 3 without disruption.

For shops that import CMM data into Net-Inspect or 1Factory, the characteristic number is the key that links the measured value to the correct Form 3 row. A numbering mismatch between the CMM output and the Form 3 means values end up in the wrong rows, creating false passes or false fails.

We cover the Net-Inspect import format in Net-Inspect Form 3 import format guide.

Mavlon Numbers Characteristics Automatically

Mavlon assigns sequential characteristic numbers following the sheet-by-sheet, view-by-view approach, with each row tagged with its reference location (sheet, view, detail). General notes are numbered first, followed by dimensions and GD&T in the order they appear on each sheet. When the drawing has existing balloon numbers, Mavlon maps characteristics to balloons for direct traceability.

Bring Your Drawing to a Demo to see how Mavlon numbers your drawing's characteristics. First drawing free.