9.5 Product Traceability, Age Control & Shelf Life

What Traceability Is and Why an Inspector Owns It

ISO 9000 defines traceability as the ability to trace the history, application, or location of an object. In an inspection department that abstract definition becomes a concrete test: if a defect is discovered today, can we determine exactly which units are affected, and can we find every one of them?

When traceability is intact, a bad heat of steel means recalling 340 shafts from three specific work orders. When traceability is broken, the same bad heat means recalling every shaft shipped in the affected period, because nothing distinguishes the suspect units from the good ones. The cost difference between those two outcomes is what justifies the entire traceability system.

Two Directions

• Forward tracing starts at a raw material lot, a purchase order, or a process record and follows it forward to every finished unit that consumed it, and then to every customer who received one. This is the containment direction: what did this bad lot become?
• Backward tracing starts at a finished unit — usually one that failed in the field — and works backward to its material lots, operators, machines, tooling, process settings, and inspection records. This is the investigation direction: what went into this failed part?

A complete traceability system supports both. Auditors test both, and section 13.2 covers tracing as an audit technique in detail.

Two Levels of Granularity

Serialization costs more and is required where the consequence of a single failure is severe or where regulation demands it. Lot control is the general-industry default. The exam distinction is that lot traceability bounds a population, while serialization identifies an individual.

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Part Numbering and Revision Control

The part number identifies what the item is; the revision level identifies which version of it. Both must appear on the traveler, the drawing, the purchase order, and the inspection record, and the inspector's first act on any job is to confirm they agree.

The governing engineering rule is about interchangeability:

• A change that leaves the part fully interchangeable with previously produced parts — a clarified note, a corrected typo, a relaxed cosmetic requirement — is issued as a revision to the existing part number.
• A change that makes the part not interchangeable — a different fit, form, or function — requires a new part number or a new dash number, because mixing the two in stock would be unsafe.

Dash numbers (for example 10025-1, 10025-2, 10025-3) identify variants within a family that share a base design but differ in a controlled way such as length, handedness, or finish.

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Lot, Batch, and Heat Numbers

A lot number or batch number identifies material produced or processed together under uniform conditions. Assignment rules must be written down, because "the same lot" means different things in different processes: one melt, one mixing operation, one shift of production, one oven load, one plating tank run.

In metals, the mill assigns a heat number to a single melt of steel or alloy. That heat number is the anchor of metallurgical traceability: it links the material to a mill test report (also called a certified material test report or an EN 10204 3.1 inspection certificate) documenting the chemical composition and mechanical properties of that melt. Aerospace and pressure-vessel work commonly requires heat-lot traceability, in which the heat number follows the material through every cutting and machining operation and is stamped or etched on the finished part.

Practical rules that generate findings when violated:

• Never break lot identity. When bar stock is cut, each piece must carry the heat number forward — by transfer stamp, tag, etched marking, or a controlled routing that never separates the pieces from their identity.
• Never mix lots in a container without recording that it is a mixed-lot container, because a mixed container inherits the containment scope of every lot in it.
• Segregate unidentified material immediately. Material of unknown origin cannot be certified and is normally scrapped rather than gambled on.

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Date Codes

Date codes encode when an item was made so that age can be computed without a separate record.

The Julian date is the one most likely to appear on the exam. In the YYDDD form, the first two digits are the year and the last three are the sequential day of that year, so 26095 is the 95th day of 2026. Day 001 is January 1 and day 365 (or 366 in a leap year) is December 31.

Two cautions. First, a date code usually marks date of manufacture, not date of expiration — the inspector must add the shelf life to compute the use-by date. Second, some manufacturers use proprietary alphanumeric codes; a written decoding reference must be on file, and an inspector must never guess at a code's meaning.

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Shelf Life and Age Control

These two terms are related but not synonymous, and the exam separates them.

• Shelf life is a property of the material: the maximum time it may be stored under specified conditions before its properties degrade beyond acceptable limits.
• Age control is the management system that enforces shelf life: dating incoming material, storing it correctly, rotating it, monitoring it, and withdrawing it when it expires.

Age-Sensitive Materials an Inspector Encounters

Three of these deserve elaboration because they are frequent exam material.

Elastomers are controlled from cure date, not receipt date. An O-ring's clock starts when the rubber was vulcanized at the manufacturer, which may be many months before the shop receives it. Aerospace practice under standards such as SAE ARP5316 and AS1933 assigns age limits from that cure date, expressed in quarters. An inspector who dates an O-ring bag on the day it arrives has silently extended its life by the entire time it sat in the supplier's warehouse.

Composite prepreg is controlled twice. It carries a freezer life measured while stored at roughly minus 18 °C, and separately an out-time budget — the cumulative hours it may spend at room temperature across all the times it is removed for layup. Out-time accumulates and is never reset by returning the material to the freezer. Prepreg with freezer life remaining but exhausted out-time is expired.

Refrigerated adhesives need documented warm-up. Opening a cold container in a warm room condenses moisture into the adhesive. Procedures specify a sealed warm-up period before opening, and the inspector verifies it happened.

Extending or Re-testing Expired Material

Shelf life extension is sometimes technically legitimate, but it is never an inspection-floor decision. Extension requires an engineering-approved procedure, defined re-test criteria (viscosity, gel time, hardness, tensile on a witness sample), a documented new expiration, and re-labeling. Absent that procedure, expired material is nonconforming material and goes through the material review process covered in section 9.3.

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FIFO and Stock Rotation

First-in, first-out (FIFO) issues the oldest stock first. It is the default rotation discipline and is enforced physically — flow racks loaded from the back and picked from the front, labeled bins, and dated tags visible without handling.

The exam-relevant limitation is that FIFO is not the same as age control. FIFO only guarantees the order of consumption; it does nothing to stop expired material from being issued if the oldest item on the shelf is already past its date. Age control adds the expiration check that FIFO lacks.

FEFO — first-expired, first-out — issues by expiration date rather than receipt date. It is the correct discipline for age-sensitive stock, because a recently received item with a short remaining life should be consumed before an older item with a longer one.

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Barcoding and Automatic Identification

Automatic identification and data capture (AIDC) removes the transcription errors that manual identification introduces.

Serialized government and defense hardware is commonly marked under MIL-STD-130 with a unique item identifier (UII/IUID) in a Data Matrix symbol.

The conceptual point the exam tests is this: an identifier is not traceability. A barcode is a fast, accurate way to read an identity. Traceability exists only when that identity is linked in records to what the item is, what went into it, who processed it, what inspection results it produced, and where it went. A perfectly marked part in a system with no linked records is untraceable.

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Traceability Records

The distinction between a CoC and a CoA is a recurring exam item and is developed further in section 9.1: a CoC asserts conformance, while a CoA reports numbers.

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Worked Containment Scenario

A customer reports a cracked bracket. Backward tracing from the bracket's serial number reaches work order 44821, which reaches heat number H-77341 of 4130 steel, whose mill test report shows a sulfur content near the top of the allowed band.

Forward tracing from heat H-77341 is now the containment activity. The material record shows the heat was received as six bars, cut into 214 blanks, and consumed on four work orders. Those four work orders produced 214 brackets: 12 scrapped in process, 41 in finished goods, 27 in work in process, and 134 shipped across nine customer deliveries. The containment is therefore 134 units at nine customers plus 68 units still in the plant — bounded, specific, and defensible.

Had the shop cut all six bars into a common blank bin without transferring the heat number, forward tracing would have stopped at "some brackets made during a two-month window," and every unit shipped in that window would be suspect. The entire difference is whether heat identity survived the cutoff saw.

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Common Exam Traps

• FIFO is not age control. FIFO controls consumption order only; age control adds expiration enforcement. FEFO is the correct rule for age-sensitive stock.
• Elastomers date from cure date, not from receipt date or from the date the bag was opened.
• Out-time accumulates. Returning prepreg to the freezer stops the clock but never resets it.
• A date code is usually a manufacture date. Shelf life must be added to it to get an expiration.
• Julian YYDDD. 26095 is day 95 of 2026, not September 5 and not the 26th week.
• A barcode is identification, not traceability. Traceability requires linked records.
• Expired material is nonconforming material. It goes to material review; it is not quietly used up.
• Unidentified material cannot be certified. Lost identity is normally unrecoverable.