Two O-Rings, One Digit
An engine shop is working to a tight turnaround. Two O-ring part numbers differ by one digit: 123-45 and 123-54. Both are stored in adjacent bins; the labels are faded, the bins are the same colour, and the shelf lighting is dim. A technician, working on his third engine of the shift, picks an O-ring without reading the label, fits it to the oil filter housing, and signs the task card. The engine is run on the test cell and a small oil leak appears at the filter housing. The leak is caught before delivery. The technician is known as careful and experienced.
1. Classify the technician's error and justify your classification.
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This is best classified as a slip – a capture error / selection error. The technician intended to take the correct O-ring and perform the correct task; his plan and intention were right. The action was captured by a similar stimulus: two adjacent bins, near-identical part numbers differing only in digit order, identical bin colour, and poor lighting.
It is not a mistake in the rule-based or knowledge-based sense, because he did not apply the wrong rule or misunderstand the task. It is not a violation, because he was not choosing to bypass a procedure – he believed he was taking the correct part.
The "careful and experienced" background is consistent with a slip: routine, highly practised actions are exactly the ones that can run on autopilot and be captured by similar-looking cues. Experience reduces some errors but does not protect against capture errors in a poorly designed storage system.
2. Identify the latent conditions in this event and explain why they matter more than the individual's carelessness.
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Latent conditions include:
- Look-alike storage: adjacent bins, same colour, labels differing by one digit transposed (123-45 vs 123-54).
- Degraded labelling: faded labels reduce the discriminability of the two parts.
- Poor lighting: dim shelf lighting makes fine label reading harder.
- Workload and shift pattern: third engine of the shift increases routine automaticity and reduces checking.
- Production pressure: tight turnaround discourages stopping to read and cross-check a simple part.
These matter because the same conditions would produce the same error in most competent technicians. Blaming carelessness would leave the bins, labels and lighting unchanged, so the error would recur. Latent conditions are organisational weaknesses that lie dormant until an active error triggers a bad outcome. Correcting storage design, labelling and lighting removes the trap for everyone, whereas a reminder to "be more careful" only asks individuals to compensate for a system that is set up to fail.
3. Propose corrective actions at three levels: part/storage, task/procedure, and organisation.
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Part/storage level
- Physically separate the two part numbers (different shelf, different bin colour, or one in a locked kit).
- Replace faded labels with clear, high-contrast labels showing the full part number and a description; add a barcode or scan step.
- Improve shelf lighting.
Task/procedure level
- Require the part number to be read against the task card and recorded before fitting.
- Introduce a duplicate or independent check for look-alike parts on oil system components.
- Add a hold point before engine test-cell run confirming correct seals fitted.
Organisation level
- Add the part pair to a look-alike/sound-alike parts register and review similar pairs across the store.
- Use a MEDA-style event investigation to capture the full causal chain rather than a single blame statement.
- Feed the finding into training and into the design of the stores layout.
The reasoning is layered defence: reduce the chance of selecting the wrong part, make any selection error detectable, and embed the lesson so it does not depend on individual memory.