An Open Bag, a Fleece, and a Dash Number

Control failures

• No wrist strap and no dissipative work surface: the engineer is not at the same potential as the card, so every contact is a discharge event.
• Bag open two days in a tool tray: the shielding bag protects only while closed; an open bag is a container, not a protected environment.
• 25% relative humidity: low humidity lets charge persist on surfaces and on the body for a long time.
• Synthetic fleece: triboelectric charging against a seat, trolley or the card's own packaging easily reaches kilovolts.
• Nearest bond 4 m away: outside normal strap cord reach, so no practical path to equalise potential - this is exactly what a portable field service kit (grounded strap plus dissipative mat) exists for.

Why BIT does not clear it

• Built-in test is a functional, low-voltage, low-stress check at ambient temperature; it has no coverage of ESD integrity.
• A human body model discharge can rupture gate oxide outright or create latent damage - trapped charge and thin spots - that shifts threshold voltage and increases leakage. Such a part often passes at 25 C and fails later under temperature and voltage stress.
• Therefore a BIT pass is not evidence that ESD damage did not occur; it only says the part works right now.

Meaning. Dash numbers denote different configurations of the same base unit - typically hardware modification status, and usually a different loaded software part number (the IPC pairs -001 with software 4.2). They are separate part numbers, not cosmetic variants, so the fit of -003 must be justified rather than assumed.

Verify before sign-off

1. Approved parts documentation: does the IPC or a service bulletin list -003 against this aircraft, or is it an approved substitute for -001?
2. Aircraft modification status: has the aircraft embodied the modification that -003 belongs to? Fitting an unapproved configuration takes the aircraft outside its certification basis.
3. Software: read the software part number actually loaded in the fitted unit - do not take the box label or the delivery note - and compare it with the aircraft's approved software configuration list.
4. Interchangeability: confirm the interface and pinout are unchanged; a dash change may bring altered power or discrete-signal behaviour.
5. Record both the hardware part number and the software part number in the technical log.

Dispatch - conditional.

• If -003 is the configuration approved for that aircraft (the relevant modification embodied, software verified by read-out and recorded), then fitting it was correct and the dash difference is an upgrade, not an error. Dispatch stands, subject to recording the part and software numbers.
• If -003 is not approved for that aircraft, the unit must be removed before further flight. Unapproved hardware or software means the behaviour is not covered by the certification basis, and the log entry is misleading.

The removed card

• Treat it as ESD-suspect: quarantine, label clearly, and do not return it to serviceable stock. It failed after open-bag, no-strap handling in dry conditions, which is exactly the latent-damage signature.
• Note that the replacement card may also have been exposed, since it was unpacked in the same environment. Check it too.

Process

• Raise an internal ESD occurrence and a shop finding; re-brief the ramp team on field service kit use.
• Where no grounded point is reachable, the job belongs in a controlled environment or must be done with a portable kit - not bare-handed because the BIT passes.