Eighteen elements, and one of them decides whether the other seventeen are any use.
AM.I.K.R4— selection of inspection techniques.
K3 and S7 list the methods. R4 asks which one to use, and getting it wrong produces a
clean result on a defective part — which is the worst possible outcome of an inspection,
because it is indistinguishable from a correct clean result and it ends the investigation.
1. What each method finds, and what it cannot
Every method has a blind spot, and the blind spot is the whole basis of selection.
| Method | Finds | Cannot find | Constraints |
|---|---|---|---|
| Visual | Surface condition, cracks, corrosion, distortion, wear, leakage, missing hardware | Anything subsurface; tight surface cracks without magnification | Access, lighting, cleanliness, and knowing the failure modes of the part |
| Dye penetrant | Surface-breaking defects | Subsurface defects. Anything in a porous material — it bleeds and gives false indications | Non-porous materials only; the defect must be open to the surface and clean |
| Magnetic particle | Surface and near-subsurface defects | Anything in a non-ferrous material — aluminum, titanium, most of an airframe | Ferrous parts only; leaves the part magnetized, §3 |
| Eddy current | Surface and near-surface defects; thickness and conductivity changes | Deep subsurface flaws | Conductive materials; sensitive to geometry, edges and lift-off |
| Ultrasonic | Subsurface flaws, thickness, disbonds | Nothing beyond a defect that reflects the beam away | Needs couplant, access, a trained operator, and reference standards |
| Radiographic | Internal condition, assembly state, corrosion under structure | Tight cracks not aligned with the beam | Access to both sides; a radiation source; regulated, and not something you will do |
| Tap test | Disbond and delamination in thin skin over core | Anything in solid laminate or metal; deep defects | Composite structures of suitable construction |
The consequence, stated plainly: dye penetrant on a subsurface crack returns clean. Magnetic particle on an aluminum spar returns clean. A tap test on a solid laminate returns clean. In every case the part is defective and the paperwork says it was inspected.
So R4 comes before the technique, and the question is always: what am I looking for, where
would it be, and which method can see there?
2. Visual inspection is a method with a technique, not just looking
S4 is the most-used inspection in aviation and the most casually taught.
It has a technique:
- Clean first. You cannot inspect through dirt, grease or old sealant, and cleaning is where half of what is found gets found.
- Light it properly, and light it across the surface rather than along your line of sight. A crack invisible under direct light is obvious under raking light.
- Get close, and use aided vision. A magnifier and a mirror are inspection tools, not conveniences.
- Know what fails on this part and where. Cracks start at stress concentrations — holes, radii, edges, weld toes, fastener heads. An inspection with no expectation is a glance.
- Work a pattern, so you cover it all and know that you did.
And the finding is only half of it. A visual inspection produces a disposition: serviceable, repairable, or scrap, against a limit from approved data. GEN-B Concepts §2 has the rest — a measurement at the limit is a rejection.
3. R1 — the part is still magnetized when you have finished
Magnetic particle inspection works by magnetizing the part so that a flaw distorts the field. When you stop, the part is still magnetized, and that is a defect you introduced.
A magnetized component collects ferrous debris from its own oil supply for the rest of its life, concentrating it at bearings and gear teeth. On some parts it will also disturb a nearby compass or instrument.
Demagnetizing is not tidying up. It is part of the inspection, and R1 exists because it is
the step most easily skipped — the inspection is finished, the answer is known, and attention has
moved on. Which is GEN-L Concepts §4 again, and
GEN-I Concepts §5's complacency, in a third place.
4. R5 — an ohmmeter is a current source
The element looks out of place in an inspection unit until you see what it is warning about.
An ohmmeter measures resistance by pushing current through the thing being measured. That current is small, and it is not always small enough:
- It can damage sensitive components — some semiconductors, some instruments, and squib or igniter circuits where any current at all is unacceptable (AF-M §1).
- Applied to a live circuit it destroys the meter, because the meter is expecting to supply the only current present.
The rule: know what is on the other end of the probes, and know the circuit is dead. This is the inspection unit's version of GEN-D Procedures Procedure 0.
5. Why the specimen library exists
A serviceability judgment cannot be assessed on a serviceable part. Nor can it be assessed on a defective one, if the candidate knows every specimen handed to them is defective — because then the task is finding the defect rather than deciding whether there is one.
So the articles used for S4, S5 and S8 are a library with known defects and serviceable
controls in every category, with identification concealed and assignment rotated
(the equipment and materials specification §B2).
That is a materials requirement, not an equipment one, and it is the single thing that makes inspection assessable rather than demonstrable.
6. A clean result is a claim
The habit this unit exists to build.
Signing off an inspection asserts absence, and absence is a stronger claim than presence. To make it honestly you need three things:
- The method was capable of finding what you were looking for (§1).
- You covered the area — a pattern, not a glance (§2).
- You knew what you were looking for and where it would be.
If any of the three is missing, the honest entry is not "inspected, no defects found". It is a description of what was actually done.
7. What consumes this unit
| This unit | Is consumed by |
|---|---|
| Technique selection | AF-A S14, AF-B S11 — repairability judgments |
| Visual inspection | Every inspection element in Airframe and Powerplant |
| Dye penetrant, tap test | AF-B, AF-E |
| Measurement and calibration | Arrives from GEN-E Concepts §6 |
| Inspection programs | AF-D, PP-C |
License
CC BY-SA 4.0 (Licensing).