Definition
A nondestructive inspection technique in which a flexible plastic or rubber bag is sealed over an aircraft part and a vacuum is drawn inside the bag. The vacuum presses the bag tightly against the part, allowing penetrant or other inspection media to be drawn into surface cracks or flaws so they can be detected.
Plain English
A way of checking a part for cracks by sealing a bag around it and sucking the air out, so the bag pulls tight against the surface and helps reveal any tiny flaws.
Context Anchor
Seen in aircraft composite structure repair and fabrication procedures, especially when a repair patch or layup must be held firmly against a surface during hardening.
Derivation
The phrase comes from the literal shop action: a flexible bagging sheet is placed over the part. In this context, “bag” does not mean a storage bag; it means an airtight cover used to apply even pressure.
Why Pilots Care
Pilots rarely perform this themselves, but understanding the term helps when reading maintenance records or discussing component inspections with a mechanic. Knowing that a part has been inspected this way means cracks too small to see with the naked eye have been checked for.
Analogy
Think of vacuum-sealing food in a plastic bag at home — the bag clings tightly to whatever is inside. Here, that same tight cling is used to press inspection fluid into any cracks on the part.
Intuition Check
Do not read this as simply covering a part to keep it clean. Here, the bag is a working tool that applies pressure by sealing the area and removing air.
Example Sentence 1
The shop used a bag over the part technique to inspect the turbine blade for surface cracks before returning it to service.
Example Sentence 2
The mechanic always bags over the part when a component will sit overnight before reinstallation.