FAA Aviation Mechanic Airframe (AMA)Airframe InspectionHard
During a scheduled inspection of a composite control surface with a honeycomb core, a technician notices a slight bulging or 'pillowing' effect in the skin between core cells. What is the most likely cause and appropriate follow-up action?
- AGalvanic corrosion between the core and skin requiring eddy current inspection
- BTrapped moisture in the honeycomb core that expanded, requiring a tap test and moisture check to confirm
- CFatigue cracking of the skin requiring dye penetrant inspection
- DNormal manufacturing variation requiring no further action
Show answer & explanationAnswer & explanation
Correct answer: B. Trapped moisture in the honeycomb core that expanded, requiring a tap test and moisture check to confirm
Pillowing of the skin over individual honeycomb cells is a classic indicator of trapped moisture in the core, which can expand due to temperature/altitude changes or freeze, pushing outward on the thin skin; a coin tap test and moisture detection method should be used to confirm and locate the extent of the problem.
Why the other options are wrong
- A. Honeycomb cores and skins are typically not a dissimilar metal pairing causing galvanic corrosion, and pillowing is a moisture indicator, not corrosion pitting.
- C. Fatigue cracking would not produce a localized cell-by-cell bulging pattern.
- D. Pillowing is an abnormal condition indicating a defect, not normal variation.
Honeycomb Panel Moisture (Pillowing)
A bulging pattern in the outer skin of a honeycomb sandwich panel caused by trapped moisture expanding within the core cells.
- Visible as a 'pillowing' or quilted pattern matching the cell pattern
- Detected/confirmed with a coin tap test or moisture meter
- Can lead to core degradation, disbonding, and freeze-thaw damage
Memory trick: 'Pillowing = the core is soaked and puffing up like a pillow.'