How are nozzle guide vane tip clearances tested and how is clearance adjusted?

• A. Visual inspection only.
• B. Dial indicators, feeler gauges, or laser-based systems; adjust by shimming the casing or replacing NGVs to restore specified clearance.
• C. NGV clearance cannot be adjusted.
• D. Replace entire engine to fix clearance.

Answer: (B)

Measuring nozzle guide vane tip clearance relies on precise metrology to determine the radial gap between the vane tips and the surrounding shroud. You’ll often see dial indicators or feeler gauges used for direct, contact measurements at critical locations, while laser-based systems provide non-contact, high-precision clearance mapping around the circumference. Once the clearance is known, restoration to specification is achieved by adjusting the position of the casing relative to the NGVs—typically by shimming the casing or mounting hardware to change the gap. If the NGVs or shroud surfaces are worn or damaged beyond what adjustment can correct, the NGVs (or the shroud) may be replaced to bring the clearance back to spec. Replacing the entire engine is not the typical remedy for NGV clearance.