The rotor tip clearance in a gas turbine engine varies throughout the engine operating regime. It has considerable influence on the engine performance. Blade to casing rub is imminent at certain operating points of the engine. Mechanical rub at high speeds could damage the total engine hardware. A precise measurement helps to obtain the optimum engine performance with safe engine operation. In this article a typical case study related to fan clearance measurement is discussed, where indications of a proven measurement system is not in agreement with the physical event during engine test. Centrifugal, thermal, assembly and wear effects can affect tip clearance measurement. Centrifugal forces untwist the blade tip, resulting in change in the effective area of the target that is seen by capacitance sensor. Relative component growths due to thermal effect result in the displacement of the sensor from its original position. This could induce error into this measurement. Assembly errors are seen during blade to disc assembly. Wear occurs under the action of centrifugal loading and vibration in compressor blades dovetail roots that are attached to the disc. This leads to wear in involved metal surfaces and it could be a source of error in this measurement. Measurement system also has its own uncertainty. During the current work all sources of errors were evaluated. Probable actual running clearance on the engine and reasons for the mismatch in indication were successfully arrived at through analytical and experimental studies. This work has provided an insight into probable sources of errors and their treatment methodologies using analytical and experimental techniques. This has helped in identifying the changes needed in the calibration procedure, methods to reduce the measurement system uncertainty band and measurement procedure.

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