Rotor tip clearance

A method of monitoring deterioration of a rotor tip clearance arrangement comprising steps:

• a) at a first time instance, measure a first rotor speed;
• b) after measuring the first rotor speed at the first time instance, alter a first position of a tip clearance control actuator comprising a cooling flow valve by a first increment to reduce a first rotor tip clearance;
• c) iterate steps a) and b) and calculate a first change of the first rotor speed between pairs of first iterations;
• d) after step c), identify a first knee point where the first change in the first rotor speed changes and record a first corresponding tip clearance control actuator position as a first calibrated position;
• e) at a second time instance, measure a second rotor speed;
• f) after measuring the second rotor speed at the second time instance, alter a second position of the tip clearance control actuator by a second increment to reduce a second rotor tip clearance;
• g) iterate steps e) and f) and calculate a second change of the second rotor speed between pairs of second iterations;
• h) after step g), identify a second knee point where the second change in the second rotor speed changes and record a second corresponding tip clearance control actuator position as a second calibrated position;
• and
• i) compare the second calibration position at the second time instance to the first calibration position at the first time instance to determine deterioration of the rotor tip clearance arrangement.