Waveguide

1. A gas turbine engine comprising:

a casing structure including a radially outer casing portion and a radially inner casing portion; and

a sensor system configured to measure a clearance between a rotary blade and the casing structure, the sensor system including: (i) a transmitter configured to transmit an electromagnetic signal, (ii) a waveguide coupled with the transmitter, the waveguide being configured to guide the electromagnetic signal radially inwards through the casing structure, and guide a reflected electromagnetic signal radially outward through the casing structure, and (iii) a receiver configured to receive the reflected signal, the waveguide including:

a radially outer portion configured to be mounted on the radially outer casing portion of the casing structure, the radially outer portion including a bore extending in a radial direction through the radially outer portion, the bore being configured to allow passage of the electromagnetic signal;

a radially inner portion configured to be mounted on the radially inner casing portion of the casing structure, the radially inner portion projecting radially outwards towards the radially outer portion, the radially inner portion including a bore configured to allow passage of the electromagnetic signal, the bore of the radially inner portion being open at a radially inner end, the radially outer portion and the radially inner portion being separate from each other; and

a sealing portion configured to extend between the radially outer portion and the radially inner portion, the sealing portion being configured to form a seal between the radially inner portion and the radially outer portion, the sealing portion including a deformable portion.

2. The gas turbine engine according to claim 1 , wherein the radially outer portion includes an enlarged sealing plug at a radially outermost end of the radially outer portion.

3. The gas turbine engine according to claim 1 , wherein the radially outer portion includes a threaded portion for mating engagement with the radially outer casing portion.

4. The gas turbine engine according to claim 1 , wherein the radially inner portion includes a threaded portion for mating engagement with the radially inner casing portion.

5. The gas turbine engine according to claim 1 , wherein the bore of the radially inner portion has a smaller diameter than the bore of radially outer portion.

6. The gas turbine engine according to claim 1 , wherein the deformable portion includes a radially-extending flexible or corrugated wall portion.

7. The gas turbine engine according to claim 1 , wherein the sealing portion is affixed to the radially outer portion and the deformable portion biases the sealing portion against the radially inner portion.

8. The gas turbine engine according to claim 1 , wherein:

the radially outer portion is mounted in the radially outer casing portion; and

the radially inner portion is mounted in or integral with the radially inner casing portion.

9. The gas turbine engine according to claim 8 , wherein the casing structure is a fan casing structure, compressor casing structure or turbine casing structure.

10. The gas turbine engine according to claim 8 , wherein the radially inner casing portion is a segment seal and the segment seal is radially spaced from the radially outer casing portion by a segment carrier.

11. The gas turbine engine according to claim 10 , wherein the radially inner portion is integral with the segment seal.

12. The gas turbine engine according to claim 8 , wherein the radially outer portion is welded to the radially outer casing portion.

13. The gas turbine engine according to claim 8 , wherein the radially inner portion is integral with the radially inner casing portion.