Method of detecting shaft break

1. A method of detecting shaft break in a gas turbine engine having a shaft system, the shaft system comprising a shaft that couples a compressor and a turbine; the method comprising:

constructing a frequency response model of the shaft system; determining a notch frequency and a first torsional frequency for the shaft system from the frequency response model;

measuring a rotational speed of the shaft;

detecting whether a feature exists at least at one of the notch frequency and the first torsional frequency in the rotational speed; and

generating a shaft break signal in response to an absence of the feature at least at one of the notch frequency and the first torsional frequency, the shaft break signal indicating that the turbine no longer drives the compressor,

wherein the feature comprises a first feature at the notch frequency comprising a notch and a second feature at the first torsional frequency comprising a peak.

2. The method as claimed in claim 1 , wherein the shaft break signal is generated in response to the absence of both the first feature at the notch frequency and the second feature at the first torsional frequency.

3. The method as claimed in claim 1 , wherein the rotational speed of the shaft is measured close to the compressor.

4. The method as claimed in claim 3 , wherein the notch frequency is proportional to square root of a shaft stiffness divided by a turbine inertia.

5. The method as claimed in claim 1 , wherein the rotational speed of the shaft is measured close to the turbine.

6. The method as claimed in claim 5 , wherein the notch frequency is proportional to square root of a shaft stiffness divided by a compressor inertia.

7. The method as claimed in claim 1 , wherein the first torsional frequency is proportional to square root of a shaft stiffness multiplied by a sum of a compressor inertia and a turbine inertia and divided by a product of the compressor inertia and the turbine inertia.

8. The method as claimed in claim 1 , further comprising applying a filter to the rotational speed before the detecting step.

9. The method as claimed in claim 8 , wherein the filter is arranged to amplify frequencies close to the notch frequency.

10. The method as claimed in claim 9 , wherein the absence of the first feature at the notch frequency is indicated by a peak having a magnitude greater than a threshold at a frequency close to the notch frequency.

11. The method as claimed in claim 8 , wherein the filter is arranged to cancel frequencies close to the first torsional frequency.

12. The method as claimed in claim 11 , wherein the absence of the second feature at the first torsional frequency is indicated by a notch having a magnitude less than a threshold at a frequency close to the first torsional frequency.

13. The method as claimed in claim 8 , wherein the filter is a model-based filter.

14. The method as claimed in claim 13 , wherein the model-based filter is a Kalman filter.

15. The method as claimed in claim 8 , further comprising subsequently processing the filtered rotational speed.

16. The method as claimed in claim 15 , wherein the subsequent processing comprises summing squares of last n filtered measurements of the rotational speed.

17. The method as claimed in claim 15 , wherein the subsequent processing comprises integrating between last n filtered measurements of the rotational speed and a threshold.

18. A gas turbine engine control system comprising a processor configured to carry out the method according to claim 1 .

19. A gas turbine engine comprising the gas turbine engine control system according to claim 18 .