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A gas turbine engine (10) comprises a fan rotor (26) having a plurality of fan blades (24) and an apparatus (34) for detecting an impact on the fan blades (24). The apparatus (34) comprises speed sensors (48,48B) arranged to measure the speed of rotation of the fan rotor (26) and the turbine rotor (

A gas turbine engine (10) comprises a fan rotor (26) having a plurality of fan blades (24) and an apparatus (34) for detecting an impact on the fan blades (24). The apparatus (34) comprises speed sensors (48,48B) arranged to measure the speed of rotation of the fan rotor (26) and the turbine rotor (58) and to produce speed signals. A processor unit (40) analyses the speed signals to determine the angle of twist between the ends of the fan rotor (26). The processor unit (40) is arranged to analyse the angle of twist to determine if the angle of twist of the fan rotor (40) oscillates by more than a predetermined level from an average level. The processor unit (40) is arranged to produce a signal indicative of an impact to at least one of the fan blades (24) if the angle of twist oscillates by more than the predetermined level and sends a signal to an indicator device (44,46).

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1. A machine comprising a rotor having a plurality of blades arranged in a set, an apparatus for detecting an impact on one or more of the blades comprising at least one speed sensor arranged to detect the speed of rotation of the rotor and to produce at least one speed signal corresponding to the s

1. A machine comprising a rotor having a plurality of blades arranged in a set, an apparatus for detecting an impact on one or more of the blades comprising at least one speed sensor arranged to detect the speed of rotation of the rotor and to produce at least one speed signal corresponding to the speed of rotation of the rotor, a processor unit being arranged to analyse the at least one speed signal to detect a rapid drop in the speed of rotation of the rotor from a correct operating speed of rotation of the rotor for a particular operating condition, the processor unit being arranged to analyze the at least one speed signal to determine if the speed of rotation of the rotor reduces by more than a predetermined rate, the processor unit being arranged to produce a signal indicative of an impact to at least one of the blades if the speed of rotation reduces by more than the predetermined rate and to send the signal to an indicator device.2. A machine as claimed in claim 1 wherein the rotor has a first axial end and a second axial end, the blades being arranged at the first axial end of the rotor.3. A machine as claimed in claim 2 wherein the speed sensor is arranged adjacent the first axial end of the rotor and the processor unit is arranged to analyse the at least one speed signal to determine if the speed of rotation of the rotor reduces above a predetermined rate.4. A machine as claimed in claim 1 wherein the processor unit is arranged to determine if the speed of rotation of the rotor reduces at a rate of more than 500 rpm per second.5. A machine as claimed in claim 4 wherein the processor unit is arranged to determine if the speed of rotation of the rotor reduces at a rate of between 500 rpm per second and 1000 rpm per second.6. A machine as claimed in claim 1 wherein the processor unit is arranged to analyse the at least one speed signal to determine if the speed of rotation of the rotor reduces at a rate of more than 7% per second.7. A machine as claimed in claim 1 wherein the machine is a gas turbine engine.8. A machine as claimed in claim 7 wherein the blades are fan blades or compressor blades.9. A machine comprising a rotor having a plurality of blades arranged in a set, an apparatus for detecting an impact on one or more of the blades comprising at least one speed sensor arranged to detect the speed of rotation of the rotor and to produce at least one speed signal corresponding to the speed of rotation of the rotor, a processor unit being arranged to analyse the at least one speed signal to determine if the speed of rotation of the rotor reduces by more than a predetermined level, the processor unit being arranged to produce a signal indicative of an impact to at least one of the blades if the speed of rotation reduces by more than the predetermined level and to send the signal to an indicator device and wherein the apparatus comprises a first speed sensor and a second speed sensor, the first speed sensor is arranged adjacent the first end of the rotor and the second speed sensor is arranged adjacent the second end of the rotor, the first speed sensor is arranged to detect the speed of rotation of the first end of the rotor and to produce at least one speed signal corresponding to the speed of rotation of the first end of the rotor, the second speed sensor is arranged to detect the speed of rotation of the second end of the rotor and to produce at least one speed signal corresponding to the speed of rotation of the second end of the rotor, the processor unit is arranged to analyse the speed signals to determine the difference in speed between the first end and the second end of the rotor to determine the angle of twist between the first end and the second end of the rotor, the processor unit is arranged to analyse the angle of twist to determine if the angle of twist of the rotor oscillates by more than a predetermined level from an average level, the processor unit being arranged to produce a signal indicative of an impact to at least one of the blades if the angle of twist oscillates by more than the predetermined level and to send the signal to an indicator device.10. A machine as claimed in claim 9 wherein the processor unit is arranged to analyse the angle of twist to determine if the amplitude of the oscillation of the angle of twist of the rotor is more than 0.5°.11. A machine as claimed in claim 9 wherein the processor unit is arranged to analyse the angle of twist to determine if the amplitude of the oscillation of the angle of twist of the rotor is between 0.5° and 1°.12. A machine as claimed in claim 9 wherein the machine is a gas turbine engine.13. A machine as claimed in claim 9 wherein the blades are fan blades or compressor blades.14. A method of detecting an impact on one or more blades of a machine, the machine comprising a rotor having a plurality of blades arranged in a set, the method for detecting an impact on one or more of the blades comprising detecting the speed of rotation of the rotor and to produce at least one speed signal corresponding to the speed of rotation of the rotor, analysing the at least one speed signal to detect a rapid drop in the speed of rotation of the rotor from a correct operating speed of rotation of the rotor for a particular operating condition, analyzing the at least one speed signal to determine if the speed of rotation of the rotor reduces by more than a predetermined rate, producing a signal indicative of impact to at least one of the blades if the speed of rotation reduces by more than the predetermined rate and sending the signal to an indicator device.15. A method as claimed in claim 14 wherein the rotor has a first axial end and a second axial end, the blades being arranged at the first axial end of the rotor.16. A method as claimed in claim 15 comprising detecting the speed of rotation of the rotor adjacent the first axial end of the rotor and analysing the at least one speed signal to determine if the speed of rotation of the rotor reduces above a predetermined rate.17. A method as claimed in claim 14 comprising determining if the speed of rotation of the rotor reduces at a rate of more than 500 rpm per second.18. A method as claimed in claim 17 comprising determining if the speed of rotation of the rotor reduces at a rate of between 500 rpm per second and 1000 rpm per second.19. A method as claimed in claim 14 comprising analysing the at least one speed signal to determine if the speed of rotation of the rotor reduces at a rate of more than 7% per second.20. A method as claimed in claim 14 wherein the machine is a gas turbine engine.21. A method as claimed in claim 20 wherein the blades are fan blades or compressor blades.22. A method of detecting an impact on one or more blades of a machine, the machine comprising a rotor having a plurality of blades arranged in a set, the method for detecting an impact on one or more of the blades comprising detecting the speed of rotation of the rotor and to produce at least one speed signal corresponding to the speed of rotation of the rotor, analysing the at least one speed signal to determine if the speed of rotation of the rotor reduces by more than a predetermined level, producing a signal indicative of impact to at least one of the blades if the speed of rotation reduces by more than the predetermined level and sending the signal to an indicator device wherein the rotor has a first axial end and a second axial end, the blades being arranged at the first axial end of the rotor and comprising detecting the speed of rotation of the first end of the rotor to produce at least one speed signal corresponding to the speed of rotation of the first end of the rotor, detecting the speed of rotation of the second end of the rotor to produce at least one speed signal corresponding to the speed of rotation of the second end of the rotor, analysing the speed signals to determine the difference in speed between the first end and the second end of the rotor to determine the angle of twist between the first end and the second end of the rotor, analysing the angle of twist to determine if the angle of twist of the rotor oscillates by more than a predetermined level from an average level, producing a signal indicative of an impact to at least one of the blades if the angle of twist oscillates by more than the predetermined level and sending the signal to an indicator device.23. A method as claimed in claim 22 comprising analysing the angle of twist to determine if the amplitude of the oscillation of the angle of twist of the rotor is more than 0.5°.24. A method as claimed in claim 22 comprising analysing the angle of twist to determine if the amplitude of the oscillation of the angle of twist of the rotor is between 0.5° and 1°.25. A method as claimed in claim 22 wherein the machine is a gas turbine engine.26. A method as claimed in claim 22 wherein the blades are fan blades or compressor blades.