Gas turbine engine

1. A gas turbine engine for an aircraft comprising:

an engine core comprising a turbine, a compressor, a fan located upstream of the compressor and comprising a plurality of fan blades, and a core shaft connecting the turbine to the compressor; and

a gearbox which receives an input from the core shaft and outputs drive, via a drive shaft, to the fan so as to drive the fan at a lower rotational speed than the turbine, the drive shaft and the core shaft forming a shaft system, wherein the shaft system provides:

a first portion which extends forward from a first thrust bearing to the fan, the first thrust bearing supporting the shaft system and being located between the turbine and the gearbox, and

a second portion extending rearward from the first thrust bearing to the turbine, such that in the event of a shaft break within the second portion of the shaft system, said shaft break dividing the shaft system into a front portion axially located by the first thrust bearing and a rear portion no longer axially located by the first thrust bearing, the rear portion is free to move axially rearwardly under a gas load;

wherein the engine further comprises a shaft break detector, configured to detect the shaft break in the shaft system,

wherein the shaft break detector includes a phonic wheel, located at the first portion of the shaft system, and a microwave sensor, configured to sense a rotational speed of the shaft system at the second portion of the shaft system axially spaced from the first portion, and

wherein the turbine has a plurality of stages, the microwave sensor being provided at a first stage of the plurality of stages that is closest to the compressor and the core shaft being connected to the turbine at a final stage of the plurality of stages.

2. The gas turbine engine as claimed in claim 1 , wherein the core shaft is supported by one or more non-thrust bearings located rearwards of the first thrust bearing.

3. The gas turbine engine as claimed in claim 1 , wherein the first thrust bearing is located such that the pathway for torque transmission from the turbine to the compressor includes the first portion of the shaft system.

4. The gas turbine engine as claimed in claim 1 , wherein the drive shaft is supported forward of the first thrust bearing by one or more further thrust bearings.

5. The gas turbine engine as claimed in claim 1 , further comprising a roller bearing supporting the drive shaft.

6. The gas turbine engine as claimed in claim 1 , wherein the shaft break detector is configured to register the shaft break when it detects a twist exceeding a predetermined threshold between two points in the first portion of the shaft system.

7. The gas turbine engine as claimed in claim 1 , wherein:

the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft;

the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor;

the second turbine, the second compressor, and the second core shaft are arranged to rotate at a higher rotational speed than the first core shaft; and

wherein the second core shaft is axially located by one or more respective second thrust bearings.