An aircraft propeller, the propeller rotatable about an axis and comprising a plurality of blades each rotatable through a plurality of blade angles, and a blade angle position feedback system, the blade angle position feedback system comprising: an annular member operatively connected to rotate with the propeller, the annular member comprising at least two circumferentially-spaced first singularities (102 A, 102 C) extending substantially parallel to the axis and at least one second singularity (102 B) extending non-parallel to the first singularities (102 A, 102c); at least one sensor fixedly mounted adjacent the annular member, the at least one sensor configured for successively detecting a passage of each one of the at least two first singularities (102 A, 102 C) and the at least one second singularity (102 B) as the annular member is rotated and axially displaced and for generating a sensor signal accordingly, the annular member and at least one sensor configured for relative axial displacement between a first relative axial position and a second relative axial position; and a detection unit connected to the at least one sensor for receiving the sensor signal therefrom, determining on the basis of the sensor signal a time interval elapsed between the passage of successive ones of the at least two first singularities (102 A, 102 C) and the at least one second singularity (102 B), and computing from the time interval a blade angle position for the plurality of blades, the detection unit configured to synchronize the propeller of a first engine with the propeller of at least one second engine on the basis of the sensor signal, characterised in that subsequent to removal from the annular member of a given one of the singularities (102 A, 102 B, 102 C), the detection unit is configured to detect, on the basis of the sensor signal, removal of the given one of the singularities (102 A, 102 B, 102 C); and in that the propeller has at least a first mode of operation where the plurality of blades are operated at a positive blade angle and a second mode of operation where the plurality of blades are operated at a negative blade angle wherein the first relative axial position corresponds to the first mode of operation and the second relative axial position corresponds to the second mode of operation. an annular member operatively connected to rotate with the propeller, the annular member comprising at least two circumferentially-spaced first singularities (102 A, 102 C) extending substantially parallel to the axis and at least one second singularity (102 B) extending non-parallel to the first singularities (102 A, 102c); at least one sensor fixedly mounted adjacent the annular member, the at least one sensor configured for successively detecting a passage of each one of the at least two first singularities (102 A, 102 C) and the at least one second singularity (102 B) as the annular member is rotated and axially displaced and for generating a sensor signal accordingly, the annular member and at least one sensor configured for relative axial displacement between a first relative axial position and a second relative axial position; and a detection unit connected to the at least one sensor for receiving the sensor signal therefrom, determining on the basis of the sensor signal a time interval elapsed between the passage of successive ones of the at least two first singularities (102 A, 102 C) and the at least one second singularity (102 B), and computing from the time interval a blade angle position for the plurality of blades, the detection unit configured to synchronize the propeller of a first engine with the propeller of at least one second engine on the basis of the sensor signal, characterised in that subsequent to removal from the annular member of a given one of the singularities (102 A, 102 B, 102 C), the detection unit is configured to detect, on the basis of the sensor signal, removal of the given one of the singularities (102 A, 102 B, 102 C); and in that the propeller has at least a first mode of operation where the plurality of blades are operated at a positive blade angle and a second mode of operation where the plurality of blades are operated at a negative blade angle wherein the first relative axial position corresponds to the first mode of operation and the second relative axial position corresponds to the second mode of operation.