A fluid system for a gas turbine engine, comprising: a fluid passage comprising a passage inlet and a passage outlet, wherein the fluid passage is configured to allow a fluid to pass through the fluid passage by entering the passage inlet as an inlet flow and exiting the passage outlet as an outlet flow; a bypass valve (350; 400) comprising a bypass inlet (348; 448) and a bypass outlet (352; 452), the bypass inlet (348; 448) being in fluid communication with the passage inlet, wherein the bypass valve (350; 400) further comprises a piston coupled to a spring, wherein the spring retains the piston in a rest position, and the spring and the piston are configured to block the fluid from passing through the bypass outlet (352; 452) in response to the inlet flow exerting a pressure below a bypass pressure threshold; and a position sensor coupled to the bypass valve (350; 400) and configured to detect a position of the piston in the bypass valve (350; 400), wherein the piston is in the rest position in the bypass inlet (348; 448), wherein the bypass inlet (348; 448) comprises an inlet surrounding wall and a portion of the inlet surrounding wall adjacent to the bypass outlet (352; 452) is a bypass slack area, wherein the piston is in the rest position at a slack area end furthest from the bypass outlet (352; 452), and the piston is configured to move along the bypass slack area toward the bypass outlet (352; 452) in response to a pressure threshold being reached by the inlet flow in the passage inlet, characterised by the fluid system further comprising a control system in electronic communication with the position sensor, wherein the position sensor transmits the position of the piston to the control system, wherein the control system comprises a processor and is configured to determine at least one of an impending bypass flow through the bypass outlet (352; 452) in response to the piston being moved to a displaced position along the bypass slack area from the rest position without allowing the fluid to pass through the bypass outlet (352; 452), or a bypass flow through the bypass outlet (352; 452) in response to the piston moving past the bypass slack area and allowing the fluid to pass through the bypass outlet (352; 452). a fluid passage comprising a passage inlet and a passage outlet, wherein the fluid passage is configured to allow a fluid to pass through the fluid passage by entering the passage inlet as an inlet flow and exiting the passage outlet as an outlet flow; a bypass valve (350; 400) comprising a bypass inlet (348; 448) and a bypass outlet (352; 452), the bypass inlet (348; 448) being in fluid communication with the passage inlet, wherein the bypass valve (350; 400) further comprises a piston coupled to a spring, wherein the spring retains the piston in a rest position, and the spring and the piston are configured to block the fluid from passing through the bypass outlet (352; 452) in response to the inlet flow exerting a pressure below a bypass pressure threshold; and a position sensor coupled to the bypass valve (350; 400) and configured to detect a position of the piston in the bypass valve (350; 400), wherein the piston is in the rest position in the bypass inlet (348; 448), wherein the bypass inlet (348; 448) comprises an inlet surrounding wall and a portion of the inlet surrounding wall adjacent to the bypass outlet (352; 452) is a bypass slack area, wherein the piston is in the rest position at a slack area end furthest from the bypass outlet (352; 452), and the piston is configured to move along the bypass slack area toward the bypass outlet (352; 452) in response to a pressure threshold being reached by the inlet flow in the passage inlet, characterised by the fluid system further comprising a control system in electronic communication with the position sensor, wherein the position sensor transmits the position of the piston to the control system, wherein the control system comprises a processor and is configured to determine at least one of an impending bypass flow through the bypass outlet (352; 452) in response to the piston being moved to a displaced position along the bypass slack area from the rest position without allowing the fluid to pass through the bypass outlet (352; 452), or a bypass flow through the bypass outlet (352; 452) in response to the piston moving past the bypass slack area and allowing the fluid to pass through the bypass outlet (352; 452).