A gas turbine engine comprising: a fan surrounded by a fan case: an inner core housing radially inward of said fan case, said fan delivering air into a bypass duct between said inner core housing and said fan case, and further delivering air into said inner core housing, said inner core housing including a compressor section, a turbine section and a combustor; and a fan case exit at a downstream end of the fan case and an inner core housing exit at a downstream end of said inner core housing, characterised in that: both said fan case exit and said inner core housing exit provide fixed nozzles; at least one of said fan case exit and said inner core housing exit is provided with a fiber optic sensing unit (85, 185); said fiber optic sensing unit (85, 185) includes a fiber optic sensing member (98, 100) surrounding a circumference of said at least one of said exits (94, 97) and communicating with a light source (98L, 100L) and a receiver (98R, 100R), said light source (98L, 100L) configured to deliver light into said fiber optic sensing member (98, 100), and said receiver (98R, 100R) being configured to receive reflected light information from said fiber optic sensing member (98, 100); and the engine further comprises a control (98C, 100C) for utilizing the received reflected light information, and measure strain from the information, said control (98C, 100C) further programmed to determine a displacement of the fiber optic sensing member (98, 100) based upon the measured strain, and programmed to calculate a nozzle area (99, 101) at said at least one of said exits (94, 97) based upon the displacement, said calculated nozzle area being utilized to update nozzle area information at an electronic engine controller of the engine, and said electronic engine controller being programmed to control at least one associated component on the engine based upon the updated nozzle area. an inner core housing radially inward of said fan case, said fan delivering air into a bypass duct between said inner core housing and said fan case, and further delivering air into said inner core housing, said inner core housing including a compressor section, a turbine section and a combustor; and a fan case exit at a downstream end of the fan case and an inner core housing exit at a downstream end of said inner core housing, both said fan case exit and said inner core housing exit provide fixed nozzles; at least one of said fan case exit and said inner core housing exit is provided with a fiber optic sensing unit (85, 185); said fiber optic sensing unit (85, 185) includes a fiber optic sensing member (98, 100) surrounding a circumference of said at least one of said exits (94, 97) and communicating with a light source (98L, 100L) and a receiver (98R, 100R), said light source (98L, 100L) configured to deliver light into said fiber optic sensing member (98, 100), and said receiver (98R, 100R) being configured to receive reflected light information from said fiber optic sensing member (98, 100); and the engine further comprises a control (98C, 100C) for utilizing the received reflected light information, and measure strain from the information, said control (98C, 100C) further programmed to determine a displacement of the fiber optic sensing member (98, 100) based upon the measured strain, and programmed to calculate a nozzle area (99, 101) at said at least one of said exits (94, 97) based upon the displacement, said calculated nozzle area being utilized to update nozzle area information at an electronic engine controller of the engine, and said electronic engine controller being programmed to control at least one associated component on the engine based upon the updated nozzle area.