A gas turbine engine comprising: a compressor section, including a low pressure compressor , and a high pressure compressor including an impeller and defining an impeller rear cavity ; a turbine section including an exhaust duct ; a bleed air passage ; and a cooler having a first passage connected to the bleed-air flow passage to allow a stream of low pressure air to flow through the cooler , the cooler having a second passage for directing a stream of high pressure air bled from the compressor section through the cooler for cooling the stream of high pressure air before delivering the stream of high pressure air to the impeller rear cavity of the compressor section; characterised by further comprising: a bleed-off valve apparatus connected in the bleed-air flow passage , for selectively bleeding low pressure air from the compressor section through the bleed-air flow passage to the exhaust duct of the turbine section when the bleed-off valve apparatus is open, wherein the bleed-off valve apparatus forms part of the bleed-air flow passage and has an air flow resistance lower than an air flow resistance of the first passage of the cooler , the first passage is connected to the bleed airflow passage in parallel with the bleed-off valve apparatus to allow the stream of low pressure air to bypass the bleed-off valve apparatus when the bleed-off valve apparatus is closed, and due to the parallel connection of the bleed-off valve apparatus and the cooler , the cooler effectively stops cooling the stream of high pressure air flowing through the second passage when the bleed-off valve apparatus is open. a compressor section, including a low pressure compressor , and a high pressure compressor including an impeller and defining an impeller rear cavity ; a turbine section including an exhaust duct ; a bleed air passage ; and a cooler having a first passage connected to the bleed-air flow passage to allow a stream of low pressure air to flow through the cooler , the cooler having a second passage for directing a stream of high pressure air bled from the compressor section through the cooler for cooling the stream of high pressure air before delivering the stream of high pressure air to the impeller rear cavity of the compressor section; characterised by further comprising: a bleed-off valve apparatus connected in the bleed-air flow passage , for selectively bleeding low pressure air from the compressor section through the bleed-air flow passage to the exhaust duct of the turbine section when the bleed-off valve apparatus is open, wherein the bleed-off valve apparatus forms part of the bleed-air flow passage and has an air flow resistance lower than an air flow resistance of the first passage of the cooler , the first passage is connected to the bleed airflow passage in parallel with the bleed-off valve apparatus to allow the stream of low pressure air to bypass the bleed-off valve apparatus when the bleed-off valve apparatus is closed, and due to the parallel connection of the bleed-off valve apparatus and the cooler , the cooler effectively stops cooling the stream of high pressure air flowing through the second passage when the bleed-off valve apparatus is open. <EOS>