An aircraft engine, comprising: a nacelle with an air inlet and an air outlet; a cavity defined within the nacelle, the cavity permitting air to pass through the nacelle from the air inlet to the air outlet; a combustion region defined within the nacelle, separate from the cavity, the combustion region receiving the air from the air inlet, combusting fuel in the presence of the air to produce combustion gases, discharging the combustion gases through the air outlet, and producing bleed air taken from at least one position between the air inlet and the air outlet; a fan disposed proximate to the air inlet for drawing the air through the nacelle; a precooler disposed within the nacelle, the precooler defining an ambient air passage and a bleed air passage, wherein the air passes through the precooler from an ambient air inlet to an ambient air outlet, wherein the bleed air passes through the precooler from a bleed air inlet to a bleed air outlet, and wherein heat is transferred between the air and the bleed air via heat exchange within the precooler; at least one pressure relief door, for increasing flow of air through the ambient air passage, disposed proximate to the air outlet; wherein the aircraft engine further comprises a controller operatively connected to the pressure relief door, and wherein the controller is arranged to at least open the pressure relief door based on a demand for increased flow of air through the ambient air passage, characterised in that the demand for increased flow of air through the ambient air passage is conditioned upon trigger variables, wherein the trigger variables comprise at least one signal indicative of an engine failure, a bleed air system failure, an ice condition, or activation of an anti-icing system. a nacelle with an air inlet and an air outlet; a cavity defined within the nacelle, the cavity permitting air to pass through the nacelle from the air inlet to the air outlet; a combustion region defined within the nacelle, separate from the cavity, the combustion region receiving the air from the air inlet, combusting fuel in the presence of the air to produce combustion gases, discharging the combustion gases through the air outlet, and producing bleed air taken from at least one position between the air inlet and the air outlet; a fan disposed proximate to the air inlet for drawing the air through the nacelle; a precooler disposed within the nacelle, the precooler defining an ambient air passage and a bleed air passage, wherein the air passes through the precooler from an ambient air inlet to an ambient air outlet, wherein the bleed air passes through the precooler from a bleed air inlet to a bleed air outlet, and wherein heat is transferred between the air and the bleed air via heat exchange within the precooler; at least one pressure relief door, for increasing flow of air through the ambient air passage, disposed proximate to the air outlet; wherein the aircraft engine further comprises a controller operatively connected to the pressure relief door, and wherein the controller is arranged to at least open the pressure relief door based on a demand for increased flow of air through the ambient air passage, characterised in that the demand for increased flow of air through the ambient air passage is conditioned upon trigger variables, wherein the trigger variables comprise at least one signal indicative of an engine failure, a bleed air system failure, an ice condition, or activation of an anti-icing system.