A surge margin control system in a gas turbine engine, the surge margin control system comprising:
- a compressor having an environmental control system bleed port having an outlet in fluid communication with an aircraft environmental control system air duct;
- the environmental control system duct configured to provide pressurized air to an aircraft, the environmental control system duct having an inlet fluidly connected to the compressor of the gas turbine engine via an environmental control system inlet valve, the environmental control system inlet valve disposed between the environmental control system bleed port and the aircraft environmental control system air duct, so that pressurized air from the compressor can be supplied into the environmental control system duct;
- an environmental control system air conditioning system that receives air from the environmental control system bleed port via the environmental control system inlet valve and the aircraft environmental control system air duct;
- an air turbine starter configured to rotate a compressor shaft of the gas turbine engine, the air turbine starter including an inlet fluidly connected with the environmental control system duct and a pneumatic line of an auxiliary power unit via an air turbine valve with an auxiliary power unit inlet valve further disposed between the auxiliary power unit and the air turbine valve, and
- a controller coupled to the environmental control system inlet valve, the air turbine valve, and the auxiliary power unit inlet valve, the controller configured to determine the surge margin of the compressor and open the air turbine valve in response to the surge margin being below a predetermined minimum surge margin so that the air turbine starter drives the compressor shaft, wherein the controller is further configured to:
- 1) in a starting mode, to open the auxiliary power unit inlet valve and the air turbine valve to supply air to the air turbine starter from the pneumatic line and 2) in a running mode, to close the auxiliary power unit inlet valve to prevent air from being supplied from the auxiliary power unit to the air turbine starter, receive an indication that an environmental control system air conditioning system associated with the aircraft is not functioning, compare the surge margin of the compressor to the predetermined minimum surge margin and, in response to receiving the indication that the environmental control system air conditioning system is not functioning and further if the surge margin is below the predetermined minimum surge margin, the controller is configured to open the environmental control system inlet valve to supply air to the air turbine starter from the compressor via the environmental control system air duct so that the air turbine starter drives the compressor shaft.