Air cycle machine bypass duct

An environmental control system comprising:

• a rotary machine comprising:
• a compressor section comprising:
• a compressor inlet;
• a compressor outlet;
• a compressor duct extending between the compressor inlet and the compressor outlet;
• and
• a compressor impeller positioned in the compressor duct;
• a first turbine section comprising:
• a first turbine inlet downstream from the compressor outlet;
• a first turbine outlet;
• a first turbine duct extending between the first turbine inlet and the first turbine outlet;
• and
• a first turbine impeller positioned in the first turbine duct;
• a second turbine section comprising:
• a second turbine inlet downstream from the first turbine outlet;
• a second turbine outlet;
• a second turbine duct extending between the second turbine inlet and the second turbine outlet;
• and
• a second turbine impeller positioned in the second turbine duct and mechanically connected to the tie rod of the rotary machine;
• and
• a tie rod mechanically connecting the compressor impeller and the first turbine impeller;
• a first bypass duct between the compressor inlet and the first turbine inlet;
• a first check valve in the first bypass duct, wherein the first check valve opens the first bypass duct when the pressure in the compressor inlet is higher than a pressure in the first turbine inlet;
• a second bypass duct between the first turbine inlet and the second turbine inlet;
• and
• a second check valve in the second bypass duct, wherein the second check valve opens the second bypass duct when an air pressure in the first turbine inlet is higher than an air pressure in the second turbine inlet.