A turbine engine is disclosed that is capable of operating in an emergency power mode, an air breathing auxiliary power mode and a bypass bleed environmental control mode. In a preferred embodiment of the invention, the engine comprises in working fluid flow order a compressor for compressing air, a combustion chamber and a power turbine for extracting mechanical work from heated and/or compressed air. The compressor and turbine are separately coupled by a shaft which is also coupled to a generator for supplying electrical power and a cooling turbine for supplying cooled air to the aircraft cabin and avionics. The combustion chamber has an upstream combustor inlet and a downstream combustor inlet, each of which may admit compressed air either to be mixed with fuel for combustion, or to pass directly to the combustor outlet which is operatively attached to the power turbine section inlet. During emergency/starting operation, the combustion chamber and turbine section receive compressed air from a stored air accumulator through a stored air inlet. During normal air breathing operation, the combustion chamber and turbine section receive compressed air from a compressor through an air breathing inlet. During bypass bleed operation, the combustion chamber and turbine section receive compressed air from the aircraft main engine through a bypass bleed air inlet. The tri-mode operation is made possible by a unique two-way bypass ring having a plurality of axial passages communicating the stored air inlet and air breathing inlet with the upstream combustor inlet and a plurality of radial passages communicating the bypass bleed inlet with the downstream combustor inlet. A valving system selectively admits compressed air into the combustor and turbine section from the stored air inlet, air breathing inlet or bypass bleed inlet depending on the mode of operation.