An aircraft environmental control system (ECS) may be configured to operate in first mode and a second mode. The ECS may include an air cycle machine (ACM), a by-pass valve positioned to allow bleed air to by-pass the air cycle machine so that when the by-pass valve is open, the ECS operates in the first mode with bleed air at a first pressure and when the by-pass valve is closed the ECS operates in the second mode at a second bleed air pressure, higher than the first bleed air pressure. A bleed air system controller and ECS controller may be configured to selectively couple high pressure or low pressure bleed air ports of an engine of an aircraft to the ECS and control the by-pass valve position.