An environmental control system, ECS, of an aircraft, the environmental control system comprising: a power turbine; a heat exchanger assembly; and an inlet plenum attached to and in fluid communication with the heat exchanger assembly, wherein the inlet plenum includes a housing comprising: a first inlet fluidly connected to the power turbine; a second inlet fluidly connected to a source of ram air; and an outlet fluidly connected to the heat exchanger; wherein the inlet plenum housing further comprises a sidewall opposite the outlet, wherein the sidewall includes a tapered contour configured to direct a flow of fluid from the second inlet towards the outlet; wherein the heat exchanger assembly comprises a bleed air heat exchanger, a fresh air heat exchanger configured to receive fresh air from a fresh air compressor, and a chiller heat exchanger; wherein the chiller heat exchanger includes a plurality of fins for transferring thermal energy between the plurality of fins and a fluid, wherein the chiller heat exchanger is fluidly connected to a bleed turbine of the ECS and via an external plenum to the bleed air heat exchanger; wherein the chiller heat exchanger is disposed adjacent to and in contact with fresh air heat exchanger; wherein the chiller heat exchanger is also fluidly connected to the inlet plenum via a cold circuit of the heat exchanger assembly that passes through each of the chiller heat exchanger, fresh air heat exchanger and bleed air heat exchanger; and wherein the chiller, fresh air, and bleed air heat exchangers are brazed together to form a single unitized tri-heat exchanger. a power turbine; a heat exchanger assembly; and an inlet plenum attached to and in fluid communication with the heat exchanger assembly, wherein the inlet plenum includes a housing comprising: a first inlet fluidly connected to the power turbine; a second inlet fluidly connected to a source of ram air; and an outlet fluidly connected to the heat exchanger; wherein the inlet plenum housing further comprises a sidewall opposite the outlet, wherein the sidewall includes a tapered contour configured to direct a flow of fluid from the second inlet towards the outlet; wherein the heat exchanger assembly comprises a bleed air heat exchanger, a fresh air heat exchanger configured to receive fresh air from a fresh air compressor, and a chiller heat exchanger; wherein the chiller heat exchanger includes a plurality of fins for transferring thermal energy between the plurality of fins and a fluid, wherein the chiller heat exchanger is fluidly connected to a bleed turbine of the ECS and via an external plenum to the bleed air heat exchanger; wherein the chiller heat exchanger is disposed adjacent to and in contact with fresh air heat exchanger; wherein the chiller heat exchanger is also fluidly connected to the inlet plenum via a cold circuit of the heat exchanger assembly that passes through each of the chiller heat exchanger, fresh air heat exchanger and bleed air heat exchanger; and wherein the chiller, fresh air, and bleed air heat exchangers are brazed together to form a single unitized tri-heat exchanger. a first inlet fluidly connected to the power turbine; a second inlet fluidly connected to a source of ram air; and an outlet fluidly connected to the heat exchanger; wherein the inlet plenum housing further comprises a sidewall opposite the outlet, wherein the sidewall includes a tapered contour configured to direct a flow of fluid from the second inlet towards the outlet; wherein the heat exchanger assembly comprises a bleed air heat exchanger, a fresh air heat exchanger configured to receive fresh air from a fresh air compressor, and a chiller heat exchanger; wherein the chiller heat exchanger includes a plurality of fins for transferring thermal energy between the plurality of fins and a fluid, wherein the chiller heat exchanger is fluidly connected to a bleed turbine of the ECS and via an external plenum to the bleed air heat exchanger; wherein the chiller heat exchanger is disposed adjacent to and in contact with fresh air heat exchanger; wherein the chiller heat exchanger is also fluidly connected to the inlet plenum via a cold circuit of the heat exchanger assembly that passes through each of the chiller heat exchanger, fresh air heat exchanger and bleed air heat exchanger; and wherein the chiller, fresh air, and bleed air heat exchangers are brazed together to form a single unitized tri-heat exchanger.