An environmental control system assembly for an aircraft, the environmental control system assembly comprising: an engine comprising a source of bleed air; a heat exchanger assembly comprising: a first heat exchanger in fluid communication with the source of bleed air from the engine, wherein the first heat exchanger comprises: a first endwall that confines and fluidly seals a portion of the first heat exchanger; a first inlet header; a first outlet header connected to the first endwall; and a first endcap located on the first outlet header; a second heat exchanger disposed adjacent to the first heat exchanger, wherein the second heat exchanger comprises: a second endwall that confines and fluidly seals a portion of the second heat exchanger; a third heat exchanger disposed adjacent to the second heat exchanger, wherein the third heat exchanger comprises: a third endwall that confines and fluidly seals a portion of the third heat exchanger; a third inlet header connected to the third endwall; a third outlet header; and a third endcap located on the third inlet header; an end sheet disposed along an end of the heat exchanger assembly, wherein the end sheet comprises the first, second, and third endwalls; and a plenum attached to the end sheet along a length of the plenum, wherein the plenum and the end sheet form a sealed conduit, wherein the plenum fluidly connects the first outlet header of the first heat exchanger to the third inlet header of the third heat exchanger. an engine comprising a source of bleed air; a heat exchanger assembly comprising: a first heat exchanger in fluid communication with the source of bleed air from the engine, wherein the first heat exchanger comprises: a first endwall that confines and fluidly seals a portion of the first heat exchanger; a first inlet header; a first outlet header connected to the first endwall; and a first endcap located on the first outlet header; a second heat exchanger disposed adjacent to the first heat exchanger, wherein the second heat exchanger comprises: a second endwall that confines and fluidly seals a portion of the second heat exchanger; a third heat exchanger disposed adjacent to the second heat exchanger, wherein the third heat exchanger comprises: a third endwall that confines and fluidly seals a portion of the third heat exchanger; a third inlet header connected to the third endwall; a third outlet header; and a third endcap located on the third inlet header; an end sheet disposed along an end of the heat exchanger assembly, wherein the end sheet comprises the first, second, and third endwalls; and a plenum attached to the end sheet along a length of the plenum, wherein the plenum and the end sheet form a sealed conduit, wherein the plenum fluidly connects the first outlet header of the first heat exchanger to the third inlet header of the third heat exchanger. a first heat exchanger in fluid communication with the source of bleed air from the engine, wherein the first heat exchanger comprises: a first endwall that confines and fluidly seals a portion of the first heat exchanger; a first inlet header; a first outlet header connected to the first endwall; and a first endcap located on the first outlet header; a first endwall that confines and fluidly seals a portion of the first heat exchanger; a first inlet header; a first outlet header connected to the first endwall; and a first endcap located on the first outlet header; a second heat exchanger disposed adjacent to the first heat exchanger, wherein the second heat exchanger comprises: a second endwall that confines and fluidly seals a portion of the second heat exchanger; a third heat exchanger disposed adjacent to the second heat exchanger, wherein the third heat exchanger comprises: a third endwall that confines and fluidly seals a portion of the third heat exchanger; a third inlet header connected to the third endwall; a third outlet header; and a third endcap located on the third inlet header; a third endwall that confines and fluidly seals a portion of the third heat exchanger; a third inlet header connected to the third endwall; a third outlet header; and a third endcap located on the third inlet header; an end sheet disposed along an end of the heat exchanger assembly, wherein the end sheet comprises the first, second, and third endwalls; and a plenum attached to the end sheet along a length of the plenum, wherein the plenum and the end sheet form a sealed conduit,