A heat exchanger assembly for use in a gas turbine engine including a core gas turbine engine having an axis of rotation and a fan casing substantially circumscribing the core gas turbine engine, said heat exchanger assembly comprising: a heat exchanger body; and a first fluid circuit comprising: a first bypass valve; the heat exchanger assembly further comprising: a first fluid circuit inlet channel, included in the heat exchanger body, in flow communication with said first bypass valve; and a plurality of cooling channels, included in the heat exchanger body, in flow communication with said first bypass valve, wherein said first bypass valve is configured to deliver a first fluid to said plurality of cooling channels during a first mode of operation to facilitate reducing a temperature of said first fluid; and the heat exchanger assembly further comprising: a second fluid circuit including: a second bypass valve; a second fluid circuit inlet channel included in the heat exchanger body, in flow communication with said second bypass valve; and a second fluid circuit outlet channel included in the heat exchanger body, in flow communication with said second bypass valve and said second fluid circuit inlet channel, wherein said second bypass valve is configured to facilitate a flow of a second fluid through at least a portion of said heat exchanger body during the first mode of operation; wherein the heat exchanger assembly has a substantially arcuate shape and is formed by a single segment or by a plurality of segments mounted end to end covering at least about 320° of a circumferential length, and wherein the heat exchanger assembly is positionable within the fan casing of the gas turbine engine. a heat exchanger body; and a first fluid circuit comprising: a first bypass valve; the heat exchanger assembly further comprising: a first bypass valve; the heat exchanger assembly further comprising: a first fluid circuit inlet channel, included in the heat exchanger body, in flow communication with said first bypass valve; and a plurality of cooling channels, included in the heat exchanger body, in flow communication with said first bypass valve, wherein said first bypass valve is configured to deliver a first fluid to said plurality of cooling channels during a first mode of operation to facilitate reducing a temperature of said first fluid; and the heat exchanger assembly further comprising: a second fluid circuit including: a second bypass valve; a second fluid circuit inlet channel included in the heat exchanger body, in flow communication with said second bypass valve; and a second bypass valve; a second fluid circuit inlet channel included in the heat exchanger body, in flow communication with said second bypass valve; and a second fluid circuit outlet channel included in the heat exchanger body, in flow communication with said second bypass valve and said second fluid circuit inlet channel, wherein said second bypass valve is configured to facilitate a flow of a second fluid through at least a portion of said heat exchanger body during the first mode of operation; wherein the heat exchanger assembly has a substantially arcuate shape and is formed by a single segment or by a plurality of segments mounted end to end covering at least about 320° of a circumferential length, and wherein the heat exchanger assembly is positionable within the fan casing of the gas turbine engine.