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 . <EOS>