Exterior aircraft light unit comprising: a housing including a mounting portion, which is configured for mounting the aircraft light unit to an aircraft; at least one light source; and a heat sink (16, 20); wherein the heat sink (16, 20) comprises: at least one first heat sink portion, which is thermally connected with the at least one light source for transferring heat from the at least one light source to the at least one first heat sink portion; and at least one second heat sink portion, which is configured to be selectively attachable to the at least one first heat sink portion; wherein the at least one first heat sink portion comprises a plurality of first cooling ribs; wherein the at least one second heat sink portion comprises a plurality of channels or grooves which are configured for receiving corresponding first cooling ribs when the second heat sink portion is attached to the at least one first heat sink portion; the first cooling ribs extend across a first support ring, which is attached to the housing of the exterior aircraft light unit; the at least one second heat sink portion comprises a plurality of second cooling ribs extending across a second support ring; and the diameter of the second support ring is set so that the second support ring of the second heat sink portion may be positioned over the first support ring. a housing including a mounting portion, which is configured for mounting the aircraft light unit to an aircraft; at least one light source; and a heat sink (16, 20); wherein the heat sink (16, 20) comprises: at least one first heat sink portion, which is thermally connected with the at least one light source for transferring heat from the at least one light source to the at least one first heat sink portion; and at least one second heat sink portion, which is configured to be selectively attachable to the at least one first heat sink portion; wherein the at least one first heat sink portion comprises a plurality of first cooling ribs; wherein the at least one second heat sink portion comprises a plurality of channels or grooves which are configured for receiving corresponding first cooling ribs when the second heat sink portion is attached to the at least one first heat sink portion; the first cooling ribs extend across a first support ring, which is attached to the housing of the exterior aircraft light unit; the at least one second heat sink portion comprises a plurality of second cooling ribs extending across a second support ring; and the diameter of the second support ring is set so that the second support ring of the second heat sink portion may be positioned over the first support ring. at least one first heat sink portion, which is thermally connected with the at least one light source for transferring heat from the at least one light source to the at least one first heat sink portion; and at least one second heat sink portion, which is configured to be selectively attachable to the at least one first heat sink portion; wherein the at least one first heat sink portion comprises a plurality of first cooling ribs; wherein the at least one second heat sink portion comprises a plurality of channels or grooves which are configured for receiving corresponding first cooling ribs when the second heat sink portion is attached to the at least one first heat sink portion; the first cooling ribs extend across a first support ring, which is attached to the housing of the exterior aircraft light unit; the at least one second heat sink portion comprises a plurality of second cooling ribs extending across a second support ring; and the diameter of the second support ring is set so that the second support ring of the second heat sink portion may be positioned over the first support ring.