The Salmson Z-9 engine, rated at 280 hp., was quite unique--although it was a radial engine--it was cooled by water. The Z-9 was installed in the Salmson 2A.2 airplane which arrived on the Front early in 1918. The U.S. Air Service purchased 705 Salmson 2A.2s from France and used them to equip many of its observation squadrons.