The Mercedes D.IIIa is a six-cylinder, in-line aircraft engine developed in Germany during World War I. It was an advanced version of the earlier D.III model, featuring improved performance and reliability. The engine produced around 160 horsepower, making it suitable for various fighter aircraft of the era. This engine was primarily used in several notable aircraft, including the Albatros D.V and Fokker D.VII. Its design contributed to the success of these planes in combat, providing them with the necessary power and speed to compete effectively in the skies during the war.