After being severely wounded in infantry service, Menckhoff transferred to Jagdstaffel 3 (Hunting Team 3) as a vizefeldwebel (staff sergeant), afterwards being commissioned as an officer.

He won the German Empire's most prestigious decoration, the Pour le Mérite ("Blue Max"), and was given command of Jagdstaffel 72 (Hunting squadron 72).

His father ran a successful linen weaving mill, the Herforder Leinen-Verein Wilhelm Menckhoff, in which Carl was apprenticed and was later employed after his own business failed.

[5] Left unfit for infantry service by his injuries, Menckhoff applied for transfer to the Die Fliegertruppen des deutschen Kaiserreiches (Aviation Troops for the German Empire), and was accepted for pilot training in February 1915.

Having qualified, he was posted in October 1915 back to the Western Front, to an airfield at Pergnies-Quessey, near Saint-Quentin, where he was again wounded during an aerial engagement in January 1916.

Rather unusually, his aircraft bore no squadron livery, but was painted with personal markings of green and white stripes curving down the length of its fuselage.

During the resulting lengthy dogfight, a red-nosed Albatros came to the aid of the beleaguered Voss, only for the savior to be driven out of the fray.

His careful leadership style conserved his men's lives; while Jagdstaffel 72 inflicted 60 losses on its enemies, it lost only one pilot.

[19][20] On 25 July 1918, three days after his 39th victory, while flying an evening patrol in one of his two Fokker D.VIIs, Menckhoff engaged elements of the United States Air Service.

During the ensuing dogfight, Menckhoff was shot down by American Lieutenant Walter Avery of the 95th Aero Squadron, United States Air Service.

[5][7] When Avery maneuvered onto Menckhoff's tail, the German ace cut his engine and dropped in a falling leaf pattern of zigzagging side-slips.

[21] Captured by French troops at the crash site, Menckhoff was chagrined to learn that Avery was a rookie pilot on his first combat flight.

Avery arrived at the crash site and respectfully refused to remove the Pour le Merite from Menckhoff's throat as a souvenir.

Instead, Avery cut a fabric letter "M" from the crashed Fokker's covering as a keepsake before Menckhoff was led away by French soldiers.

[5][22] Following interrogation, Menckhoff was held as a prisoner of war, along with many other German pilots, at Camp Montoire, near Orléans, France.

Travelling on foot, by rail, and at one point in a stolen car, he managed to reach Switzerland eight days later, crossing the border near Mont Salève and making his way to Geneva.

[24] He established a second residence in Switzerland, but in October 1938, when crossing the German-Swiss border at Basel, he was found to be carrying an illegal quantity of Swiss francs.

When she learned Karl Gerhard lived nearby, she decided to surprise him by returning the fabric souvenir 'M' from the Fokker D.VII to the Menckhoff family.