Girly Air Force

(Contains minor spoilers below) A lot of common points, both purely physical / visual and also related to the story. Futuristic universe (this one shows up some combat airplanes with a futuristic armament & design) ; main character with pink hairs (can't miss this detail) ; strong growing up relationship between her & the other male character Main character having an issue to pilot (in Ditf : a mecha ; in GAF, an airplane) until the male character appears, etc For now, many more similarities but I won't spoil. Just consider GAF is a younger (meaning closer in the time from reality) scifi anime, showing up air fights, strong male/female relationship, etc read more 

Close story , sci-if anime , in darling there's a robots but in air force there's a air fighters , also close anime start but better end , there's aliens too wants to destroy the Humanity , and the heroine can't ride the air fighter without the hero , in short they are like a same anime  

They both have females that pilot machines to fight, in darling in the franxx you had Hiro who wanted to pilot a FranXX but in girly air force you have gripen who has issues piloting a jet, and in both shows, the duo pair they create make them push foward in the fight against the main villian, both shows are really good 

Similarities : - a squad of pilots composed only with females fighters - air fights - more funny / relaxing than serious (even with the war context) Differences : - Girly Air Force takes place in an alternative current world (including high technology planes & "upgraded pilots" - Kouya no Kotobuki Hikoutai takes place in an alternative older world (I would say XIXth century approximately) - GAF has fast & explosive fights while Kouya no Kotobuki Hikoutai figures lower fights but more skilled (lot of maneuvers) - Kouya no Kotobuki Hikoutai's animation and design are really different (especially about movements). 

In both series, humanity faces a threat and is forced to use parts of the enemy to create defense weapons. In Girly Air Force, there is girls who represent planes, in Aoki Hagane no Arpegio, there is girls who represent boat.