As in most of the European countries, France have traditions coming from centuries and its armed services make no exception.
In the French services the hierachy comes into 3 classes :
1 - Officers (officers)
2 - Sous-officiers (under officers or NCOs starting with the rank of Sergent)
3 - Troupe (enlisted men which include the rank of caporal-corporal).
During WWI, the French Air Service (Aéronautique militaire), was ENTIRELY manned by officers, NCOs, corporals and soldiers, posted from other corps. This explain the variation of appellation and uniforms. It is characteristic of this era to see a picture of officers of the Air service, they all wear a different uniform... This lasted until 1917, when the "Bleu horizon" uniform became compulsory for everybody, except the colonial troops which where dressed in khaki. With of course the odd exceptions, for instance
Charles Nungesser who was always wearing the light blue jacket of his corps of origin, the Hussars, in which he started the war as a "Brigadier", then a "Maréchal des Logis"...
Now back to the ranks. Because of the above mentionned traditions, the "mounted arms" of the French Army (Cavalry, Artillery, Transportation corps, etc...) had different appellations for some NCOs and troops.
- A caporal is called a "brigadier",
- A sergent is called a "maréchal des logis".
but they are exactely the same ranks.When transferred in the Air corps, the men holding those ranks kept them as well as their uniform.
It has been like that for centuries and it is still the same to day. Even the "1er Régiment étranger de cavalerie" (1st Foreign Legion Cavalry Regiment) uses those Maréchal des logis (MdL) and Bigadier (Brig) ranks.