Originally Posted by
TemboOne
Perhaps I should have been more specific. I am referring to the civil registration (such as C-GHPQ for fin 801) and not corporate logos.
As to Aer Lingus. I am familiar with almost every single airliner they currently have in use, particularly their Airbus fleet, and they all have their EI-XXX registration on the underside of the port wing near as practical to the wing tip. This location seems to be an industry standard. Can't really question any other examples you give but it's interesting that AC seems to have voluntarily elected to not have this optional identifier on their metal.
While it is true that airborne aircraft are not normally identified by their civil registration such identifier could be helpful in an emergency such as an airborne intercept because of radio and/or transponder failure or an accidental emergency squawk.
Why exactly do you think AC should do this? You do know that all airliners have to have the ID painted on the fuselage near the rear door, right? As for the underwing being "industry standard", you say that EI has it, and based on looking at a few shots on Planespotters, so do WS and BA, but UA, AA and DL don't, nor does LH.
You will also find the airline's fin number on the tail and the nose wheel door.