Livery Details
A partial SunExpress livery is applied to Avion Express A320s. SunExpress and Avion Express have singed a long-term wet-lease contract such that several Avion A320s are painted in SXS livery and operate SXS flights during the summer. This has been the case for several years due to the highly seasonal flight schedule at SXS, and to my knowledge will continue for the foreseeable.
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Why should this livery be added?
Due to the seasonal nature of SXS flights and the almost set-in-stone nature of these Avion A320s showing up every year, they can almost be treated as a part of the fleet. Furthermore, PRG is one of several destinations that SXS deploys the Avion Express operated A320s and thus would also go to enhance realism.