Is it permissible to pray behind a flight attendant when it is not known whether he serves alcoholic beverages or not?

The scholars differed regarding who is most entitled to lead the prayer (Imamah). Abu Hanifa, Al-Thawri, and Ahmad held that the most entitled person to lead the prayer is the one most knowledgeable of Allah's Book (the best reciter), citing the hadith of Abu Mas'ud Al-Ansari as proof.

On the other hand, Malik, Al-Shafi'i, and their companions maintained that the most jurisprudent (faqih) is given precedence over the best reciter. This is because what is needed in terms of recitation is limited and well-defined, unlike jurisprudence. Furthermore, during prayer, situations may arise where only a person with complete jurisprudential understanding can correctly handle the matter. For this reason, the Prophet, peace and blessings be upon him, gave precedence to Abu Bakr in leading the prayer.

According to both schools of thought, if a less qualified person leads the prayer while a more qualified person is present, the prayer is valid, though leading the prayer by a morally corrupt person (fasiq) is disliked. Therefore, your prayer behind the aforementioned flight attendant is valid, especially since you trust his religiosity, knowledge, and character.