In this case, the expression "unaccustomed to flying" is a parenthetical phrase and should be represented between commas.
Patenthetical phrases are used as an interruption in the sentence to complement or emphasize pre-existing information or to introduce pertinent new information.
Family separation was a routine practiced during the Holocaust because the males and females were always separated and thus splitting up families. This is an extreme challenge for a united family because they are unable to see each other ever again.
