There are several ways to be absolved of sin. Baptism removes ALL previous sin but is done only once. The Sacrament of Confession removes sin. Outside of confession perfect contrition can also remove sin (perfect contrition: sorrow for sin due to love of God, Imperfect contrition is due to fear of hell) Absolution is given near the time of death, however when a plane is crashing its kinda hard for people to line up in front of a priest to get confession or absolution one at a time.
The priest can give conditional absolution to everyone on the plane on his own, it would apply to any person on the plane who would have if conscious of mortal sin sought absolution.
I wouldn’t suggest betting my soul on that situation.