It was the last gift under the tree, and Jennifer Rosiak was thrilled.

Her husband, James, a senior airman at Eielson Air Force Base, Alaska, had surprised her with airline tickets for her and 8-month-old son Nicholas to visit her mother near Detroit.

The visit was to have been special, said her aunt, Josephine Yackaboskie, of Spotswood, N.J. It would have been the first time for her mother to meet her new grandson.

All three Rosiaks had spent New Year's in New Jersey. Last week, they said goodbye at the Philadelphia airport. James would fly to Luke Air Force Base, Arizona, the family's next assignment. Mother and son were to go to Detroit - via Cincinnati - before reuniting in Phoenix.

''It's a sad story,'' said Capt. Sandy Troeber, public information officer for Eielson Air Force Base.

Said Mrs. Yackaboskie: ''She was a sweet girl.''