(S (S (S (BASENP (DT The) (NN plane)) (VP (VBD was) (PP (IN over) (NP (BASENP (DT a) (JJ mountainous) (NN area)) (VP (VBN covered) (PP (IN with) (BASENP (JJ dense) (NN forest)))))))) (SBAR (WHADVP (WRB when)) (S (BASENP (NN air) (NN traffic) (NN control)) (VP (VBD lost) (BASENP (NN contact)))))) (PERIOD .))