They had been responding to a report about an aircraft that did not exist.
( They ( ( had ( been ( responding ( to ( ( a report ) ( about ( ( an aircraft ) ( that ( ( did not ) exist ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBD had) (VP (VBN been) (VP (VBG responding) (PP (TO to) (NP (NP (DT a) (NN report)) (PP (IN about) (NP (NP (DT an) (NN aircraft)) (SBAR (WHNP (WDT that)) (S (VP (VBD did) (RB not) (VP (VB exist)))))))))))) (. .)))
The aircraft they had been responding to did not exist.
( ( ( The aircraft ) ( they ( had ( been ( responding to ) ) ) ) ) ( ( ( did not ) exist ) . ) )
(ROOT (S (NP (NP (DT The) (NN aircraft)) (SBAR (S (NP (PRP they)) (VP (VBD had) (VP (VBN been) (VP (VBG responding) (PP (TO to)))))))) (VP (VBD did) (RB not) (VP (VB exist))) (. .)))