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 report referred to an aircraft that existed.
( ( The report ) ( ( referred ( to ( ( an aircraft ) ( that existed ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN report)) (VP (VBD referred) (PP (TO to) (NP (NP (DT an) (NN aircraft)) (SBAR (WHNP (WDT that)) (S (VP (VBD existed))))))) (. .)))