At 8:51, the controller noticed the transponder change from United 175 and tried to contact the aircraft.
( ( At 8:51 ) ( , ( ( the controller ) ( ( ( ( ( noticed ( the ( transponder change ) ) ) ( from ( United 175 ) ) ) and ) ( tried ( to ( contact ( the aircraft ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (CD 8:51))) (, ,) (NP (DT the) (NN controller)) (VP (VP (VBD noticed) (NP (DT the) (NN transponder) (NN change)) (PP (IN from) (NP (NNP United) (CD 175)))) (CC and) (VP (VBD tried) (S (VP (TO to) (VP (VB contact) (NP (DT the) (NN aircraft))))))) (. .)))
The transponder code changed and the controller was eventually able to contact them.
( ( ( ( ( The ( transponder code ) ) changed ) and ) ( ( the controller ) ( ( was eventually ) ( able ( to ( contact them ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT The) (NN transponder) (NN code)) (VP (VBD changed))) (CC and) (S (NP (DT the) (NN controller)) (VP (VBD was) (ADVP (RB eventually)) (ADJP (JJ able) (S (VP (TO to) (VP (VB contact) (NP (PRP them)))))))) (. .)))