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 on United 175 stayed the same.
( ( ( The ( transponder code ) ) ( on ( United 175 ) ) ) ( ( stayed ( the same ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN transponder) (NN code)) (PP (IN on) (NP (NNP United) (CD 175)))) (VP (VBD stayed) (NP (DT the) (JJ same))) (. .)))