But another aircraft was heading toward Washington, an aircraft about which NORAD had heard  United 93.
( But ( ( another aircraft ) ( ( was ( heading ( toward ( ( Washington , ) ( ( an aircraft ) ( ( about which ) ( NORAD ( had ( heard ( United 93 ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (CC But) (NP (DT another) (NN aircraft)) (VP (VBD was) (VP (VBG heading) (PP (IN toward) (NP (NP (NNP Washington)) (, ,) (NP (NP (DT an) (NN aircraft)) (SBAR (WHPP (IN about) (WHNP (WDT which))) (S (NP (NNP NORAD)) (VP (VBD had) (VP (VBN heard) (NP (NNP United) (CD 93))))))))))) (. .)))
There was another plane going toward Washington at that time.
( There ( ( was ( ( another plane ) ( ( going ( toward Washington ) ) ( at ( that time ) ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBD was) (NP (NP (DT another) (NN plane)) (VP (VBG going) (PP (IN toward) (NP (NNP Washington))) (PP (IN at) (NP (DT that) (NN time)))))) (. .)))