It is appropriate, therefore, to reconsider whether United 93 would have been intercepted.
( It ( ( ( ( ( ( is appropriate ) , ) therefore ) , ) ( to ( reconsider ( whether ( ( United 93 ) ( would ( have ( been intercepted ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (ADJP (JJ appropriate)) (, ,) (ADVP (RB therefore)) (, ,) (S (VP (TO to) (VP (VB reconsider) (SBAR (IN whether) (S (NP (NNP United) (CD 93)) (VP (MD would) (VP (VB have) (VP (VBN been) (VP (VBN intercepted))))))))))) (. .)))
The notion that United 93 would have been intercepted should be reconsidered.  
( ( ( The notion ) ( that ( ( United 93 ) ( would ( have ( been intercepted ) ) ) ) ) ) ( ( should ( be reconsidered ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN notion)) (SBAR (IN that) (S (NP (NNP United) (CD 93)) (VP (MD would) (VP (VB have) (VP (VBN been) (VP (VBN intercepted)))))))) (VP (MD should) (VP (VB be) (VP (VBN reconsidered)))) (. .)))