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))))))))))) (. .)))
It is not appropriate to suggest that United 93 would have been intercepted.  
( It ( ( ( is not ) ( appropriate ( to ( suggest ( that ( ( United 93 ) ( would ( have ( been intercepted ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (RB not) (ADJP (JJ appropriate) (S (VP (TO to) (VP (VB suggest) (SBAR (IN that) (S (NP (NNP United) (CD 93)) (VP (MD would) (VP (VB have) (VP (VBN been) (VP (VBN intercepted)))))))))))) (. .)))