Before 9/11, it was not unheard of for a commercial aircraft to deviate slightly from its course, or for an FAA controller to lose radio contact with a pilot for a short period of time.
( ( Before 9/11 ) ( , ( it ( ( ( was not ) ( ( unheard ( of ( for ( a ( commercial aircraft ) ) ) ) ) ( to ( ( ( deviate slightly ) ( ( ( ( from ( its course ) ) , ) or ) ( for ( an ( FAA controller ) ) ) ) ) ( to ( ( lose ( radio contact ) ) ( with ( ( a pilot ) ( for ( ( a ( short period ) ) ( of time ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN Before) (NP (CD 9/11))) (, ,) (NP (PRP it)) (VP (VBD was) (RB not) (ADJP (JJ unheard) (PP (IN of) (PP (IN for) (NP (DT a) (JJ commercial) (NN aircraft)))) (S (VP (TO to) (VP (VB deviate) (ADVP (RB slightly)) (PP (PP (IN from) (NP (PRP$ its) (NN course))) (, ,) (CC or) (PP (IN for) (NP (DT an) (NNP FAA) (NN controller)))) (S (VP (TO to) (VP (VB lose) (NP (NN radio) (NN contact)) (PP (IN with) (NP (NP (DT a) (NN pilot)) (PP (IN for) (NP (NP (DT a) (JJ short) (NN period)) (PP (IN of) (NP (NN time))))))))))))))) (. .)))
It wasn't unheard of for a commercial aircraft to change their route a bit.
( It ( ( ( was n't ) ( ( unheard ( of ( for ( a ( commercial aircraft ) ) ) ) ) ( to ( change ( ( their route ) ( a bit ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBD was) (RB n't) (ADJP (JJ unheard) (PP (IN of) (PP (IN for) (NP (DT a) (JJ commercial) (NN aircraft)))) (S (VP (TO to) (VP (VB change) (NP (NP (PRP$ their) (NN route)) (NP (DT a) (NN bit)))))))) (. .)))