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))))))))))))))) (. .)))
Commercial airlines can change their route whenever they feel like it.
( ( Commercial airlines ) ( ( can ( ( change ( their route ) ) ( whenever ( they ( feel ( like it ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Commercial) (NNS airlines)) (VP (MD can) (VP (VB change) (NP (PRP$ their) (NN route)) (SBAR (WHADVP (WRB whenever)) (S (NP (PRP they)) (VP (VBP feel) (PP (IN like) (NP (PRP it)))))))) (. .)))