This could be a third aircraft.
( This ( ( could ( be ( a ( third aircraft ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (MD could) (VP (VB be) (NP (DT a) (JJ third) (NN aircraft)))) (. .)))
The airplane is the third aircraft, beyond all doubt.
( ( The airplane ) ( ( ( ( is ( the ( third aircraft ) ) ) , ) ( beyond ( all doubt ) ) ) . ) )
(ROOT (S (NP (DT The) (NN airplane)) (VP (VBZ is) (NP (DT the) (JJ third) (NN aircraft)) (, ,) (PP (IN beyond) (NP (DT all) (NN doubt)))) (. .)))