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)))) (. .)))
This plane is possibly the third one.
( ( This plane ) ( ( ( is possibly ) ( the ( third one ) ) ) . ) )
(ROOT (S (NP (DT This) (NN plane)) (VP (VBZ is) (ADVP (RB possibly)) (NP (DT the) (JJ third) (NN one))) (. .)))