They seem to think that he is descending.
( They ( ( seem ( to ( think ( that ( he ( is descending ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBP seem) (S (VP (TO to) (VP (VB think) (SBAR (IN that) (S (NP (PRP he)) (VP (VBZ is) (VP (VBG descending))))))))) (. .)))
The airplane is being flow downward.
( ( The airplane ) ( ( is ( being ( flow downward ) ) ) . ) )
(ROOT (S (NP (DT The) (NN airplane)) (VP (VBZ is) (VP (VBG being) (VP (VBP flow) (ADVP (RB downward))))) (. .)))