( ( A ( low-flying plane ) ) ( ( ( would suddenly ) ( ( ( lose altitude ) ( as ( ( the density ) ( of ( the air ) ) ) ) ) ( under ( ( the wing ) ( decreased ( causing ( ( an ( imminent loss ) ) ( of lift ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (JJ low-flying) (NN plane)) (VP (MD would) (ADVP (RB suddenly)) (VP (VB lose) (NP (NN altitude)) (PP (IN as) (NP (NP (DT the) (NN density)) (PP (IN of) (NP (DT the) (NN air))))) (PP (IN under) (NP (NP (DT the) (NN wing)) (VP (VBN decreased) (S (VP (VBG causing) (NP (NP (DT an) (JJ imminent) (NN loss)) (PP (IN of) (NP (NN lift))))))))))) (. .)))
A low-flying plane would suddenly lose altitude as the density of the air under the wing decreased causing an imminent loss of lift.
(ROOT (S (SBAR (IN As) (S (NP (DT the) (NN altitude)) (VP (VBZ decreases)))) (, ,) (NP (NP (DT the) (NN density)) (PP (IN of) (NP (NN air)))) (VP (VBZ decreases)) (. .)))