That aircraft was United 93.
( ( That aircraft ) ( ( was ( United 93 ) ) . ) )
(ROOT (S (NP (DT That) (NN aircraft)) (VP (VBD was) (NP (NNP United) (CD 93))) (. .)))
United 93 was the aircraft's identifier.
( ( United 93 ) ( ( was ( ( the ( aircraft 's ) ) identifier ) ) . ) )
(ROOT (S (NP (NNP United) (CD 93)) (VP (VBD was) (NP (NP (DT the) (NN aircraft) (POS 's)) (NN identifier))) (. .)))