When Flight Engineer Dudley Dvorak notices that the hydraulic quantity drops to zero, Captain Haynes asks him to turn on "backup systems", but they are also found unworkable. Haynes then states that "all three hydraulics lines at once" must have been destroyed. This exchange shows that the screenwriter misunderstands the way hydraulics work: the DC-10, like all airliners, has three different hydraulic lines (marked as red, blue and yellow), but they are used simultaneously in flight, as each of them controls different control surfaces of the aircraft - while the movie implies one of three is the main one and the other two are backup, used only when the main hydraulic line fails. (In fact, the DC-10 did not have any backup hydraulics: they are running through different parts of the airplane, so it was found improbable to have all three taken out at once with the exception of a total disintegration of the airliner; however, in case of UA232, when the turbine disc on the tailfin engine disintegrated, it ruptured shrapnel pieces all around, severing all three lines, as the red one ran just under the engine and blue and yellow lines ran through the tail fin in fairly close proximity.)
