Late '50s / early '60s sci-fi movie. Helicopter crashes on a jungle island. Survivors fight acid-secreting, walking plant monsters (not Triffids). Survivors defend with ditches filled with gasoline. Scene where plant rips off man's arm. They escape after repairing helicopter.
You might be thinking of The Navy vs. the Night Monsters (1966).
It's set on a tropical island named Gow Island, according to the Wikipedia plot synopsis:
Unloading the cargo, Dr. Beecham recommends planting the trees to ensure their survival in the island's tropical conditions. That night, a tropical storm ravages the island. Somewhat later, Gow Island's bird population becomes disturbed by something unknown. The weather station's scientists try to figure out a connection between this event and a corrosive residue that begins turning up at various island locations.
And it features acid-secreting killer trees, according to the IMDB plot summary:
Operation Deep Freeze, a scientific expedition to Antarctica discovers unusual tree specimens. When specimens are shipped out for further study, the trees are accidentally introduced to a south seas Navy base, soon revealing themselves to be killer, acid-secreting monsters that live by night.
The trailer below also shows a man having his arm torn off:
Could it be The Land Unknown (1957)? It has them crashing a helicopter on an island and being attacked by plants (and dinosaurs) before again escaping by helicopter.
A small crew led by Commander Harold Roberts and reporter Maggie Hathaway are on an expedition into Antarctica for the United States Navy. During a helicopter flight, they are called back to their ship via radio because of an unexpected storm approaching. At first they try to fly around the storm, but low on fuel, they fly into the storm, where they almost collide in mid-air with a man-sized pterosaur. Their rotor control rod is bent. Unable to stay in the air, they start to descend and are surprised when they end up landing well below sea-level in a warm volcanic crater. Inside, they discover a steamy tropical jungle with living dinosaurs, giant flesh-eating plants, and fresh human footprints. They cannot contact the ship by radio. When they try to straighten the bent rod, it breaks. They are trapped. The crew encounter many dangers and perils in the jungle in a fight for survival.
The crew meet Hunter, the lone survivor of a plane crash from the 1947 expedition. He has learned to survive in this land with the aid of a conch that drives off the animals and by raiding the dinosaurs' nests. He offers the remains of his airplane to repair the helicopter, but only if the crew agree to leave Maggie with him. The crew refuses, but they also know that after 25 days their ship will have to leave before the Antarctic winter sets in. Unsuccessful in finding the remains of the plane, hidden by Hunter, the crew debate leaving Maggie, or forcing the information out of Hunter by torture. Commander Roberts refuses to sink to either low. Maggie is later attacked by an Elasmosaurus, but Hunter rescues her. After a fight and learning that the crew refuse to torture him for the location of his plane, Hunter gives them the map to its location.
After repairing the helicopter, the crew take off in a hurry as a Tyrannosaurus rex attacks their base. They fly to pick up Maggie, who is with Hunter at the time. Hunter is ambushed by the Elasmosaurus, and the crew come to his rescue. They fly out of the lost world with him. Once clear of the crater, the crew are able to communicate again by radio with their ship; however, the helicopter runs out of fuel and crashes into the ocean before it reaches the vessel. The crew are rescued, and once safely on the ship Harold and Maggie declare their love for one another.
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50s film helicopter island plants attack