A model is built to a scale of 1 ft as 87 ft. That means that actual size is 87 times

as mush as the model size.

Let the actual length be x ft.

Hence for model airplane which is 3 feet long , actual plane length must follow the equation 

87/1 = x/3

i.e.   x = 3*87 = 261 ft long.

Hence actual airplane is 261 ft. long.