 Model tests of the YF-16 conducted by the Langley Research Center revealed a potential problem, but no other laboratory was able to duplicate it.YF-16 flight tests were not sufficient to expose the issue; later flight testing on the FSD aircraft demonstrated there was a real concern.In response, the area of the horizontal stabilizer were increased by 25% on the Block 15 aircraft in 1981 and later retrofitted to earlier aircraft.