Found this on Quora, written by Navy pilot John Chesire:
Fighters can fly safely and extremely low with proper training and a large amount of incremental practice. The only question is, as one flies ever lower, when does it become less safe, or even unsafe.
I have had a lot of low-level training. In training as a Naval Aviator we flew canned low-level training routes or, ‘sand blowers’ across the western US. These were done at 500 feet/152 meters and as I recall at 360kts/667km/h. (We had a waiver for flying over 250kts below 10,000) However in training for combat, in between combat line periods in Southeast Asia, we practiced more incredibly low and incredibly fast flights over water.
More than anything else, unseen obstacles are the greatest threat to low level flying safety. At 500 feet and being on the planned and authorized canned routes flown by many others, obstacles were not too much of a problem. These planned routes avoided them. However other aircraft were sometimes a problem, mostly light civil aircraft. I also do remember one time I dropped down from 500 feet to perhaps 200 feet over the flat desert, only to have an F-111 fly perpendicular and underneath me at an even lower attitude.
We flew a lot out of the Philippines. There the main problem was the unseen obstacles of logging cables, stretched across a valley from one mountain to another. We had one aircraft have part of his wing sliced off by one of these cables as he flew a low-level through a valley. He landed safely but was in a lot of trouble with his CO. Months later we lost an aircraft hitting a logging cable, but the pilot safely ejected.
I remember practicing low-level flight over the water off the Philippine coast. We flew as fast and as low as we possibly could. We maybe rationalized that it was for training to dodge SAMs over North Vietnam, but I think we did it mostly because it was thrilling. We used to leave wakes in the water, then pull up, look back to see who had the best wake.
There is an aerodynamic effect called g
round effect. It is the reason that sometimes an aircraft will tend to “float” down the runway after flaring, instead of touching down sooner. On some of these extreme low-level flights, I would experience ground affect. I would get so low that I could feel resistance and pressure if I gingerly pushed the stick slightly forward and any lower. I knew that was as low as it should ever get! I had hit, the ground effect barrier!
[Ground affect occurs at one half the aircraft’s wingspan. My F-4 had a wingspan of 38ft. Half of that meant I was only 19ft/6m above the water at a very fast speed. It was a cushion. Was it safe? Probably not since one of my friends and squadron pilot actually lost his centerline tank doing this. But his F-4 was otherwise undamaged.]
Years later flying an F-14, I found I could not fly anywhere near this low. And thankfully, I never had to. 500 feet was low enough! And safe!