Low passes become dangerous when the pilot decides to go into a steep turn as part of his low fly-by show. When an airplane goes into a steep turn, all the lift is going sideways.
Visualize: put your hand out as if to shake hands. That’s a wing in a steep turn. Now, with your other hand, poke a finger into the first palm. The finger shows the direction of the lift: sideways. Lift needs to be vertical to keep an aircraft in the air.
The other thing: the stall speed of the wing goes way up in a steep turn. Meaning, you have to fly much faster to keep the wings flying (generating lift) than in straight & level flights.
Low-pass flybys are cool. We would sometimes do them at the ship in order to let the maintainers see their hard work in action. Those hard-working sailors toil for 18 hours a day so we can go fly, and all they get to see is if the jet catapulted away and then, hours later, watch it trap on the carrier. The attached picture is me conducting a fly-by for my mechanics during our deployment in 2000
In the Navy and Marine Corps, they still teach the art of low-level flying. This is both a somewhat viable tactic, as well as a valuable skill developer. Typically, the instructed techniques involve flying at 500 feet with a speed somewhere in the 300-400 knot range. As airspeed increases and/or altitude decreases, the danger of that type of flight increases.
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The scan requirements for the pilot are limited to 3 secs of looking at any single facet. In old-school aircraft, where the HUD or helmet didn’t exist, looking inside the cockpit for altitude or navigation was limited to a 3-second glance. Within that 3 seconds, the aircraft could begin to dive or roll to such an angle that recovery might not happen before ground impact.
So picture this: you’re in your sweet F-14 Tomcat, and you are zorching around at 500 feet and 500 knots, having a blast. You see a glimpse of a warning light. You look down at the light on your right side and see an oil pressure light, then you look at the oil pressure gauge to confirm that the light is legit. While you are heading down, you ease up on the stick and the nose drops 15 degrees. When you put your eyes back outside, you can see that you are diving, the trees are getting pretty big, so you begin a max G pull-up, but the arc of your turn is too large and you impact the ground.
Now with a HUD or a helmet, the pilot is able to keep his eyes outside more and do a better job of terrain avoidance, but there are other hazards when down there; birds, support wires for towers, power lines, drones, etc. So you get to deal with more objects in the air when closer to the ground.
There are a few old sayings that are pertinent to low fly-bys:
In an emergency, the altitude above you is useless. You can only tie the low altitude record.
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