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I can attest to that, in the service acquiring a sunburn severe enough to limit your duty, was at the commanders discretion, sufficient for an Article 15. Basically a fine, my first shirt used to say you had damaged government property by burning yourself.As with an aircraft, this might not affect its efficiency in any way, but it has damaged the surface and, just like your car, it's not something you want - more so if it's Government Property !
Anyone who works on or around aircraft knows there's certain parts you don't step on, or use as a hand hold to push the aircraft around, like control surfaces, but sometimes you're using unskilled labor. Those no step, and no hand hold stickers are mainly meant for them.
But sometimes ground crew get in a hurry, or may not be familiar with a particular aircraft, and need reminders.
There's not just a danger to the aircraft. A lot of people have walked into spinning propellers, sucked into jet intakes, or walked too close to a jet exhaust.
I know of a lot of people seriously hurt or killed by aircraft that were not even off the ground.
Desk jockeys never take responsibility for much of anything in my experience. When I was young back before cars you know, there was a VERY small airport near my home town of Tarrifville Connecticut. The runway was so close to the little frontage road that there were occasional car strikes from landing aircraft.Sooooo ... wing walk markings show only as far as someone sitting behind a desk is willing to go to say there shouldn't be a problem walking there. After that you're on your own, he ain't responsible (they never are - lol).
In a very few cases, mostly on training and low performance aircraft, the areas where you could walk to get into the cockpit were marked with black paint or possibly anti-skid adhesive tape or period equivalent. Waxed or wet metal wings are slippery and at an angle on a tail-dragger aircraft, so there were good safety reasons for doing so this.
I'd imagine that airframe mechanics knew exactly where they could and couldn't step on a particular aircraft. On a tough aircraft like the C-47 or B-17, or when maintenance speed mattered more than anything else, they might step or crawl all over an airplane. On a more lightly-built aircraft like the B-24 with a very efficient, but not very strong wing, they'd probably be much more careful.
Another thing that you see in the cockpits of fighter aircraft is "no hand hold" on the gunsight mount, to keep knuckleheaded pilots and ground crew from grabbing the gunsight and damaging it or knocking out of alignment as they got into out out of the cockpit.