where are the maths boffins?
get a protractor....... if your assumption is correct in saying the FF on the groung is standing straight up then measure the angle between him and the back corner of the truck. and see what you come up with.....
the Hino's have a safe working angle of 35deg. and the centre of gravity is getting better the more the water goes down.....
lets just see what sort of angle we come up with (id do it myself, but I dont have a protractor here)......
I dont see the FF on the back of the truck hanging on for grim death, and I dont see the FF standing directly beneath it panicing that hes about to get squished!
Now I know i didnt mention anything about fluid movement when the truck moves again, but dont think its a huge issue.
Oz_fire has it right.... they know the vehicle, they know the terrain.... dont knock what you werent there to see!