Originally Posted by
DaveReidUK
Gz is vertical acceleration at the CofG.
To get a picture of what's happening at the back end, you would need to factor in pitch data.
True. The vertical g, Gz measurement is almost always offset to the aircraft mean cg or its instantaneous CG, as well as aerodynamic centers. It is strapped down to the aircraft references, so is impacted as well by attitude vs external reference, and the rates will affect the readings dependent on the offset from where the aircraft is actually pivoting in all xis. Convention gives the rotation around the actual CG.... which is not quite true but is near enough for passing a written exam. There are occasions that the aircraft will not rotate around the CG at all, a statement that can start an argument or not, but it is interesting reading, and even more interesting to look at the flight data. One of the most obvious cases of that is a good crosswind, like 45-50kts on a B777 and watch where the alignment puts the cockpit. Other planes behave slightly differently but the rotation is not around the CG. (moment around the CG is what is used in the end for calculations, but the moments are around the AC not CG, so have to be corrected by those offsets to get the moments around the CG.... which is kind of odd, as the AC is generally more stable in position than the CG is).
where:
Anyway,
This case, the aircraft had a mild pitch up approaching the min soeed case, as the AP was atttempting to respond to the commended mode of the crew, as speed reuduced. (forget about the fact that the plane had already been cleared to a lower level, so the vertical mode is pretty much irrelevent, it is just options). As it got to peak pitch of just under 10 degrees, at 184.5kts, it then started to pitch down, at around 6 degrees a second, 0.1rad, and recorded a Gz lowest of -.025g +/-, which would have an actual almost zero g loading at the aft galley... Thereafter there was a pitch recovery and a return of the Gz towards normal levels.