Airbus: G-Load demand at FL350

[F/O Junior]

It was just an idea: In normal law, all the A32x family planes as well as the A330 (and I guess the A340's absolutely the same) the pitch input made with the sidestick is a G-Load demand.
If I'm not completely wrong, G-Load decreases with higher altitudes. The change may look negligible, but after switching off the A/P at FL350, the plane built up an v/s of +500 ft/s within 20s, three times in a row. Met conditions were stable.
Although the plane did what I've expected, I'm a bit puzzled here. Is this behavior really according design, or was the plane just 3 times slightly out of trim, what led to the climb ?

[Checkboard]

You are talking about the decrease in Gravity with furthur distance from the centre of the Earth? Negligable effect :)

G-load is related to the amount of acceleration an aircraft undergoes. The amount of G you can pull is limited at altitude, but the G for any given situation is the same.

The climb to pitch ratio is roughly proportional to the Mack number. At M 0.5, one degree in pitch give around 500 fpm. At M0.83 one degree gives around 800fpm. This is why it is difficult to fly accurately at altitude, and some companies specify autoflight abouve a certain altitude.

In the situation you describe, it would take only half a degree or so of pitch to induce the climb, perhaps you applied that slight movement without noticing it?

[F/O Junior]

Definitely no movement, as the AP was disconnected via the FCU.

[WOK]

Is it not the case that at, say, 5NM above the Earth's surface travelling at 500kts, in order to follow a trajectory parallel to the Earth's surface (let's assume it to be sperical) it will be necessary to follow a trajectory which will result in a load factor very slightly less than 1g?

Obviously there is middle ground between the orbiting satellite (0g) and the stationary object at the surface (1g); maybe a cruising airliner is in a regime where the effect is measurable?

I confess to being too lazy and dim to do the sums but it may be the case that a nominal "1g" trajectory at FL350 would result in a slight barometric climb?

As an aside, the conc flies with about 1degree of bank in the cruise due coriolis effect, so these things are measurable.

Sorry - no answers just questions, any clever b*gg**s out there?

[manuel ortiz]

Not clever... just read abaut it some time ago.
Yes, the tendency for this aircrafts to pitch up a bit at cruise is due to the fact that the accelerometers are set on the ground at sea level for a load factor of 1 g.
As g decreases with altitude pitch law reacts to recover the chosen value of 1g stick free providing the climb we notice.

Shure made me scratch my head for quite a while until I found the answer.