A320 FBW: Why 0.96G at the gate?
 

I pulled up the VRTA AIDS code at the gate a couple days ago. The two accelerometers showed 0.959 G.

Why wouldn't they show 1.0?

Sensor tolerance?
 

I am not familiar with these particular sensors or their use within the A320 systems. Any individual sensor will have a certain tolerance so I would not expect them to read exactly cosine(pitch attitude) as expected when sitting on the ground. For very accurate comptuation of normal acceleration there is probably some equalization logic that looks at multiple sensors and has logic to identify and correct for sensor error/bias.

Could be... i'l check again on another plane next week.

And now that I think about it, I think they showed 0.979 g.

I don't know how sensitive the accelerometers are, but Earths gravity is not constant on the entire surface, there are small variations depending on your location.

Maybe it reflects the difference between axis of the G sensor at the gate and at some nominal value in flight. The axis of the G sensor will correspond to a particular pitch attitude will it not. The Pitch attitude at gate, say -1 degree and 2.5 in flight for example. Although I have never set foot on an a320 so NFI what respective angles are..

I think Roo got most of the answers.
But yes, the g-sensor (vertical accelerometer) is aligned with what? An aircraft structural datum, not necessarily fully related to the aircraft on-ground attitude.
The on-ground attitude of the aircraft is affected by the fuel load, pax and baggage load, landing gear shock damper pressures, and I'm probably forgetting some.

Too late at night here, and I'm too lazy to do all the calculations.

CJ

Gravity variation with position on Earth
 

Another thing to consider is that gravity is not the same at every point on earth. The biggest variation is with lattitude. Altitude also has a small influence.

From Wikipedia:

"... the equatorial bulge and the effects of centrifugal force mean that sea-level gravitational acceleration increases from about 9.780 m·s−2 at the Equator to about 9.832 m·s−2 at the poles, so an object will weigh about 0.5% more at the poles than at the Equator."


This variation is not as much as the difference between what was noted in the opening of this thread and the expected ~1.0, but it shows that there are other reasons besides sensor tolerance and sensor alignment that would lead to a measurement other than cosine(pitch attitude).

Thanks, FCeng84.
You're obviously less lazy than me....
I still wonder about the actual cos(theta) in terms of attiitude on the ramp (loading, and even ramp angle)... looks as if I have to get my calulator out tomorrow.

CJ

Maybe the airplane was full of hot air... :}

That would surely contribute to the deviation but would not be sufficient.
It would take an angle of 16° to get 0,96 g.

My Vote would be for primarily sensor bias.
But I assume that Airbus FBW corrects for that by crosschecking with other inputs as @FCeng84 already pointed out.

Is it possible that the system defines "g" as 10.0M /s^2?

I'm quite confident that even our French friends wouldn't violate Newton that badly... :}

I think ROO got it...

Tried it in a different aircraft, same result. Looked at VRTA in flight and saw that at 2.5-3 degrees it held 1.0G. At the gate it is -1 -> +1 degree pitch depending on how it was loaded.

Thanks for the help guys.

Interesting !
And a bit strange/surprising.
Would have expected that the readings of these sensors should follow (at least roughly) the cosine of the pitch angle !?
That would require as I mentioned before a pitch angle of 16°.

Anyone any idea why they apparently don't ?

I'm not sure if you caught it, but I corrected my original post by saying that at the gate the sensor showed about .979 G. The cosine of 3 degrees is 0.9986, so it still seems a little off.

Ah, OK, now I got it !
Indeed missed that somehow...

Thanks!

Still a bit more than could be expected from the pitch angle alone.
Interesting find therefore.

Could you see any variations while taxiing or sitting on a different ramp?
In other words how sensitive is that thing in general regarding attitude?