Bus-Normal and Alternate law

[alexban]

From other topic :

As the A320 is "trajectory stable" in Normal or Alternate Law it is perfectly acceptable to fly a manual approach with the autothrust engaged unlike "conventional" aircraft where the effect of thrust change will affect pitch trim and may be destabilizing. (Direct Law there is a pitch/trim couple so autothrust use is not recommended)

I'm a Boeing pilot,but maybe in the near future I'll have to saddle on the Bus ,so I'm curios about this.Can any Bus pilot explain the above to me?
How does it work? The thrust change will not affect pitch trim on a Bus? It autocompensates? And what are normal or alternate laws?
Thks
Alex

[BigHairyBum]

Auto trim my dear chap

The airbus auto trims in pitch and roll up to 33 degrees of bank. It does this in normal and alternate law. In direct law the pilot must trim as in a conventional a/c.

The auto trim therefore eleminates any pitch power couple. This means that the throttle is a fast/slow selector and the stick an attitude selector.

So, with A/THR engaged and A/P disengaged in normal and alternate law the A/THR will fly either manual or selected speed while you simply point the thing where you want it to go.

[scroggs]

Normal Law gives full envelope protection for speed/G/attitude, and includes the auto-trim function which means that speed and attitude are essentially not related. In this mode, you select the attitude you want to achieve your required vector, and the electronics then ensure that the aeroplane maintains that vector without further input from you (within certain limits).

Alternate Law is a reversionary mode which gives some envelope protection after certain failures, and still includes auto trim.

In Direct Law there are no artificial protections or interpretations, and the aeroplane responds purely conventionally to control inputs.

[square leg]

I don't think that A/THR works in Direct Law, but I must check that one.

EDIT: It does work! (my mistake)

The theory about the above statement (from alexban) is correct, but in practice, you'll always have to correct a wee bit to get the trajectory to where it was. The same applies for a FPA/TRK (flight path angle/track) approach. Invariably wind changes, thrust changes and ultimately there will be a shift in the FPA and thus small corrections/adjustments will be needed.

But yes, on the whole, once the flight path vector (FPV) has been set (by giving pilot inputs through the side stick) the A/C maintains that vector. So you'll find that once flying and stabilised as you'd like it to be, it becomes "hands-off" flying until you want to change you FPV. It needs a change in thinking from the pilot's point of view.