A320 Flare Mode vs. Ground Mode

[Airmann]

There has obviously been a lot written in these forums regarding the flare in an A320, it is something that is a great mystery to those that are new on the bus.

My question is regarding the Flare vs. Ground Mode.

In the FCTM the following two definitions are given
GROUND LAW: The control law is direct law
FLARE LAW: The control law is a pitch demand law

Having looked in the FCOM for a more detailed description of the latter I find that it mentions
1) Pitch trim is frozen at 50 feet RA and this pitch is used as the initial reference for pitch attitude control
2) at 30ft RA The aircraft begins to pitch down to to -2 degrees nose down over 8seconds
3) Flare Mode is "essentialy" a direct stick-to-elevator relationship

The first couple of points are self explanitory. Its the last point that I would like to clarify. Reading the FCTM description of Flare Mode I take from it that the side stick is now commanding a pitch attitude. However, this is not the traditional direct stick-to-elevator relationship that we find in direct law (and consequently in ground Law/Mode). Is this why airbus calls it "essentially" a direct stick-to-elevator, because it isn't but they seem to feel that it is close enough to consider it such?

[DozyWannabe]

Some useful info on this older thread:
http://www.pprune.org/tech-log/444765-a320-flare-technique.html

It's important to get your head around what happens at 50ft on the Airbus. The aircraft enters its flare law, auto-trim ceases and the system memorises your pitch attitude.

What this means in practice is that if your pitch is less and your ROD is more than it should be at this stage it can be hard work to prevent an unpleasant arrival.

...Which is one of the reasons that hand-flown approaches are best flown using manual thrust on Airbus FBW aeroplanes like the A320. Sadly, this seems to be widely discouraged, or even forbidden.

As has been said above, the best way to achieve a smooth AND accurate touchdown on the A320 was always to flare as appropriate for an accurate, moderately-firm touchdown, and then (provided the nose is high enough) briefly release the back pressure on the stick to reduce the pitch-attitude slightly just before main-wheel touchdown.

On touchdown, resume back-pressure to avoid thumping the nose-wheels on. After touchdown, however, ground-spoiler deployment tends to cause some degree of pitch-up: this has to be counteracted. If you are using medium autobrake, it will try to thump the nose-wheels on.

Keep flying the aeroplane. This applies in pitch and roll. Don't be tempted to take your hand off the stick as soon as the nose is down, as I sometimes saw in my line-checking days. It's not over yet. Ground spoilers kill most of the wing lift, but some pitch and (limited) roll control remain available initially. You may still need them.

Chris

For what it's worth, my reading of point 3 is that the stick will directly command elevator deflection (as opposed to pitch/C* in Normal/Alternate Law), but with the offset caused by the gradual pitch-down aspect. Flare mode was intended to mimic the slight increase in backpressure required with conventional controls, as it felt more natural to the pilots during testing.

[CONF iture]

Is this why airbus calls it "essentially" a direct stick-to-elevator, because it isn't but they seem to feel that it is close enough to consider it such?
I would think there is a direct stick-to-elevator, but in the meantime we should not forget how the System is also commanding the elevators in order to obtain the -2 deg nose down attitude over the 8 sec period.
So both the pilot and the System have an action on the elevators.

[A33Zab]

3) Flare Mode is "essentialy" a direct stick-to-elevator relationship


Flare mode is a proportional stick-to-elevator demand but the NZ and body rate feedbacks remain.

Direct mode is a proportional stick-to-elevator demand, but max deflection is limited by CoG and S/F selection.

Ground mode is the only true proportional stick-to-elevator demand.

[gums]

I gotta go with AZ33Z.

From reading all the stuff about the flight control laws, seems they wanted to have the pilot continue to hold some back stick versus just letting the plane continue on a Nz and attitude until touchdown. So you have to apply back stick to have a "normal" touch.

All the control laws and modes of the 'bus seem to get in the way, and it appears that the jet uses a RA altitude to go to the "flare mode". Don't like that, but never flew heavies or had that implementation.

I preferred the Viper implementaion that only used Nz ( biased for AoA) and pitch/body rates. Our aero gains were also used unless in a "standby gains" mode when the air data was FUBAR. The jet behaved like every other one we had flown. You could drive it on or flare a bit for an easier touch.

Still seems to this old fart that folks don't know when exactly they have direct control of the elevator unless in the pure "direct" mode. The gains and Nz inputs and such are there in all other modes and sub-modes and AP modes and on and on....

[vilas]

The word essentially is used because as far as relationship between side stick and the control surface (elevator) is concerned it is same as direct law as stated below but that is not all and there is more to it.
The pitch direct law is a direct stick-to-elevator relationship (elevator deflection is proportional to stick deflection).