Airbus FBW - some technical q's

[Ka8 Flyer]

Hi folks!

Not being an airbus driver, I was wondering about the FBW system - especially what target values you set with the Sidestick (as that is what I understand you do)
So basically, if you pitch up to, say, 10°, and let the sidestick go, what will it hold?
Will it be those 10° or will it be a 1 G load?
If its pitch, then theoretically, as you speed up, the aircraft will start to climb and if you fly slower, she will descend.
If its G Load, then the aircraft should start a descent if you increase bank angle, correct?

Or is it something completely different?

Regarding the X axis, I guess you simply set the angular acceleration and if you let go at a specific bank angle, it will hold that.

Thanks,

Mark

[bantios]

Buddy,

Dont get so complicated here.

When stick is neutral it will hold 1G.

Hold the stick back till 10deg is reached; let go and it will hold it.

Bank at any angle (up to 67deg) and you will maintain altitude.

Hope that helps

[FlightDetent]

Ka8, correct.

Pitch 10, release. It will stay at 1 G - keep the pitch.

Roll is roll rate, just as you suggest. For 1 g keeping during accelerated manoeuvres, i never thought of it. It defenitely does not do the silly things you say, perhaps the sidestick 1 g keeping is in fact 0 g change keeping, measured along vertical? It trims automatically for sure, but definitely yo do have to pitch up in turn to keep it level (I doubt in manual it would adjust pitch itself - uncommanded manouevre).

Bank at any angle (up to 67deg) and you will maintain altitude.

Will it, really? Honest question.

Cheers,
FD.
(the un-real)

[DBate]

Bank at any angle (up to 67deg) and you will maintain altitude.

Will it, really? Honest question.

Well, as long as you do not exceed the bank angle protection limitations, the aircraft will maintain altitude without pitch input from the pilot. Let's go a bit into detail:

If the pilot releases the stick at a bank angle greater than 33°, the bank angle reduces automatically to 33°. Up to 33° the system holds the roll attitude constant when the stick is at neutral.

If the pilot holds full lateral stick deflection, the bank angle will go to 67° and no farther.
But - if the pilot exceeds 33° he will have to make a pitch input, since automatic pitch trim is only active from 0-33°. So you will not maintain altitude if you exceed 33° bank and the pilot does not increase the pitch input on the stick.

Furthermore, and just for some more info: If the bank angle exceeds 45° (while autopilot is active), the autopilot disconnects and the FD bars disappear. The FD bars return when the bank angle decreases to less than 40°.

So long,
DBate

[FlightDetent]

The entire posting EDITED - DELETED because manufacturer says:

...
In addition, if the bank angle is less than 33 degrees, pitch compensation is
provided...
...
During a normal turn (bank angle less than 33 degrees), in level flight:
· The PF moves the sidestick laterally (the more the sidestick is moved
laterally, the greater the resulting roll rate - e.g. 15 degrees/second at
max deflection).
· Not necessary to make a pitch correction
· Not necessary to use the rudder.
...

Tea, coldrex, thermometer, socks...

FD
(the un-real)

[XPMorten]

Airbus FBW (http://www.xplanefreeware.net/morten/DIV/FBW.pdf )

Cheers,

M

[Ka8 Flyer]

So is it vertical acceleration that is kept constant (ie vertical speed)?

If you are in level flight and you let go of the sidestick and then pull back the thrust levers, will the aircraft start to pitch up to maintain altitude?

[Aztec Kid]

Stick free the A320 series aircraft will essentially maintain 1G. However the auto-trimming may not be able to totally compensate for all pitching moments.

Since the FBW system is maintaining a 1G load rather than holding a specific pitch attitude, the Airbus has a number of interesting handling traits. If you happen to be hand flying in turbulent conditions, you will see the aircraft pitch up and down slightly as the FBW system attempts to maintain 1G in the vertical gusts. Some pilots, new to the Airbus, have mistaken this characteristic as an uncommanded pitch changes and made an unnecessary entry on the aircraft maintenance logbook.

When in a turn up to 33 degrees of bank, the FBW system will maintain the appropriate G load for the bank angle (i.e. 1.14G for a 30 degree bank). This minimizes the need for any pitch inputs during normal turns.

Another fascinating characteristic of the A320 series is due to the high resolution of the vertical accelerometers. When cruising at higher altitudes, the local force of gravity is slightly less than 1G. Since the FBW system is sensitive enough to discern this difference, the aircraft will slowly pitch up, in order to maintain 1G. The greater the altitude, the more pronounced the pitch rate. Even at FL390, this characteristic, while slightly annoying is easily controlled.

While I don’t have any experience on the A330/340, I have been told its FBW system is more sophisticated. On the A330/340, the system takes the aircraft altitude into account, and will reduce the stick free G load so the aircraft will not exhibit the minor pitch up tendency of the A320 series. Unfortunately, I don’t have any documentation regarding the A330. Possibly some of the pilots who fly the A330/340 could comment.

Best regards,
The Kid

[bantios]

Ka8 Flyer,

Answer to your question.

If you are flying level and you retard the thrust levers to idle,
the FBW system will maintain level flight. Once a minimum safety speed is reached, the autothrust will advance the thrust to prevent the plane from stalling.

I am facinated by the FBW system everytime i fly it. I am new to the Bus and i love it.

Something else that you might like to know:

In a single engine failure, the FBW will keep wings level regardless of thrust settings.

If you lived in Australia I'd take you on a simulator. An experience you can't miss.

[spleener]

Some off the cuff thoughts:
I don't have the FCOMs handy, but I think engine inop the aircraft will roll to 7deg AOB max without aileron input in normal law. Regardless of thrust setting? Hmm, if speed greater than the applicable Vmca then yes.
The ATT vs G discussion is interesting, but generally not relevent as flight/environmental conditions are normally more dynamic so corrections [ S/S inputs] are inevitable. As expected, the big engined [ie non CFM] aircraft appear more affected by thrust changes.
The turbulence response is a factor, one reason why Airbus suggest "global" sidestick inputs.
Can't say that I've thought it prudent to play with the sidestick at high altitude [330/340] but A/P disconnect does not appear to produce any pitch up tendency.
Overall, Airbus FBW is generally a delight to fly. The Pilot just needs to remember that the aircraft still obeys the physical laws of Aerodynamics and Gravity should he get it wrong!

[Carnage Matey!]

KA8 Flyer - I don't think Bantios' explanation is quite correct. In level flight if you close the thrust levers the airspeed will reduce to a minimum level then the autoflight system will trim the aircraft nose down to descend so as to maintain that speed. Left untouched the aircraft will descend into the ground at that speed. If you attempt to intervene by pulling the stick back you will reduce the airspeed further into the alpha-protection range, then the engines will go to full power to prevent the stall.

[lgw bean flicker]

Following on from the above,........

Is there a simple way to fly a rate 1/rate 2 turn?!

Mr Bean

[Hand Solo]

You don't fly rate 1 turns in an airliner unless you like pulling lots of g.

[lgw bean flicker]

So how do you fly a MANUAL hold?! (without automatics)

Is it follow track bar on the inbound and outbound, and then vary the bank angle depending on wind?!

Mr Bean

[FlapsOne]

Why would anyone want to re-join the dark side and fly manual holds without automatics.....................perish the thought;)

[lgw bean flicker]

:ugh: Its been a while!! just wondered how its done in a 'bus (AP failure/FMGC failure/Emer Elec..... just a few scenarios that could lead to manual flight!!

Any ideas?!

[Hand Solo]

Do you really think you'd be holding in the emergency elec config?

[145qrh]

No holding, but in BA a good chance of continuing to destination..

[JamesT73J]

Bantios, is the inverse true of that as well, in that if you advance the throttles, does the aircraft simply increase spead rather than want to pitch up?

[Obi Offiah]

Buddy,
Dont get so complicated here.
When stick is neutral it will hold 1G.
Hold the stick back till 10deg is reached; let go and it will hold it.
Bank at any angle (up to 67deg) and you will maintain altitude.
Hope that helps

I just wanted to revive this thread as it is very interesting. :)

Airbus apparently use C* control laws with g-load for pitch response correct?. If the aircraft is climbing (autopilot/climb schedule computers off) at a velocity vector angle of 15 degrees, in order to maintain 15 degrees of pitch, i.e zero pitch rate, the aircraft must fly with a g-load less than 1, (In theory if the plane was flying straight up at 90 degrees to hold this angle 0g is necessary).
So is Airbus using G-command or pitch rate command, where neutral stick equates to zero pitch rate.

Thanks
Obi

[Ka8 Flyer]

Obi,

The way I understand it now, is that the FBW system will maintain a constant vertical acceleration when the sidestick is neutral (in the Y axis) (as long as the bank angle is less than 33°)

Therefor, if you are climbing at 15° pitch up attitude, and your vertical speed is constant, the vertical acceleration will remain constant aswell (=0)
I think the whole concept of "1G" is a simplification Airbus made to ease transition to the FBW Airbus. 1G simply doesn't make any sense as soon as you have a bank angle...

So, at least as I understand it, the FBW will maintain a constant vertical speed when you let go of the stick. Does this make sense?

Regards,

Mark

[Obi Offiah]

Ka8 Flyer

I think I understand now.

Usually whenever a discription of the Airbus FCS is presented 'maintains 1g flight and pitch angle' is often times quoted in some form :) , this is what I found confusing.

Considering that the FCS will maintain pitch attitude with neutral stick, while bank angle is < 33 degrees, it must vary the G load. It didn't help that I was observing pitch and roll separately.

My passion for aircraft mainly lies with military aviation and this has contributed to my confusion. My knowledge or perhaps more accurately, my lack of knowledge regarding FCS :} is biased towards military fighter applications, this skewed my thinking.

With fighter aircraft the FCS/CAS is designed to provided constant stick force per g, so a pilot flying at 300kts would apply the same stick force to achieve a 4g pitch as would be done at 600kts. This feature I guess enables the pilot to fly by 'feel' and perform accurately as such, by keeping/feeling constant g on his/her body. Because stick force per g is constant hands-off stick force is usually trimmed for 1g i.e zero pitch in level flight, the most common occurance.

Unfortunately a side affect of this system is that in order to fly at any other pitch angle (velocity vector referenced), the pilot would have to hold a constant forward stick pressure, ranging from a g command equal to or greater than -1g, to a command less than 1g depending on pitch attitude. The only other option would be to re-trimmed for hands-off in said situation, but this would increase the pilots work load.
Trimmed for 1g level flight, if the pilot were to pitch up e.g to +1 degree, the aircraft would very slowly (initially) begin to pitch upwards. Pitch rate will increase as pitch angle increases and would be greatest at 180 degrees (inverted), thereafter pitch will begin to decrease until a pitch rate of 0 is reached at 0 degrees of pitch.

Airbus FCS logic demands that stick force/deflection is not constant per g and is dependent on pitch attitude. At 0 degrees of pitch a forward stick deflection could cause a 0.5g dive, whereas at 45 degrees of pitch (in theory) the same stick deflection would cause a 0.0g dive. Although the Airbus C* laws use inputs primarily from the accelerometer, it could be that the system infact acts as a pseudo or hybrid pitch-rate command system?.
With the Airbus FCS, the pilot will have to vary stick displacement to maintain a particular g load as pitch varies.

Airliners such as Airbus are limited regarding g loading and attitude ranges. Transtitioning between g loads in an airliner occurs relatively slowly, g-loads are low and changing g loads in pitch happens slowly, with rates that are not sustained for long periods, unlike fighter aircraft. Due to the limits mentioned, the smooth, comfortable and manoeuvre friendly aspect of airline flying, maintaining accurate flightpaths with little deviation and precise control of pitch should be easy to achieve.

In closing I guess my confusion was caused by a fighter plane bias, I failed to realise that two completely different aircraft, designed to accomplish two completely different tasks would have differences that also manifested in their flight control law design philosopies.
It seems to me, that while fighter aircraft perhaps have FCS's tailored to make precision flying less demanding while manoeuvring in a dynamic environment, Airbus has a FCS which is focused on reducing pilot work load for tasks such as cruise/climbs and descents.

Edited: After thinking some more about this, it would be wrong to label the Airbus FCS as a pitch rate command system (in normal law at least).

Obi