Boeing vs Airbus - FBW design philosophies

[Flight Safety]

Hi, I'm a new Ppruner and this is my first post. Sorry if this topic has been covered previously, but I'm interested in a comparative discussion between the Boeing(777) and Airbus(320,330,340) approach to FBW design. While admittedly I'm a Boeing fan, I'm really more interested in the best possible design of FBW aircraft from a flight safety perspective.

There are several issues that concern me about the current FBW designs. First, why did Boeing chose airspeed stability in the control laws for the 777, while Airbus chose pitch stability in their control laws? I'm still trying to decide which approach is better.

Second, why on earth did Boeing decide to design the 777 with "relaxed" pitch stability in the basic airframe and did Airbus do something similar? To me this works fine for military fighter aircraft where FBW was originally developed, but why on earth would you want "relaxed stabiliy" in an airliner?

Third, the 2 companies took very different approaches to the flight dynamics pilot feedback issue. Boeing chose to use a control column with AFCS feedback servos and thrust levers with servos, while Airbus chose to use a joy stick and detents on the thrust levers, without any feedback. Which approach do you think is safer?

I have a lot of comments about these issues but I'll save them for later. Your comments would be greatly appreciated, thanks.

[This message has been edited by Flight Safety (edited 11 January 2001).]

[Juliet November]

Here we go again, the usual pi**ing contest ...

[Flight Safety]

Juliet November, I didn't mean for this post to be a "who's better, Boeing or Airbus" contest. I was really much more interested in a comparison of the design decisions that each company made regarding their respective FBW implementations, and how they relate to flight safety.

I'll chose a better topic title next time.

[This message has been edited by Flight Safety (edited 11 January 2001).]

[Juliet November]

FS,

That's understood but it'll probably end up in a contest never the less. You use the word "comparison" and that has historicallly started quite a few heated discussion on this board.

But it's probably going to be interesting, as it usually is when the Boeing lovers gang up on the Airbus huggers or vv.

I'm not a position to offer any information on the subject you refer to. I'm sure somebody will though :)

[7times7]

FS,

just curious, where did you get the info that the B777 is "relaxed" pitch stability.

Is it the same as the F16's Relaxed Static Stability, where the CG is behind the CP and the artificial longitudinal stability provided by the rapidly moving stab. I think not.

[Flight Safety]

7x7,

I'm not sure, I went looking for the material but couldn't find it, when I find it I'll post it. I'm not sure what airframe design feature causes the "relaxed pitch", the material I read didn't specify how it was achieved. I did read however that since FBW was "faster" in response, that meant the 777 horizontal control surfaces could be smaller in design, thus saving weight. However I don't see how that helps the pilot if reversion to Direct Law occurs.

What bothered me about the "relaxed pitch" was both the reversion to either Direct Law or mechanical reversion. Can you imagine what it would be like in mechanical reversion to be rolling the airplane with the number 4 and 11 spoilers (why spoilers 1 and 14 weren't chosen I don't know), and then trying to control the pitch with the stab trim? My undertanding is that ANY change in airspeed requires retriming the stab. While changes in horizontal trim generally occur in all aircraft with changes in airspeed, I'm talking about a much more pronounced effect with "relaxed pitch".

If "relaxed pitch" was indeed designed into the 777 airframe, then I see this as part of an overall problem in FBW implementations where pilots are being isolated from some of the "real world" aerodynamics acting on their airplanes. To me, this can pose safety problems when "envelope protections" are lost when either pilots turn one or more of them off, or partial system failures turn some of the "protections" off.

To me it seems dangerous for the FBW pilot to lose his/her "daily awareness" of the aerodynamic limitations of his aircraft, that pilots of conventionally controlled aircraft can never afford to forget. FBW aircraft are still subject to the same aerodynamic limitations as any other aircraft, and if a failure causes a FBW aircraft to lose some of it's "envelope protections", then the pilot is being asked in an emergency to suddenly "recall" those aerodymanic limits in an aircraft where he's not often exposed to them, and where the failures provide reduced controllability.

All three issues that I brought up at the beginning of this post revolve around this central issue of isolating the pilot from intimate knowledge of the aerodynamic limitations of his airplane.

[This message has been edited by Flight Safety (edited 11 January 2001).]

[mkaram]

By the way the 777 does not have autotrimming in normal law like Airbuses right?

[SchmiteGoBust]

Flight,
The A321 was also designed with relaxed stability in pitch ,or so I have been told!The reason was so that the area of the horizontal stabiliser could be reduced along with the downward forces on it. Obviously this produces less empennage drag. If you look at a plan view of a 321 compared to a 757 it is noticeable.
With the F16 the aircraft was designed to be unstable in pitch to make it more manoeuverable(as said CP in front of CG).Fly by wire was used to allow a mere human to control it!
The A320 has CG in front of CP, but they are nearer (static margin I think ?) to each other than is usual so slightly less stable.
Funnily enough the Panavia Tornado was built so it was stable. Massive tailerons and fly by wire were used to increase it's manoeuverability--opposite way round to F16. Different design concept I suppose?

[Haas_320]

From A boeing lover on Airbus, Let me start by saying I gained a lot of confidence in the A FBW sytem I started very sceptical. I would not like to go into the technical side too much, expect to state that all the relaxed stability involves a greater allowable C of G on a smaller stabelizer, and indeed this gives problems in case of loss of functionality in one of the elevators.

I compare FBW on the bus to driving a Mercedes, You are very comfortable doing it but ,you pay by losing contact with your vehicle. The good thing is that the car doesn't revert to anything and the aircraft does. I envision a future generation of pilots beeing required to fly a certain number of hours of conventional flight in order not to forget standard Aerodynamics and Navigation just like there is a requirement for recurrent training today.

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