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The A320 and A330 family are FBW aircraft that use C* longitudinal control laws for longitudinal sidestick command tracking. In essence, stick neutral commands 1G flight. When stick is deflected longitudinally, a blend of pitch rate and normal load factor is commanded to provide a consistent G-feel to the pilot; once the stick is released, 1G flight resumes, but the airplane is steady-state at the new pitch angle (or flight path angle to be more exact).

Unlike B777, B787 and A220, however, the A320/330 family do not have artificial speed stability. Once the aircraft is steady-state at the new pitch attitude, and with the autothrottle disengaged, the airspeed will settle to a new trim point. In another word, unlike a conventional aircraft, the A320/330 only has neutral speed stability.

This is why the envelope protections of the Airbus aircraft are not nice-to-have features, but essential features to ensure the airspeed does not rapidly bleed off to stall or increase to overspeed, all while the stick is neutral.

My question is, why is a C* control law without speed stability desirable for a transport category aircraft? The A320 does not need to perform precision target tracking as a fighter aircraft would, and it changes the flying characteristics drastically from those of a conventional aircraft.

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  • $\begingroup$ Are you saying that the 320, if you are doing say 220 kt, and you pitch up 10 degrees keeping power the same, and the airplane slows to 180 kts and stabilizes there, it will now trim to 180 kt and if I make a power reduction without touching the stick it will pitch to maintain 180kt? $\endgroup$
    – John K
    Dec 30, 2019 at 16:06
  • $\begingroup$ @JohnK The first part, yes. Reducing power does not nominally change the flight path angle and instead reduces the speed further. That's my understanding. $\endgroup$
    – JZYL
    Dec 30, 2019 at 16:35
  • $\begingroup$ I had a chance to try out a C series (A220) development sim once and I remember that it was designed to behave like a normal a/c. It has a trim speed bug on the speed tape and when you work the trim it moves to tell you what your new trim speed will be and will behave like you'd expect it to. Very convenient. I know that Boeing's overall philosophy is more "pilot centric" whereas AB, at least on their early FBW, preferred to grant more authority to the computers. Bombardier tried to split the difference on the C series, going with side sticks but trying to maintain traditional behaviour. $\endgroup$
    – John K
    Dec 30, 2019 at 16:52
  • $\begingroup$ Don't know for sure but I would theorize that a system that maintains FPA would tend to make it easier to hand fly a glide slope, where the priority is to pitch to the FPA (the glide slope) instead of pitch to speed. You just work the thrust for speed and let the computers take care of trim. On a normal a/c it's a bit more work because you'll be pitching to slope and retrimming and managing thrust all at the same time. $\endgroup$
    – John K
    Dec 30, 2019 at 17:07

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Many accidents have shown over and over that once the handling becomes muscle memory, the cue that having to pull harder means you are losing speed is easily missed anyway, so it's not as important as it seems.

So the artificial neutral stability instead decouples the controls: stick for flight path angle, thrust levers or A/T for speed. Nice and easy and you don't have to fiddle with the trim button that most pilots end up pressing rather mindlessly.

Large aircraft tend to be flown with auto-thrust all the time anyway, including e.g. a 777 which is stable in pitch and has trim. And when they are, the logic is rather like Airbus—control column for flight path angle, maintain flight path angle when released, just after changing speed you have to fiddle with the trim to get it stable again, which Airbus Normal Law does for you.

Note that aerodynamically all Airbus aircraft are still longitudinally stable, so if the sensors fail and the system degrades to Direct Law, the trim behaves like in any other aircraft.

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  • $\begingroup$ Do you have sources to back up your statement regarding accidents and static long stab? Static long stab requirements are written into the basic Part 23/25 rules and all the major OEMs, except for Airbus, have adopted the C*U-like approach. What of the case with maximum TLA applied, for max climb, for example? $\endgroup$
    – JZYL
    Jan 1, 2020 at 15:23
  • $\begingroup$ @JZYL, not as an explicit statement. However note that there are still many accidents when pilots keep pulling up until the plane stalls despite the increasing force. Also note that the unreliable airspeed procedure does not utilize the trim, but simply defines pitch and power. Big part of the problem is that thrust setting, flaps and gear change the trim a lot, so the force is not really indication of your speed or your margin from stall in direct trim aircraft. It only does that in the Airbus alpha-limit mode. $\endgroup$
    – Jan Hudec
    Jan 1, 2020 at 22:08
  • $\begingroup$ @JZYL, note that the requirement only states that force to keep the nose up must increase when approaching stall, which Airbus satisfies with the alpha-limit mode. $\endgroup$
    – Jan Hudec
    Jan 1, 2020 at 22:10
  • $\begingroup$ @JZYL, maximum climb in Aibus is easy—set maximum continuous thrust and pull on the side-stick until the speed trend stops indicating increase. In direct trim aircraft with underslung engines (the typical configuration) you have to push the yoke when adding power to avoid stalling (at least one A310 crashed because the alpha-floor A/T logic added power and the pilots didn't compensate), arrest the phugoid oscillation and then re-trim for the new power setting, so you'll be referring to attitude and airspeed indicators anyway and not the feel anyway. The Airbus way sounds easier. $\endgroup$
    – Jan Hudec
    Jan 1, 2020 at 22:15
  • $\begingroup$ The static long stab applies to all airspeeds from 1.3Vsr to Vfc/Mfc (25.173). It's separate from stall characteristics. I assume A320/A330 got an ESF from the respective cert agencies. $\endgroup$
    – JZYL
    Jan 1, 2020 at 22:57
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I did not find much about the decision process airbus went through, but my layman idea about their motivation is reduced pilot workload - with pitch stability it gets easier to hold a constant altitude also in manual flight, without the need to use trimming input. flying a hold gets compareably easy - demand some roll rate to the intended bank and go back to neutral. Pitch stability will keep the plane more or less at the current altitude, only little corrections needed. I guess reducing pilot workload is always a potential selling point for new designs (whether they actually do or not)

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It's not desirable. Airbus had to get special permission from at least the FAA if memory serves, just to get their FBW aircraft certified with such a design. It's simply to allow for workload reduction in not having to trim the horizontal stabilizer after a pitch up input is given where the autotrim will trim the trimmable horizontal stabilizer (THS) automatically to give neutral stability. It makes life easier for the pilot. It certainly does have it's disadvantages as you pointed out where it would stall itself if airspeed bleeds off too much or too fast. I'd also like to say it makes stalling the FBW Airbus much easier while in alternate law as it'll maintain your nose up attitude and you don't have any artificial force feedback from the FBW to have to trim for.

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