View Full Version : Positive Stability in Fly by Wire Aircraft

26th Feb 2019, 08:55
Was wondering if anyone knew of the how positive stability characteristics of a fly by wire aircraft such as the Airbus' works compared to an aircraft with conventional flight controls.

Namely after an initial upset to the aircraft do the flight control systems leave the aircraft to correct using it's natural positive stability or does it induce flight control movements in order to return it to it's previous state?


26th Feb 2019, 09:29
As far as aerodynamical positive stability is concerned, all airbuses are very conventional aircraft.

For the flight path attentuation / augumentation function of the FBW (called normal law), in an upset it works like this (AP off):
- the disturbance causes a motion of the airframe around one or more of its axes
- the FCS senses that motion and compares it against the pilots’ input (stick/rudder displacement)
- a combination of control surfaces movement will be commanded so that the resulting motion mirrors the input on the controls and not the outside interference .

In simple terms: the FBW will stop an external protuberance, but then keep the A/C displaced from its initial state. It won’t come back without pilots steering.

26th Feb 2019, 10:50
If the FCS becomes inop during flight, will there be inherent positive stability of the static airplane configuration, i.e. no input from the pilot?

26th Feb 2019, 14:40
So is it fair to say that Airbus fly-by-wire aircraft are made to have neutral stability as the flight computers maintain the pitch and bank after an external force changes the flight path?

26th Feb 2019, 15:04

Thanks, "Detent", you have it right.

From what we saw in all the 447 data traces, the 'bus is aerodynamically solid as a rock. In fact, if it was less aerodynamically "stable" on its own, without a lotta help from Hal, maybe the crew would have associated the descent rate with a stall. Some designs would have been shaking and shuddering and possibly have wing rock and other indications that you were stalling or approaching a stall.

@Airmann! Fly by wire has nothing to do with static or dynamic stability or stall characteristics. Zip, nada, nyet, no-way dot com. Attitude hold is not a stability thing, but the 'bus appears to have neutral speed/longitudinal stability as a function of its control laws, not its aerodynamic characteristics. In Direct Law, I bet it slows up with increasing AoA and speeds up with lower AoA. Just like Tiger Moth or Aeronica!!

What FBW allows for fighters as I flew is less stability in order to have better mission performance like nose rates, roll rates, stall protection ( to some extent). smoother tracking of your target and so forth. For the heavies, it reduces workload and likely makes checking out in a new plane a lot smoother, quicker.

Hal is quicker than we carbon-based lifeforms, so you see a very quick and smooth recovery after hitting a thermal or maybe bumping the stick/rudder. As pointed out above, the plane usually will not try to go back to the exact attitude that existed before the upset. You can see this when the Thunderbird solo planes do a snappy roll, because there's no wing rock when the pilot releases presure on the side stick. No roll command? So Hal stops immediately. As light as the Viper was, it felt like a much bigger, more solid plane down low going thru thermals and gusts.

Hope that clears up questions.

Gums sends...

26th Feb 2019, 17:52
Airmann i think it is safe to say that the aircraft, as a result of a closed feedback loop of the Normal Law FCS function, behaves like a one with neutral stability. After a disturbance it is not diverging, nor coming back but stays steady instead.

I just did a search of the FCOM and the term “positive stability” shows up few times only where the protections are described. I.e. if you are getting near an edge of the envelope, some positive stability (various types) is re-introduced to support staying inside.

Without the FCS interfering at all, in the so called Direct Law, the aircraft has all the usual and appropriate stability characteristics by the virtue of its aeromechanical design.

Gums’ covered it actually.

26th Feb 2019, 17:52
How a FBW airplane responds to a disturbance is a function of how the FBW control system has been designed and what control system mode is active. With autopilot engaged controlling to altitude and heading a disturbance that perturbs the airplane away from either will be met by corrective action from the autopilot to control back to the selected altitude and heading. In a similar manner, if the autopilot is in an approach mode following glide slope and localizer signals the system will control back to the center of both guidance beams if disturbed by a gust.

That same airplane operating with the autopilot turned off, but an "augmented manual flight" control system active will reject disturbances per the system design. Most FBW systems have been designed to provide conventional flying qualities wherein the pilot's pitch command calls for pitch rate, normal acceleration, or a combination of both and possibly some level of positive speed stability. In the roll axis, roll command calls for roll rate. In the yaw axis, pedal input calls for sideslip and may or may not simultaneously command roll rate. With no pilot input in any of the three command axes, the FBW system with autopilot disengaged is typically designed to command 1g/zero pitch rate, zero roll rate, and zero sideslip angle. As result, the response to a disturbance that generates pitch rate, roll rate, and/or sideslip will generally return the airplane to zero pitch rate, zero roll rate, and zero sideslip angle but will not make an attempt to return to the pre-disturbance vertical flight path angle, roll angle, or heading. If part of the FBW system, positive speed stability features may command long term nose up or nose down pitch in response to variation in speed away from a trim point.

26th Feb 2019, 18:03
If the FCS becomes inop during flight, will there be inherent positive stability of the static airplane configuration, i.e. no input from the pilot? On the ‘bus the FCS at worst degrades to that Direct Law, which is your scenario, it does not really fail. The answer for that case is yes.

If it did fail completely the game would change to a Manual Reversion mode where only the trimmable horizontal stabilizer and pitch couple of the underslung engines are left to steer the aircraft, maybe with a tiny little bit of spring loaded rudder. But even there the answer is yes. Just a funny fact. :ok:

26th Feb 2019, 18:19
The Boeing FBW commercial transports provide integral control in the full-up Normal Mode such that differences between pilot commands and airplane response are driven to zero in steady state. The backup control laws provided in Secondary and Direct Modes do not involve integral control and thus will not drive command errors to zero. These backup modes provide very simple feedback to increase "tameness", but will not stabilizer otherwise unstable characteristics. At some conditions (such as aft cg at higher angles of attack) the unaugmented airplane will not be stable, but the combination of the bare airplane handling characteristics and the simple feedbacks provided by the backup modes will provide for continued safe flight and landing.

26th Feb 2019, 23:28

@washoutt...... from way early on the thread, and I did not opine.

A fully FBW plane basically comes in two varieties:
- hydraulic lines run to actuators that operate/move the control surfaces. That;s elevators, ailerons, rudders, spoilers and high lift devices. The actuators could be commanded/controlled by remote terminals on a digital data bus or directly controlled by wires carrying signals from a central box.
- electrical commands are executed at the control surfaces with zero hydraulic lines from the aircraft. In other words, volts and amps move things. The F-35 is one such beast( no kidding, electric pumps and such at the actuators, with no hydraulic lines) , and prolly many space payloads use a similar architecture.

The commercial airline folks have never gone to a completely "electric jet" like the Viper, Raptor or Stubbie(F-35). after all, they fly completey different missions and we lies have the nylon let down option if things go bad. So we have seen a variety of flight control systems by the commercial folks that have various degrees of mechanical things, assiciated/ taylored control laws and then specific degradation fault sequences such as we saw in the Airbus ( 447 et al). No commercial system that I have seen goes directly from normal to "direct law".
I have expressd my concern on several threads about the hybrid systems that mix some FBW concepts, including sftwe and hdwe, with basic hydraulic and cable/pushrod control linkages.
The 737 fiasco is best example to date. A basically good design has had one after another thing added as the aerodynamics of the jet were different after each new engine or tail design or whatever. The best example of a call/hint to start over happened last October.
I completely understand the rationale of the heavy folks developing and fielding new planes. Unlike the military requirements that have a vastly different element of risk, the heavies should have a more robust control system. And it could or should have some basic maechanical aspects if there's computer problems or electrical problems or even structural damage.
My beef is the mix of FBW and actual mechanical architecture has not evolved to an accepted, reliable, understandable state.
And BTW, for many of the youths here. As with most of the military professionals here, I flew four different jets from 1965 to 1984 that had zero mechanical linkages from my stick or rudder to the control actuators or surfaces, All were irreversible hydraulic lines/actuators with zero aerodynamic feedback to me as I had years before in Luscombs, Champs and Taylorcraft when I really learned how to aviate.
'nuff philosophy from this old man.

27th Feb 2019, 00:28
It might be worth exploring the 'why' of FBW. On the newer military aircraft, FBW is pretty much a necessity due to the lack of inherent stability needed to provide great maneuverability. For a human to fly such an aircraft without the computer assistance provided by the FBW system, it would very difficult (in some cases actually impossible) and quickly exhausting to the pilot - not exactly beneficial for someone who needs to be alert and ready for combat. Plus, those military aircraft have a 'get the hell out of Dodge' lever they can use if the system malfunctions or things otherwise go south.
Commercial aircraft have moved to FBW for fundamentally different reasons. Cable flight control systems are high maintenance, require very careful design to get the control column forces right, and are non-fault tolerant. As computer systems have become cheaper and more reliable, it's actually cheaper to design a FBW system, they have multiple levels of redundancy and hence are highly fault tolerant, are easier to maintain (basically pull out the black box and put in a new one), and allow much better integration between the flight control systems and the rest of the aircraft systems (e.g. autothrottle/autothrust).. But you still want good stability to make it easier on those human pilots to handle it if things go south, - there is no 'get out of Dodge' option for either the pilots or the passengers, so you want the aircraft as easy to fly and land as possible when things aren't working right.

27th Feb 2019, 02:57
Great points, TD, about the commercial heavy move to FBW, even with a very limited mechanical connection or two.

First, the mechanical stuff would be the backup, heh?

Second, no question about degree of maintenance on the very old systems with pulleys, pistons, tubes, cables, jackscrews, and so forth.

Third, with FBW you can control each surface separately because you don't have cables and such routed to them from a single control wheel/column/stick, This can really help if you incur physical damage from hail, birdstrike or have a fire/hydraulic line rupture and so forth. Amazing what you can do, and just look at my picture on profile bio or the 610 thread.

Lastly, the control laws can realy make the ride smoother and at the samw time let the pilot yank to the limit in order to get evrything the plane can give you without departing or stalling.

Gums sends...

27th Feb 2019, 04:24
In addition to what tdracer and gums have put forth above as reasons for FBW on commercial airplanes are the performance benefits that FBW supports. It is certainly the case that FBW heavy transports must be designed to support continued safe flight and landing if system failures lead to use of simplified, backup control laws. The handling qualities that are achieved with those backup laws would, however, not be certifiable for every day usage. The open loop aero characteristics plus the limited amount of augmentation provided in backup modes do not have to be good enough for regular usage. They are allowed to be Level 2 for those familiar with handling qualities rating scales. This is permitted because of the high reliability and availability of the full-up up normal systems that produce desired response characteristics.

Relaxing the requirements on the inherent aerodynamic handling qualities allows for designs that employ relaxed static stability and high performance wings that do not present desired stall characteristics on their own. Relaxed static stability permits loading further aft such that trim drag is reduced. This also allows reduction in the horizontal tail area as it does not need to generate as much down force to balance wing lift and CG location. Look at the ratio of horizontal tail area to wing area across a number of models: 737-200 (28.4%), 737-800 (26.3%), 747-200 (26.7%), 787 (23.8%). A smaller horizontal tail is one measure of static stability. The 787 has a comparatively smaller tail and along with that lower trim drag thus better fuel economy. Being able to rely on augmented FBW to provide desired stall characteristics allows use of thinner, higher aspect ratio wings that again have improved trim drag performance.

Another added benefit of augmented FBW is envelope protection and load alleviation functions that further increase performance by enabling lighter weight designs. Those are two large stories on their own that I will not dive into with this posting.

27th Feb 2019, 06:01
If it did fail completely the game would change to a Manual Reversion mode FD may be you meant MECHANICAL BACK UP? Because it still needs hydraulics unlike Manual reversion in 737 which doesn't.

27th Feb 2019, 06:15
. No commercial system that I have seen goes directly from normal to "direct law"
Well it's possible in Airbus. In a benign way it happens when you have lost both Radio Altimeters aircraft remains in normal law but when gear is lowered it goes to direct law. The other examples are with IR faults. When there's double IR failure with the second not self detected it goes into direct law till faulty IR is switched off and ELACs are reset. With tripple IR failure from normal law it will irreversibly go into direct law.

27th Feb 2019, 15:26
Salute Vilas !!
I stand corrected and should have gone back to the documentation I have on the 'bus reversion sequences. Thanks, as bad answers are worse than no answers.
The "flare" law/mode reversion is logical and I should have remembered that one for sure. My 777 data is less detailed than for the 'bus, BTW, but that beast looks to have more mechanical connections.

Gums sends...

Chesty Morgan
27th Feb 2019, 17:43
In simple terms: the FBW will stop an external protuberance,

How to kill a stiffy...

27th Feb 2019, 20:05
777 has three manual primary flight control law modes: Normal, Secondary, and Direct. Normal Mode is required to support autopilot engagement and/or flight director. C*U and envelope protection control laws are provided only in Normal Mode. Secondary Mode continues to use the same primary flight control computers (PFCs) as Normal Mode, but is much simplified due to detected loss of certain signals needed to support Normal Mode. Direct Mode has essentially the same functionality as Secondary Mode, but uses only the Actuator Control Electronics (ACE) computing hardware/software bypassing the PFCs. Normal Mode is maintained for loss of radio altitude, but reversion to Secondary Mode will occur if input signal selection / fault detection logic declares air data or inertial data invalid. (Note that redundancy supports continued operation in Normal Mode for loss of individual sources of either air or inertial data.)

There are two ways to get to 777 Direct Mode. The first is via pilot selection using overhead primary flight computer disconnect switch. The second is as a result of loss of valid commands from all three PFCs simultaneously. (Each PFC employs multiple redundant computing elements so that operation with a single PFC is permitted provided in-line monitoring continues to declare is commands to be valid.)

From the handling qualities / pilot training perspective 777 Secondary and Direct Modes are sufficiently similar to not require dedicated training with one vs. the other.

The story for 787 and the coming 777X with regard to primary flight control modes is similar to the current 777.

28th Feb 2019, 04:18
FD may be you meant MECHANICAL BACK UP? Yes, thanks for correcting.

FE Hoppy
28th Feb 2019, 18:58
CSeries is the same as 777 with the same C*U. Secondary and Direct are known as PFCC direct and REU direct. FD and AT are still available in PFCC Direct but lost in REU Direct. There is one further drop to Alternate Flight Control Unit Direct. This is completely independent. Starting with it's own RVDTs in the inceptors via ARINC to the AFCU computer which is connected directly to the hydraulic PCU electronic input. Only fitted to one PCU on each primary flight control surface but with a damping signal to the others.Also can drive the stab via one channel for trim. The architecture allows AFCU to be powered by the RAT both hydraulics and electrics. Sim handles pretty much the same in all 3 direct modes if a little less responsive in AFCU. Can't comment on the aircraft handling.