[anovikov]

Obviously stability has to be achieved through fly by wire tech. Doing it through natural aerodynamic stability is a waste of resources of insane proportions as such airframes induce more drag and burn more fuel. If this sounds too scary for people in a forum of software developers it only puts a shame on our profession, from ourselves.

[XMPPwocky]

The software itself isn't necessarily the issue, though- it's also all the sensors and actuators involved.

Suppose, for instance, that an aircraft needs more yaw stability.

There's all sorts of design choices that could be made, but consider either A: a larger vertical stabilizer or B: automatic application of the rudder to damp oscillations.

The vertical stabilizer here is essentially a bit of metal. We know very, very well what can go wrong with bits of metal. Fatigue, corrosion, manufacturing defects, bad repairs... But, in 2019, we've pretty much figured out the failure modes of big bits of metal on an aircraft, and we generally know how to prevent and/or minimize them.

Now, the dynamic stabilization approach. We'll need gyroscope data (from the IRS, probably), a software model of flight dynamics (which almost certainly already exists and is running), and possibly faster servo valves for the rudder actuator.

This can work! We can formally verify that the control system we've created damps oscillations throughout all normal flight regimes. The gyroscopes are already redundant and well-tested. And you might not even need the faster servos.

Problem is, now avionics failures are even scarier. Will the stabilization here still operate when you get dropped into secondary mode? Probably not- so now, in unexpected situations, pilots need to keep in the back of their minds that yaw oscillations are more possible, that they may need to damp them manually, etc, etc.

Now you throw in some extra factors- turbulence, IMC (which would probably make detecting those oscillations manually that much more stressful), and trying to solve whatever problem dropped you into secondary mode in the first place... and you have something a bit concerning!

A bit of metal won't do that to you. We can make much better estimates of a bit of metal's reliability, and its failures are also less correlated- they aren't much more likely to crop up when you already have another problem.

[anovikov]

Well military jets have been doing exactly that - maintaining stability through software on inherently unstable planes that would break up even in straight and level flight in a split second if computer crashes - for 40 years now. And Boeing builds both kinds of planes so they have the experience.

[joe_the_user]

No one knows better than software engineers how difficult it is to make inherently reliable software and how much complexity can add to the difficulty of making reliable software.

That said, the cost of not using latest fuel efficient airplane would indeed be huge and the actual reliability of modern aircraft is very high and has been increasing over the years in which fuel efficiency also increased.

Sometimes, human can hit on a formula that produces objects that satisfy all the given parameters more fully rather than compromising on any of the requirement. But it's quite plausible that these formulas cannot be milked forever - thus the "Max" may be the point where tradeoffs stop working.

[cco]

Could you clarify your assertion that it would be massively more expensive?

A quick look at the numbers suggests that a 737 MAX 8 is about 10% more efficient on fuel burn compared to a 737 300. That is not "massive" in my book and I'm more than happy to pay a little bit more per ticket if it means a higher safety margin.

Did you mean something older and less efficient than a 737 300?

[edoloughlin]

such airframes induce more drag and burn more fuel

As a curious bystander, I assumed using fly by wire tech to achieve stability would involve using control surfaces, which increase drag by their nature. How would an airframe that's naturally stable and doesn't require control inputs burn more fuel?

[AWildC182]

It's more about preventing a stall with a swept wing which is needed to achieve high mach numbers.

That said, an easy (but different) case to visualize is a traditional tailplane. The center of gravity on an airplane is in front of the wing so it wants to pitch down slowly. The tail pushes DOWN in the back to keep the nose up. Nose heavy planes are stable and forgiving but you induce drag because the wing needs to supply some lift just to counteract the tail which is producing negative lift. If you move the CG backward, you get less stability because the airplane wants to pitch up/down more violently with a control input but you have less negative lift from the tail.

[zaroth]

That’s a great explanation, even if it is oversimplifying.

We don’t build planes with training wheels anymore because the performance cost was too high. Planes are still the safest way to travel even without the training wheels.

I don’t think 737 MAX 8 pushes the envelope too far. I think they screwed up on re-training the disengage, and they may have screwed up on redundancy by only using a single AoA sensor, but I also am guessing the latest crash has absolutely nothing to do with trim.

[cmurf]

There are two alpha vanes on 737s, including the MAX 8, that measure angle of attack. Also we don't know the exact source of the error (in the Lion Air case; in the Egyptian Airlines case we don't know at all). The vane itself could be the source or some other part of the system.

[kakaorka]

I think you mean the Ethiopian airlines case, not Egyptian.

[cmurf]

Yep! Thanks for the correction.

[rocqua]

So the stable airframes we know are stable because a dynamic aerodynamic force opposes a static one.

Makes sense then, that those opposing aerodynamic forces induce drag.