Other airliners have had undesirable parts of the flight envelope. The 727 for example has a high T tail, and it's possible to get into a deep stall, which is not recoverable because at very high angles of attack the elevators are in the turbulent air behind the wings. In the UK, the CAA required 727s to be equipped with stick pushers to prevent them ever getting close to this regime. This was due to their experience with a BAC 111 that deep-stalled and crashed in 1963. I don't think the FAA ever required this though.
(For those who don't know, a Deep Stall happens when the horizontal stabilizer gets in the "air shadow" of the wings, hence you can't get airflow there to move the plane)
Some interesting discussion on t-tails and deep stalls http://www.rbogash.com/Safety/deep_stall.html
It won't "take itself" but possibly just a very light pull would stall the plane in some situations, and that is the very safety problem which Boeing tried to solve with MCAS, while keeping silent on its existence, because the plane is behaving differently than the one for which the pilots trained. Which is the opposite of how the plane was advertised and sold.
So any time when the MCAS is off the plane behaves differently than the one for which the pilots trained, therefore "just keep it turned off" was also not an option.
And we don't know how "light" is light, it can really be the case that it's even dangerously light.
Was the need for MCAS really so infrequent ("never during the pilot's career" -- are you aware how seldom it is?) the planes would simply have MCAS completely disabled and fly as we speak. That would be the full content of the Boeing's "software fix.
It's obvious that MCAS has to be kept turned on as soon as no autopilot is activated. (I'd also like to know how non-redundant (i.e. max 2) faulty AoA sensors affect the autopilot, by the way -- having just read that the modern Airbuses have four of combined AoA-speed sensors).
"But it's just because of the regulations" -- don't forget these regulations are there to minimize the number of crashes. And the regulations eventually in this case reduce to as simple as "the pilots have to be trained for the plane if the plane behaves differently." Which they weren't, and the latest Boeing claimed on that topic, after the two crashes was that the "one hour iPad training" will be enough(!?) And we now even know that Boeing was even allowed to do the certification process for themselves instead of FAA doing this.
The only really stable commercial aircraft are, perhaps, the bushplanes. They are not fast and not fuel efficient over any real distance.
This doesn't follow. Not even fighter jets were inherently unstable before modern control electronics. Using the electronics to make the flight characteristics better is not the same as the airframe being inherently unstable.
Fighter jets are also, in comparison to airliners, not very aerodynamically efficient. They can be fast but burn lots of fuel while doing so. They are like a motorcycle compared to a bus.
Until they hit an AOA that puts the tail in the wind shadow of the wings. Then some will pancake. They can slip sideways into scary turns/dives and will not recover on their own. And they are delicate. It doesn't take too much force to twist them out of shape.
In a deep stall you can’t recover from, it’s because the aircraft’s stability is greater than its control authority, not because it’s unstable.
> I don't see why it matters if they are airliners as long as instability does not translate to a higher safety risk
We don't know what the extra safety issue might be.
> I don't know if any other airliners are unstable or how unstable they might be.
We don't know if anyone has ever done this before, or how completely unusual it might be.
> In this case the instability appears to have been far less significant than the faulty corrective mechanisms.
And finally we don't even know the level of instability. The system was there possibly correcting for huge issues. After all the fleet is grounded instead of just disabling the system.
It'd approach criminal liability if the FAA certified a plane they didn't know the level of instability of. This is why test flights are performed.
As the other commenter mention, something that gets lost in this discussion is the fact that MCAS was there to make the control feel approximate other airframes.
The same aero adjustments could be accomplished by the pilot, but regulations prohibit their being necessary. So, MCAS.
We're talking 'stability that adheres to control forces required by regulation' rather than 'F-117 drop out of the sky' instability.
The question isn't if they didn't know, it's if this is an unusual level of instability for an airliner.
>We're talking 'stability that adheres to control forces required by regulation' rather than 'F-117 drop out of the sky' instability.
Hopefully yes, but we don't know how much instability there is. If you assume the answer there's no point having the discussion.
Because we don't. You've only corrected a strawman, that we know this plane isn't a fully aerodynamic unstable design like an F-16. No one thought that.
I never even specifically said aerodynamically unstable, just that we don't know the level of instability MCAS is correcting for to know if it is unusual for an airliner design. From what I had read I had assumed it was actually a power-on issue and not aerodynamic. But as it turns out the issue is indeed apparently aerodynamic from the engine shape generating more lift at higher angle of attack and thus leading to pitch up.
And no, we don't know. We the public at least. We don't know how big the flight envelope is where MCAS is needed or how much of an issue it would be if MCAS didn't exist. From public information we don't know if without MCAS the plane wouldn't enter an unrecoverable pitch up attitude in normal flying conditions. That's not knowing if the plane isn't inherently unstable.
I’m not responding to a straw man, I’m responding to what you wrote. If it’s not what you meant, well, I can’t really do anything about that.
Hopefully yes, but we don't know how much instability there is. If you assume the answer there's no point having the discussion.
I was making the point that we don't know what was actually certified. Having F-117 level instability is obviously not the case but apparently the instability it does have is actually aerodynamic so your argument doesn't even work. The actual opposite is happening. The non-fly-by-wire design was modified with an automatic control adjustment to fix an aerodynamic issue. All your argument proves is that the MAX didn't magically turn the 737 into an inherently unstable design which is not relevant to the issue of if the MCAS is covering up a large or small instability.
- Boeing did a good enough job with the airframe that the plane flies mostly fine but there are no miracles and some specific angle-of-attack and airspeed situations make it fall outside the rules.
- To bring the plane back into compliance they implemented a fairly simplistic solution in MCAS. Because the situation that is being fixed was fairly benign they didn't engineer it with triple redundancies.
- The lack of redundancy was understood to not be a big issue because the worst possible outcome is runaway trim and that's something pilots already train for anyway.
- The MCAS runaway trim presents itself in such a unique way that even experience pilots that are already warned about issues with the plane fail to diagnose it.
It's in this context that I would then like to know how unusual this situation is. If modern airliners all have these kinds of issues solved by control software then this is just another failure mode that needs to be fixed. If instead it was indeed unusual to fix the aerodynamic issue with control systems then the discussion about Boeing forcing through with the 737 instead of redesigning becomes more important.
McDonnell Douglas also implemented something similar one of their DC' family of jets. I believe it was either the 9, 10, or 11. In that case it was to accommodate a smaller tailplane to decrease drag.
"I don't know" certainly does not translate to we don't know. Plenty of people know.
That is FUD. The level of instability was absolutely known to the manufacturer and certifying agencies before delivery, and even before first flight. The reason why I said "appears to have been far less" is because the summaries of this instability I have read did not describe it as major, and even now, nobody is pointing to the instability as a significant issue. The issue is with how it has been dealt with. Until information comes to light contradicting the information known publicly so far, it is unreasonable for me to be afraid of something I have zero evidence is an issue when evidence to the contrary exists.
In the strict sense of the term, we the public will never know. The same people who built the plane will provide a fix, the same people who certified the plane will do so again, the same people who were flying them will do so again, and we will trust them or we won't. Some more people will have their eyes on it, but again, we still won't know. That is the way most science and technology works. Everyone has their small domain of expertise and must trust others in their own. Mistakes were made, and most mistakes look stupid/obvious on hindsight, but they don't negate the capacities of the parties involved. All car manufacturers have safety recalls extremely regularly and we don't make them out to be criminals, nor do we make the certifying agencies pariahs. I personally find it an exaggerated response to lose all trust in everyone involved - especially since new models of planes regularly have more accidents in their first few years of flight, because they are hard to make and they have bugs. It also seems unreasonable for me to criticize a manufacturer for taking economic/market concerns into account when historically even the largest manufacturers are on the verge of bankruptcy semi-regularly. Realistically, they must. At the same time this specific decision does not appear to have been the correct one, but I don't demonize them for making it and even at my most cynical, they obviously did not think people would be dying because of it.
> Since there is no evidence the problems you are bringing up are actually problems
I didn't bring up any problems. I just said that the things you were claiming were true are not things we actually know to be true.
>In the strict sense of the term, we the public will never know. The same people who built the plane will provide a fix, the same people who certified the plane will do so again, the same people who were flying them will do so again, and we will trust them or we won't.
This is precisely true. Which is why I responded to you by describing the bunch of things we don't know. Chances are this will be fully fixed and we will get a very nice detailed report on how that was done. The actual engineering tradeoff of more inherent stability versus more control is something we don't even know enough to conjecture well about. That's what I responded to in your comment.
Recent 737 models, including the MAX 8, make partial use of fly-by-wire systems, such as the MCAS that's elicited so many hot takes recently. Airbus aircraft, by comparison, make heavy use of fly-by-wire systems, and have for years. In terms of functionality, it's perhaps the single largest difference between their product line and Boeing's.
On the other hand, the widebody 777 is the first Boeing product to go fully fly-by-wire, completely separating the pilots' control inputs from the aircraft's control surfaces in the way that all modern Airbus aircraft do.
If fly-by-wire is the problem - if, as you seem to suspect, these systems exist to compensate for the inherent flaws of aircraft designs which otherwise would not be able to stay safely in the sky - then these Airbus models, and the 777, should have pretty rough safety records, shouldn't they? We should expect to see these other aircraft fail due to problems of the same sort as affected the 737 MAX 8's MCAS, right?
Luckily, we don't actually need to expect anything in this case, because we have good information on the frequency, severity, and causes of airliner accidents more or less all over the world. Indeed, that information is so good, and so available, that it's a major scandal, and a fecund ground for conspiracy theories, in the rare case where it's not available, as with Malaysia Airlines flight 370.
Thanks to that information, we know that the safest airliner on the planet, by flying hours, is the Airbus A340, which has not yet had a fatal accident. Second is the Boeing 777, which has seen only 541 fatalities in its almost quarter century of revenue service - and that's counting the 298 souls aboard MH 17, shot down five years ago over Ukraine.
Both of these aircraft are about 25 years old. Both have full fly-by-wire control systems. And both are, by any measure, among the safest commercial airliners in the world. If fly-by-wire were intended to cover up for the deficiencies in basic aerodynamic design you seem to lump into the term "instability", is this the safety record you'd expect to see?
I also am unconvinced computers can mediate an airplanes aerodynamic design in order to achieve certification under this part, because again by my reading the expectation is that we (pilots and the flying public) can expect static and dynamic stability dynamic stability, laterally and longitudinally. Can computer control improve these behaviors? Sure. Can computer control make them more linear or consistent or docile or predictable? All of those things. But is it acceptable to have negative static stability, naturally occurring by design, in transport category aircraft moderated by computer? I don't think that's allowed but then I'm not deep diving these regulations either. What happens if all computer control goes down? You can't fall back on the natural static and dynamic stability of the airplane, because it doesn't exhibit those characteristics? That seems bad and not consistent with FAR 25.
However, if any such computer safeguards can be disabled (either by the computer itself becoming confused and removing those safeguards; or by pilots disabling them explicitly), I expect the pilot to be trained as part of the type certification requirements, how the airplane behaves when the safeguards are enabled and disabled.
I'm quite sure Boeing knows the answer to these questions. I suspect the FAA knows the answer to these questions. And when you say "we don't know" I think you mean HNer's engaged in discussion, who maybe aren't even aware that quite a lot of things are mandatory for obtaining transport category aircraft airworthiness certification.
I mean that the general public doesn't know that indeed. And thanks for the references, I'll look them up. The original post I replied to was specifically saying that what you are now describing is not allowed (these types of planes being unstable) should be acceptable.