Also see @salawat above on nacelle lift-induced pitch instability. We now have airliners that begin to resemble the F-117 in their dependence on computers for controllability in flight.
What's problematic with 737 MAX is that the stability assist was obviously installed as an after thought, and even worse, it's dependent on instrumentation that does not have several layers of fault tolerance as would be the case if it was actually built with the care that planes actually require to be as dependable as they are.
So the instability is not a problem. That boeing tried to get around it with a cheap patch of jury rigged software that is depending on input from non-fault-safe instrumentation, is.
For military purposes, I agree. There is an end objective to fighter jet development. To own the sky at all costs.
With civilian aircraft, there is too much to lose. You can't have an unstable air frame for civilian commercial mass purposes.
https://duckduckgo.com/?q=maple+seed&t=ffab&iax=images&ia=im...
maple tree seeds resemble auto-rotation in a helicoptor, not powered flight.
It's a nice mental model as to why you won't fall out of the sky necessarily with a loss of power in a heli, though.
The reason you don't see it in nature is that macroscopic organisms never evolved a continuously rotating joint. It's a hard problem for evolution to solve; how would blood vessels cross that boundary?
But of course machines have no such constraints, and the wheel and axle is hardly a new (or scary) invention.
Is it possible we're talking about two different things? Making a circle with your arm isn't the same as rotating it, and just making a circle isn't particularly useful for flight or locomotion. Imagine attaching a wing to your arm and making a circle -- half the time isn't going to be attacking the wind the wrong way. This wouldn't work to, e.g., fly a helicopter. You need real rotation.
Swap out "instability" for "drab paint jobs" and see:
> I disagree that "drab paint jobs are not a problem". In military aircraft that have drab paint jobs it's common for everyone in the plane to be in an ejector seat or to have a parachute. Can you provide statistics on how many (or what proportion of) military planes with inherent drab paint jobs routinely carry passengers without parachutes?
In this case the argument is not that a is evidence of b, it is that a ameliorates b. And that the risk of b is therefore priced differently where a is also present. So where a is not present, we must price the risk differently.
How that assertion relates to the historical record may be of interest.
To blithely say "oh well, military planes do X so X must be fine for civilian aviation as well" is either intellectually dishonest or fundamentally naive.
E.g. higher redundancy of HW & SW, stricter testing/checks/validation of flight envelopes, more feedback to the pilots about why a computer is performing certain actions, etc... than military aircrafts using the same systems.
Do any modern military non-fighter jets have bail out capabilities? It was my understanding that you couldn't even put on a parachute in flight to jump out. You either had an ejection seat, or no capability at all.
Fehrm, a former jet-fighter pilot and an aeronautical engineer: https://www.seattletimes.com/business/boeing-aerospace/faa-e...
That said there are airliners out there which are unstable in the longitudinal axis and require a yaw damper to prevent Dutch roll.
My intention was not to claim 737 MAX had the same aerodynamic nature as, say, F-16, but in the scope of the discussion point out that fixing instabilities with flight software can be made to work reliably.
The main argument was that it was inherently a bad idea to require computer assist to maintain stability and decades of military planes demonstrate that you can engineers such systems to be more or less fault safe.
There was a sibling comment somewhere in this post that stated that airbus doesn't introduce a layer between the pilot and the aircraft. This couldn't be further from the truth (assuming normal control laws). Applying full back stick on an airbus will hold the aircraft right on the edge of a stall. Applying full left or right on the stick will neatly roll the aircraft to 60 degrees of bank. Releasing the stick at this point will roll the aircraft back to a perfect 30 banked turn in the same direction. This isn't normal behaviour by any stretch.
If anything I think Boeing are suffering from not going to fly by wire control earlier. That would have made this whole subsystem irrelevant. You'd just apply the desired behaviour in the response curves in the control software.
https://en.wikipedia.org/wiki/Accidents_and_incidents_involv...
Having an unrecoverable stall mode is not a desirable flight characteristic, for sure, but I don't think it's unheard of. For sure it's nothing like inherently unstable airframes as used in fighter jets.
My rather limited understanding is that the airframe certification path Boeing used was one meant for fast-tracked recertification of minor configuration changes of the same hull. The main idea being that the manufacturer need only prove that the reconfiguration meets the same or similar flight characteristic requirements as the hull it was derived from.
This lets them produce/sell a technologically refreshed product without having to pass the rigorous testing normally associated with type-certifying a new airframe. The important characteristic of this is that the onus for this certification is left to the manufacturer. I do not know if the FAA actually independently confirms the recert is justified, or just plays it via honor system.
Old 737 hulls definitely didn't have this characteristic onstability, or do-or-die reliance on functioning AoA sensors, so while I agree there isn't anything wrong with incorporating instability into a design to satisfy some specifications per se, I'd still contend that the addition of that instability should disqualify an airframe from being recertified as a reconfiguration of an older one, and should require more extensive testing/training before pilot's are cut loose on it.
If anyone else has more accurate understanding of type certification, I'd be thrilled to learn the specifics.