But, I'm not sure if the concept of "full manual" is anymore relevant with modern passenger jets. Are they even flyable without any computer intervention or too unstable / have too complex flying charasteristics?
Commercial airliners are stable when flying. The advantage of an unstable aircraft is high maneuverability which you don't need in a commercial airliner. Also in full fly by wire aircraft there is a "red button" that will drop the flight controls into a fail safe mode. This mode usually has much simpler control law, and disables other functionality like auto land/auto pilot/tail strike prevention.
In a non-fly by wire air craft though like the 737 max, a full manual mode is basically when you have auto pilot disengaged. There might be a mode you can enable to disable other functions like tail strike prevention/MCAS but I a haven't ever heard of it.
[0] explains that fuel efficiency considerations prompted Boeing to push the altered 737's larger new engines well forward compared to prior iterations, causing the airplane to pitch nose up in some maneuvers in unstable fashion. The MCAS system was introduced to compensate, and is the component that failed in both crashes.
Basically all planes have flight profiles where they are stable and unstable. Just because a plane is unstable in certain conditions, does not make it an unstable plane. A stable plane is a plane that is aerodynamically stable for most of its flight profile.
The 737 MAX is just not stable in every condition that the old 737 NG was. And it probably goes the other way, there are most likely flight conditions where the 737 MAX is stable but the 737 NG is not.
The difference is that pilots had been trained and have flown the old one for many years and trained to avoid the unstable conditions for the old 737.
*Flight conditions meaning: air speed, AOA, bank angle, total thrust, altitude.
> The difference is that pilots had been trained and have flown the old one for many years and trained to avoid the unstable conditions for the old 737.
I don't think that's the case. MCAS was added to satisfy a requirement in the airworthiness certifications that requires positive control forces to increase AoA. The lift generated by the engine nacelles caused this to not be the case at high AoA.
None of what has been revealed so far indicates that the pilots had issues related to the symptom, rather they had issues with MCAS. In a way the cure has been worse than the disease.
> Also in full fly by wire aircraft there is a "red button"
There is no red button as such (at least as far as Airbus is concerned). The computer decides that something has gone wrong and invokes an alternative law. You'd have to start pulling circuit breakers to manually trigger a reversion.
> Commercial airliners are stable when flying.
This is not necessarily true. All modern airliners have aggressively swept wings. This generally makes them susceptible to dutch roll in some phases of flight. They are fitted with yaw dampers to control this tendency.
Learn something new every day. Looks like Boeing has a switch the pilots can select for direct mode. But for an Airbus airplane you would have to pull the circuit breakers for the Flight Computers to drop to Direct Law.
The 737 MAX is not. That's why it has the MCAS system in the first place: the engines are too large for the airframe, making the aircraft fundamentally unstable, so they came up with MCAS to try to make it seem stable to the pilots, and then they didn't even bother telling the pilots that this MCAS system even existed.
Ignore what you have read about the new engine making the MAX unstable. It’s more nuanced: It’s unstable near stall.
Commercial aircraft are not fighters. They don’t need to be unstable with computers making them flyable. An unstable airliner would be an uncomfortable airliner and hence a commercial disaster.
What has changed is whether or not there is a manual connection between the yoke and the control surfaces. For reasons of cost, that connection has been going away.
But, I'm not sure if the concept of "full manual" is anymore relevant with modern passenger jets. Are they even flyable without any computer intervention or too unstable / have too complex flying charasteristics?