[VBprogrammer]

> Why is MCAS needed? If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? The most relevant question is “is the MAX aerodynamically stable?” If it is, why do we need the MCAS? That needs to be investigated.

This isn't even a question. The 737 Max is completely aerodynamically stable. It does however exhibit control behaviours which are undesirable.

A fair analogy for this is probably a car which oversteers, in general a normal family car is designed to understeer, because for your average driver that is safer. In the 737 Max the controls get lighter close to the stall because of lift generated by the engine nacelle. The certification requirements require that the controls don't get lighter. Boeing applied what now appears to be a poorly thought out fix.

What might surprise you is that there are plenty of certified aircraft which are actually aerodynamically unstable, at least along the longitudinal axis. For example the 757 has dual yaw dampers and at least one of them needs to be serviceable before flight. The consensus is that at cruise altitude it would depart controlled flight without one of them working.

[mikejb]

I think a fairer car comparison would be this: a new power train is fitted to the car, but it sometimes causes torque steer to the right. To overcome that, the steering wheel gets trimmed to the left when torque steer is detected until torque steer is neutralized.

The system detecting the torque steer sometimes has false positive.

[ajnin]

> The certification requirements require

Eh, let's not spin it as "the requirements made them do it", they chose to make the new model stick with the existing 737 certification because building a substantially different new plane would require pilots to be retrained, and they didn't want that as Airbus was ahead of them in the development of the A320neo and they needed that commercial advantage to remain competitive.

[VBprogrammer]

I'm not sure where you draw the line though? None of this is new, making modifications to existing designs is the aviation equivalent of developing a new feature. Hanging new engines from an existing fuselage, replacing the avionics and lengthening a fuselage have all been done before with reasonable results. Conversely, clean sheet designs have had terrible safety records initially.

Asking a manufacturer to make a clean sheet design every time they make a change is probably going to result in more accidents than it fixes (see the bathtub curve).

[dirkg]

> The certification requirements

The 737 is self certified by Boeing, which is really a joke. Basically a Boeing engineer can sign off and say LGTM with no oversight or independent audit as I understand.