Originally Posted by
BDAttitude
So I will not call it "unstable" but "smaller than required or negative static margin hence not stable enough to be certifyable".
Thanks for the link, a lot of good information there. However, this document does not specifically address the 737. It does reference flight control augmentation on the MD-11 and 777, but I believe those aircraft actively manage c.g. for cruise performance and hence reduces the longitudinal stability.
The text you need to focus on is this:
The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots. As the CG moves aft, it reaches a point where the stick force per knot drops to zero, then reverses.
The reported performance of the 737MAX approaching a stall is one of stick forces
decreasing, but never stick forces
reversing. Thus we can say that the longitudinal stability decreases at high AOA, but it does not go negative. MCAS does improve the longitudinal stability at high AOA, but the 737MAX still demonstrates positive static stability in this area of the envelope.
There's probably another important point worth mentioning here. No one is claiming that the MAX doesn't need MCAS to meet the stability requirements of the FAR's. Well guess what? The 737NG doesn't meet the stability requirement without its Speed Trim System. I also suspect that just about every swept wing commercial transport doesn't meet the stability requirements without a yaw damper. Having a stability augmentation system (SAS) is nothing new in commercial aircraft - they've literally been in use for decades. It is worth noting, however, that the FAR's also place requirements on these same aircraft in case of failure of a SAS, and those requirements would basically preclude negative stability characteristics in absence of the SAS.