Yes, there was a 400, USAir had them, not sure who else in the US( maybe Alaska?) but almost 400 were built. 500/600 were not stretches, they were in the same size range as the 200 100/200 (orig) 300/400/500 (classic) 600/700/800/900 (next gen) and now max very few 100 built before the 200 took over, very few 600 also
A total of 486 734s were delivered.
So the problems are sorted, which is great news. Does anyone actually know why it took so long?
Any idea what has been altered or is it just a new computer programme?
Here is the new Airworthiness Directive (AD) from the FAA. It describes everything that needs to be done before each aircraft and each crew is allowed to fly one of these. It is a PDF document.
https://www.faa.gov/foia/electronic_reading_room/boeing_reading_room/media/737_AD_2019-NM-035fr.pdf
Here is the blurb from FAA that went with the overall release of the modified setup:
https://www.faa.gov/news/updates/?newsId=93206
It took time first to figure out exactly what to change to reduce the risks to acceptable levels, and then to incorporate them and test them. Also there was some time involved in doing the Flight Simulator incorporating the actual expected behavior of the plane and make sufficient numbers available so that airlines could train their pilots for the MAX, something that was not required previously, but is required now.
One very major change is that MCAS does not have unlimited authority to override the pilot any more. It is allowed to intervene once and then take itself off line. So no more positive MCAS directed flights into terrain, which happened to be a feature before the fix, if the pilots failed their test to determine the exact cause of the problem and pull the select circuit breakers in a timely manner.
The other major change is that the MCAS now uses input from both attitude detectors and if they don;t agree, raises an alarm instead of blindly believing whatever its gets from a single detector.
So there has been changes in both hardware and software and in training and certification of crew requirements.
Speaking of using automation to enhance stability of aircraft, Yaw Dampers have been used in swept wing aircraft since the early days of the jet age since several of the jet aircraft, specially those with T-tails were unstable and susceptible to go into dutch rolls unless spontaneous yaw motion was countered in a timely manner. The aircraft were flyable without the yaw damper, but it was a dicey exercise even for extremely proficient pilots. But the problem was severe enough on T-tailed aircraft that they required a fail safe Yaw Damper system to operate commercially. So to claim that a pilot can do better than automation in all cases just shows a certain level of lack of knowledge on part of the one claiming so.