Problem is - this is exactly describing ugly stall characteristics.
The way things work, arm controls force rather than position. Did you ever try non-spring loaded joysticks? Nightmare...
So, the way trouble goes is: pilot pulls stick to what would be 10 deg AoA from 8 deg AoA, force goes down; arm moves further to 12 deg AoA, force goes down, arm moves to 15 - and plane stalls before pilot realizes what goes wrong. You need to train some very counterintuitve muscle memory to avoid that.
I have flown an 172 in Turbulence where the controls go negative - it's not that hard to counter and all training tells you to not rely on feel - rely on your instruments and outside visual reference.
Where your controls went negative was not a function of the plane, but a function of atmospheric conditions.
I wonder how your plane would have felt if it ALSO had controls going light by design (which is just a different way of saying there isn't enough stability margin) ON TOP of the atmospheric conditions you experienced.
It may not have been hard to counter, but that is by design of the plane. The 172 is a very stable and predictable platform (the jury is still out for that on the unaugmented MAX).
You may have felt is was not that hard to counter, but that was on a very stable and forgiving platform. If that part of the design is insufficient, you may find that to be very hard and unsuitable for airline ops. I suspect that may be an important reason why this FAR exists today.
The 172 has very clear stall identification and post-stall behaviour. It is really hard to hold the nose up once the wing stalls. Also it does not have an issue with control lightening on approach to stall as it isn't a swept wing aircraft with a sophisticated aerofoil. The fact that the 737MAX does have these issues is not, in and of itself, a problem. They are present on every other commercial transport being delivered today. The issue is can you create an augmentation system that alleviates these issues and does not present catastrophic new issues?
The fact is any aircraft that uses stab trim to help with stick force and stall identification can present a case where the stabiliser trims nose-down in an inappropriate condition. Just like any aircraft that forces elevator nose down to prevent a stall can do the same. These systems have been and are currently acceptable means of compiling with the letter and intent of the regulations. The questions that need to be answered for 737MAX, and may have been answered already in some cases.
- Is there are point in the flight envelope where the pilot would have to push the stick forward to counter an AoA upset from a trimmed condition? – Basic static stability
- Does the trim rate of the stabiliser allow for appropriate stick force gradients through the flight envelope?
- Is stall, or approach to stall, clearly identifiable by the pilot?
- If the augmentation system fails what are the failure modes and what can those lead to if handled improperly? A fail passive mode that allows stick force lightening or makes stall identification harder is very different than a fail active that noses the aircraft into the ground
- If the system fails active can other systems and/or the flight crew take appropriate action to counter said failure, and can they take such action fast enough to prevent catastrophic outcomes
The fact is that 737NG STS is a simplex system with a similar failure rate of the primary sensor to that of MCAS. Boeing assumes that is fails 1E-5 per flight hour. This was deemed fine by the regulator (it was done at the behest of the JAA), and has worked acceptably in practice. The reason for this is that the failure modes don't lead to catastrophic outcomes at a rate that is unacceptable. MCAS, in the stall identification mode, breaks that condition. The combination of increased authority, plus different presentation of failure leads to catastrophic outcomes far too often. The solution to this is:
- Reduce the fail active rate, if you can move it from 1E-5 to 1E-9 you are good from a purely regulatory point of view
- Increase the ability of flight crews to counter the failure mode
This is what Boeing is proposing to do with MCAS 2.0. They will then need to ask another question.
- As MCAS will now be a duplex system will the increase in system unavailability lead to other unacceptable conditions happening too often?
If the answer to that is no, then you should be fine, if the answer is yes, then some form of triplex or psuedo-triplex system would be required
It seems that Boeing has finally acknowledged that simulator training will be required for the NG to MAX conversion. There are a number of reasons why this is likely to be the case. However, they will most likely centre around the behaviour on MCAS failure
- Since the MCAS system is now likely to be unavailable 2E-5 per flight hour, the reactions of the pilots need to be considered to ensure that a fail passive does not enter the Major or Hazardous realm
- Pilots reactions to the fail-active condition need to be improved
The potentially good thing for Boeing is that the failure modes are unique to MAX and in addition to the ones shared with the NG, eg both still have the same speed-trim runaway. Further, the actions countering a failure on the MAX would also counter similar conditions on an NG. This makes the training simpler as you can actually just work it into the reoccurring training regime for NG pilots prior to MAX delivery.
PS I still think Boeing would be well served to change the stab trim cutout switches back to one controls auto-trim and the other manual electronic trim. This allows the flight crew to use electric trim in many of the failure modes above, reducing workload.