FTA: "MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation."
FTA: "As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics"."
Here's FAR §25.203(a): "(a) It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls." For anyone that's interested, here's the link for FAR Part 25. Para §25.173, §25.175 and §25.203 are well worth reading.
Here's my interpretation...if anyone disagrees, please feel free to comment.
The 737 Max:
1. Is not longitudinally unstable.
2. Engine nacelles produce lift forward the CG at High AOA. This lift causes a pitch up effect as the aircraft approaches stall due to reducing stick force (in the fighter world, we called this stick force lightening).
3. Would not receive Airworthiness Certification without MCAS as required by FAR §25.203(a)
My conclusion is that using MCAS to correct the stall pitch up effect is perfectly acceptable. However, the original MCAS design is faulty.
Also, Boeing's proposed fix is a bandaid. It disables MCAS whenever AOA gages disagree. So, the FAA would not certify the Max without MCAS and yet Boeing's fix will disable MCAS under certain conditions.
IMHO, the proper fix would include a third AOA sensor so the flight control computer could determine a faulty sensor and lock it out. MCAS always stays on.
Comments please...
If one relied only on the popular press you would miss a great deal of the subtlety involved.
Longitudinal stability is not a binary thing. There's a continuum from stable to unstable plus an interaction with maneuverability which effects certification.
AFAIK, all swept wing transport category aircraft require some sort of artificial control augmentation in order to comply with FAR §25.203(a) when operated at high altitude/high velocity conditions.
This is compounded by the financial incentive to move CG aft which saves money, but decreases stability.
IMHO, that sentence nails the situation perfectly.
Boeing stated that it didn't highlight MCAS to pilots because it was supposed to be transparent to them, and that the conditions that MCAS would activate would be extremely rare in normal operation.
What Boeing failed to do is to activate cross checking of both AoA sensors on aircraft that did not have the Angle Of Attack display option on the Flight Displays. I read somewhere, (and I don't have the link and don't feel like finding it) that Boeing's engineers were supposed to keep the cross check in the software for all versions, but it inadvertently was dropped on non AoA display versions of the software. THAT is an admission of liability.
For those who fear that Boeing will never recover from this, Boeing has already finished updates on the software, flight tested it, showed it to their major airline customers, and submitted it to the FAA and EASA. They are just waiting certification to resume operation of the MAX.
Boeing estimates that they will have that certification by the end of July or early August.
I see the problem of designing a safe easy to operate aircraft as adding expensive designs-a problem when you are in competition to sell your product, and have competition.(Airbus)
It is competition, not for the safest easiest aircraft regardless of price, but for the least expensive to purchase, more fuel efficient aircraft, that can pass Governmental requirements.
That is understandable since Airlines are in business to make money. Fuel efficiency and cost , and maintenance of aircraft is a big factor.
Looks to me like the Govt. will have to pay more attention to the safety aspects of commercial aircraft that undergo alterations.
It appears to me flying is safe but if you alter a design of an existing aircraft the design should get a very cloes inspection.
It is the old story of "Today's solution is tomorrow's problem - Tom