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Old 15th Nov 2018, 03:17
  #1245 (permalink)  
Join Date: Sep 2018
Location: Laredo, TX
Posts: 121
Originally Posted by silverstrata View Post

What Boeing were trying to guard against with MCAS, is a repetition of the Sept 2007 pitch up event at Bournemouth, where a 737 pitched to 44 degrees nose up during a stall event. This happened because as the aircraft approached the stall the pilot gave full thrust to alleviate the stall, and because the engine pitch moment is greater than the elevators (when flying at slow speeds), the engines can overcome elevator authority and pitch the aircraft up uncontrollably.

To alleviate this nose up pitch, you need to either:.. a. reduce power, which is a counter-intuitive action during an incipient stall; or b. trim forward. The trim operates on the stab, not the elevator, and can give you a lot more control authority to oppose the engine pitch-up moment.

But here is the problem - the 737-max has much bigger engines, and a larger thrust-pitching moment. So at slow speed approaching the stall, giving too much recovery power may simply flip the aircraft over backwards. Which would really spoil your day. To (partially) mitigate this, you want an auto-forward stab-trim, to give the pilot more nose-down control authority at slow speeds.

Enter the new MCAS system.

However, MCAS can generate its own litany of problems and failure modes. If it gives full nose-down pitch during a stall, you still have that forward trim applied during the dive-recovery, which will hamper the recovery. (Which is why a stick-pusher combined with a thrust limiter might be a better option.). Likewise, if MCAS gets false information and operates at 250 kts, it is possible it will overpower the elevator and pitch you down. And if the auto-trim was not noticed, that trim will still be applied while you are trying to recover from a dive, hampering any recovery.

So in attempting to alleviate one problem, you have generated several others. MCAS looks like design firefighting, rather than a clean-sheet holistic design philosophy. And in making this fix, nobody went through all the possible failure scenarios - including a failure giving full forward trim at 250 kt.

I think MCAS needed to be much more sophisticated, than a simple stall alleviation device. In fact, it is likely that the 737 needs a complete rethink and revision of its control system, which was designed in 1963 for the 727. There is only so much design-firefighting you can do, with a 60 year old design.

So the MCAS is needed on all 737s starting with the 737-300?
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