Originally Posted by
silverstrata
Actually, this 'talk' is perfectly correct. When the MCAS system is activated erroneously, the pilot will inevitably make a trim input. And if your trim input is insufficient to counter the MCAS input, as was the case on the Indonesian, you rapidly end up with full nose down trim.
And I have to ask again, because nobody has answered - why would anyone invent a system that gives full nose-down trim? It goes against all aviation history and procedures. Remember the old adage - NEVER FLY WITH THE TRIMMER. And yet here is Boeing effectively using the trimmer as a primary flight control - deliberately placing the aircraft out of trim, instead of keeping it in trim.
That is why all other aircraft have stick-pushers, so you fly the stall with the primary flight controls, not the trimmer. The difference? Assisting the stall with the trimmer leaves you out of trim for the subsequent dive - with 60 kg of pressure on the stick, and 60 turns of the trim before you get back to something that is flyable. Conversely, when a stick pusher relents, you are still in trim, and can recover and fly away with ease. So why did Boeing chose such an absurd system for the 737-Max? Are Boeing really penny-pinching to such a degree? They should have taken a leaf out of the British Midland book - quote: "if you think safety is expensive, try having an accident..."
Silver
Silver - Your comments above are all worthy of review and consideration. The 737 is not a fly-by-wire airplane and thus does not have an augmented elevator command when flying manually. Earlier models (pre-MAX) used electric stabilizer control to address speed stability (STS). This approach was expanded to address otherwise uncertifiable 737MAX Cm-alpha characteristics by driving the stabilizer through the MCAS function. MCAS underwent extensive analysis and testing prior to certification. An assumption that was made throughout was that if the crew did apply manual trim, they would not stop their manual trim inputs for 5 seconds or longer without having offloaded column forces to (or near) zero. As mentioned via earlier posts in this thread, modifications to MCAS have been developed and will be out to the fleet this spring to improve MCAS safety. One of the key factors is removing the assumption that the crew, having activated manual trim, will provide complete manual trim to zero column force.