Originally Posted by
FGD135
All of those sources would be media, seeking to explain the system to the layman as simply as possible. For the media, trying to explain in terms of pitch-down moments would be way too technical. You won't hear Boeing, the FAA, or anybody on the inside describing MCAS as being for stall prevention.
No. A system that pulls the nose down would be for stall prevention/recovery. If certification standards required that on the MAX, then Boeing would have simply added a stick pusher. The stick pusher is a vastly more simple and reliable system and has been in use on airliners for many decades now. Why would Boeing have adopted the new and unproven MCAS when they could have simply used a stick pusher?
Check how MCAS is described by insider sources. Here is one such source:
When you say "effecting", I think you mean "affecting". That first sentence is false. The second is true. For the first, if you had said, "influencing the pitch attitude", then it would have been true.
"5 degrees of nose-down movement" is an official statement by Boeing. It wasn't from the media. I think that clearly implies the system is designed to create nose-down movement.
Right that's better wording. MCAS
affects pitch attitude which can, in the scenario of avoiding a stall, effect the pitch attitude. Since it's changing the stab position it has incredible amount of pitch authority, so I think it isn't really that inaccurate to say "effects". Or MCAS has an effect on pitch attitude. It may not if the pilot uses the elevators to counteract the pitch down force. But that's technically adding another force outside the MCAS system itself. All other things being equal, if MCAS activates, the pitch attitude will change / the nose will drop (more so than if it wasn't active).
Put another way, your disagreement would be like saying the aerodynamic forces on the tail have no
effect on pitch attitude. They only have an
affect. I'm not sure that's more accurate but if it is it seems pedantic.
From the link you pasted (which I've read at length before): "as the nacelle is ahead of the CofG this causes a pitch-up effect which could in turn further increase the AoA and send the
aircraft closer towards the stall. MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching
stall."
I don't think it's inaccurate to think of MCAS as a system to avoid a possible stall. Effectively, pulling the nose down by changing stab position.
I would post a good video of an airliner pilot explaining it as essentially a stall prevention system but I can't because I don't have enough posts to paste links. His take is the engines change the stall characteristics of the plane such that under a stall condition the elevator forces may not be enough to bring the nose down, so an adjustment to the stab is required.
So nose down trim affects pitch attitude. Which can and certainly in the latest crashes
effect the pitch. By describing it the way you do you seem to imply MCAS has no pitch authority. Changing the position of the entire "tail wing" has a profound effect on the lift forces generated by the tail and the final pitch attitude of the plane.