FCeng84FCeng84 , 12th Mar 2019 22:26
Quote:
Originally Posted by
hans brinker
You would be making things worse by adding power. To get out of a high AOA, you lower the nose before adding power.....
Hans - your patience is amazing. You are correct about the relationship between 737MAX engines and need for MCAS. It has nothing to do with the thrust pitching moment!!!
There is a cert requirement that as AOA increases, the nose up pilot command required must not decrease. This is demonstrated at fixed thrust levels so there is no change in thrust pitching moment. The 737MAX issue here that gives rise to the need for MCAS is that as AOA increases the lift provided by the engine cowling that is so large and mounted so far forward of the wing causes a nose up pitching moment that results is a decrease in the column pull needed to maintain a steady positive AOA rate. That characteristic is not compliant with the requirements. MCAS comes active during this maneuver putting in nose down stabilizer that must be countered by the column. The net effect of engine cowling lift and MCAS nose down stabilizer as AOA increases is that the column needed to complete the maneuver does not decrease part way through the range of AOA for which characteristics must be demonstrated. 737MAX without MCAS fails the cert demo. 737MAX with MCAS passes the cert demo.
I know there's little evidence at this stage for this tragedy, but to elaborate and hypothesise about MCAS function:
1. MCAS activated by errant AoA vane data. (This same data may also lead to warnings such as stall etc.)
2. So, by the time you run through checklist, source the problem and stop the MCAS from further inputs by flicking the Stab trim cutout, the stab is already at X° nose down.
3. At relatively low speed you're able to manage this by pulling back on the yoke, and the now fixed stab angle may even go unnoticed and forgotten about for a while
4. Additional engine power may already have been applied, but, if not, you do so now as you need some height, especially with advancing terrain and the loss of altitude that you'd suffered.
5. The effects of the engine cowling on aerodynamics, as stated in the comment above, helps lift, as does the thrust moment created by the engines
6. You think you have the problem somewhat under control compared to the situation you were in a minute ago. You've now been able to climb and seem to have relatively stable manual control.
7. Now you're at Y feet, (still with X° nose down stab trim) with an increased airspeed of Z, up until now has been controllable due to your elevator inputs being assisted by thrust moment and engine aerodynamics, but, at this new increased airspeed and increased altitude the yoke is becoming even more difficult to keep holding back. The stab is still in the same X° nose down it was when you switched the cut-outs, and up until now you mightn't have thought about it because you'd disabled it- in accordance with the checklist.
8. You think about returning and getting this back on the ground
9. Once you level off, or even before then, with that stab STILL at X° nose down (now with a much higher airspeed component) there's only one place you're going. Once this vertical direction change has momentum there's now no chance, no elevators in the world are going to help you.
Look at the memory item for stab runaway- i.e.- not told to manually wind back trim wheel, the instruction is to "grab and hold".
All this could be caused by one errant sensor? Madness that it was certified.