Hi. For some reason I haven’t been able to respond to any messages and the help desk hasn’t been able to, you know, help. So even though I may be an elitist snob I really haven’t been ignoring everyone who’s been messaging me about this.
Here’s what I know about this so far:
1. The 737 Max has a tendency to pitch up when power is applied due to the more powerful engines and the forward placement of those engines. The longer pylons give the engines a longer lever arm and thus aggravate the problem. This is common any time you have engines slung under the wing, but I guess it’s especially bad on the 737 MAX.
2. Once the plane starts to pitch up, the engine cowlings themselves will start to generate lift and cause the pitching moment to get even worse.
3. The MCAS system, as everyone now knows, was meant to be a software fix to this problem. We also now know it takes data from a single AOA (angle of attack) sensor and will run the stabilizer trim nose down to prevent a stall.
4. A malfunctioning AOA sensor can cause the MCAS to “think” there’s a stall and start running nose down trim.
In general, I don’t like the idea of having to use software to fix a problem with the basic design of the aircraft. However, that’s the world we live in.
Using a single AOA sensor for something that important seems, well I don’t want to use the word “criminal” and have a team of Boeing lawyers breathing down my neck, but it’s bad. In the airline world we like to have two or preferably three of anything important.
5. The stabilizer trim has more pitch authority than the elevators (pilot’s yoke). You can’t pull back on the yoke hard enough to beat the stabilizer.
6. For this reason, most heavy jets have trim cutout logic that prevents the stabilizer trim from running opposite the yoke. In the 767, if I’m pulling nose-up, the trim can’t run nose-down. The MCAS however will override this.
7. There is a procedure in the 737 QRH (Quick Reaction Handbook) for “uncommanded stabilizer trim” that will eventually tell you to turn off the stabilizer trim using the cutout switches. If that fails, pilots are directed to forcibly grab the trim wheel to stop it from moving.
So far so good.
However, this doesn’t appear to be a “Phase One” procedure that you have to memorize. Shutting off the stab trim is about halfway through the procedure.
Just imagine the pilots suddenly fighting a whole handful of airplane that’s trying to pitch nose down while going through a checklist. I like to think I’d figure it out on my own, but in the heat of battle you never know.
8. Boeing is reportedly working on a software fix that will reference both AOA sensors and also limit how many times the MCAS can override the pilot’s inputs.
I hope that helps.