transportbiz

I have no idea where to post this, but being neither an engineer nor a physicist, nor an aeronautics expert, I was trying to understand what Boeing is saying about the MAX.

They say they developed a piece of electronics wizardry to counter a physics problem, I.E. that the engines were mounted forward on the wings causing an upward push (which I don't understand either, as wouldn't this cause a downward push?) Anyways, that's the explanation as to why they created the anti-stall algorithm to the software.

I'm sure many here at one time or another have been told not to change seats, that the weights and balances have been entered into the computer as they are, and any movement could cause issues with those balances, when flights are light. Clearly this means nose to tail balances are important?

Dumbing this down: Does this basically mean Boeing tried to fix a physics problem with electronics? If so, isn't it inevitable every so often physics will win?

Curious

tjl

The change to the engine mounting affects aerodynamic characteristics, not just weight distribution. The aerodynamic characteristics may have affected the stall risk.

The change to the engine mounting was due to the original low ground clearance (which meant less stairs for passenger and baggage handlers to climb before there were jetways and baggage loading belts) making the new larger engines too close to the ground (more risk of scraping). Media reports indicate that the nose landing gear with lengthened slightly, but do not mention anything about whether lengthening the other landing gear was considered as a solution to this problem.

CPRich

The electronics provide an opposing force - which is physics. Auto pilot, yaw dampers - there are many, many electronic systems on an airplane that aim to use computers to provide counteractions to physical forces.

Physics will "win" only when the electronics fail or break down. Pilots typically can disconnect the electronics and just fly the plane manually - throttle, stick, pedals. I think the claim is that pilots weren't trained on this malfunction type, so didn't know how to react/what to disconnect.

Velocipediste

The engines are below the center line of the airplane, or more technically, below the center of gravity (CG). The may now also be farther ahead of the CG. The uncentered thrust tries to rotate the aircraft about the CG. It is like a car with a bad wheel alignment that wants to pull you into the ditch every time you stomp the throttle. To compensate, you need to steer crooked. The problem on the MAX is that the correct amount of needed crookedness varies depending on flight conditions.

Fighter aircraft are intentionally built unstable for more nimble handling. Computer control can compensate - if done correctly.

pinniped

So it sounds like the solution proposed yesterday (in super layman's terms is): (a) train everybody about MCAS, (b) fix these two sensors that appear to be faulty at an apparently alarming rate, (c) add some sort of indicator light in the cockpit, and (d) only let the MCAS do the nose-nudging once - after that, if the pilot pulls the nose up the software lets the pilot "win".

I'm still concerned about these sensors that were apparently off by 20 degrees even while the plane was sitting on the ground.

And I certainly hope we never have a MAX crash where the plane actually does stall. From what I understand, taking away the MCAS entirely is not a feasible option.

transportbiz

I knew a lot of people here would have educated ideas of all this. I only know a bit about cars, enough to know that when the issue is electronic, it's a bear to track down. OBDII really helped that, and soon I think we'll have OBDIII. But, the other thing that's not making me not feel too comfortable is how electronics can work fine in one moment, and suddenly not in the next, but then mechanical things can break at a moments notice as well...I guess is just the grace of god that keeps us safe. Thank you all...

MSPeconomist

LarryJ

AoA vanes can not read accurately on the ground as the airplane doesn't have an angle-of-attack when it isn't moving to measure. The vanes just blow around in the wind and, since the wind on opposite sides of the aircraft will be different, it would be normal for the two vanes to produce significantly different readings on the ground.

As you approach flying speed on takeoff, the relative wind (half of the AoA calculation) becomes greater than the surface wind and the AoA data becomes meaningful.

tjl

Regarding (b), wasn't the fix also supposed to have the MCAS pay attention to both AoA sensors, instead of just one?