Boeing knew about problems with the 737 Max the year before Lion Air crash and did nothing about...

2 out of 3 is one common way that fault-tolerant digital systems determine what is intended in the case of a fault.

I guess there is one other factor that needs to be covered, which would be wind attack at some angle that is not parallel to the ground. Or can these other factors be used to figure it out (i.e. the airplane is acting as its own wind vane with known characteristics)?

And in turn the MAX engine configuration made MCAS necessary. If the pilot just “puts the hammer down” (I heard a pilot say that once) willy nilly, it will cause the plane to pitch its nose up because of the leverage of the jet exhaust with respect to the midline of the plane. That could cause the plane to stall.

Mentour Pilot makes recovery look really easy — allow the plane to descend while taking care of the airspeed problem — but the pilot has to know exactly what to do. And one thing not to do is to hit the throttle hard.

The only way to avoid this danger totally is to scrap the MAX.

The only way to avoid this danger totally is to scrap the MAX.

No, Boeing is not going to scrap the MAX, nor should they. You are being overly dramatic.

The flight envelope where MCAS is used to adjust horizontal stab trim is a very narrow one that the pilots will rarely, if ever actually achieve. The MAX is not an inherently unstable aircraft, and MCAS was not conceived in order to make the plane stable. It was conceived to make the MAX handle just like the NG and Classic during certain high AOA manual flight, in order to keep one type rating for the pilots. Without MCAS the aircraft handles differently, but not unsafely, than the NG or Classic.

The problem has been when MCAS was given erroneous AOA information and acted inappropriately. This too is mitigatable if the pilots follow an already taught emergency procedure for runaway stab trim. The Ethiopian pilots were warned by Boeing after the Lion Air crash to be familiar with the runaway stab procedure. The Ethiopian pilots initially followed that procedure, then deviated from it.

The software update to cross check both AOA sensors and disable MCAS if they disagree has already been written, flight tested, demonstrated to the airlines, and submitted to the FAA and EASA for certification.

The MAX will be back in the air by August.

How much deviation or disagreement between AOA sensors will be allowed? 1%? More?

How much deviation or disagreement between AOA sensors will be allowed? 1%? More?

5.5 degrees.

Here's a brief explanation of what was wrong and what has been done to correct it:

http://www.b737.org.uk/mcas.htm#fix

The Proposed Fix

Boeing have been working on a software modification to MCAS since the Lion Air accident. Unfortunately although originally due for release in January it was not released due to both engineering challenges and differences of opinion among some federal and company safety experts over how extensive the changes should be.

Note that as MCAS is an FCC function, the modifications to MCAS are made in the FCC software. The revision will be known as FCC P12.1

There are three significant changes to MCAS software being worked on by Boeing:

(1)To give the system input from both angle-of-attack sensors, Currently MCAS only uses data from the angle of attack sensor on the side of the active FCC, (see AoA source). The system will have split vane monitor and Mid Value Select (MVS) input. This will both enhance detection of erroneous AoA vane behaviour and the MVS signal selection will pick the average of ADIRU L & R and the previous MVS output. If the output of the two AoA vanes differ by more than 5.5 degrees MCAS will be disabled.
(2) To limit how much MCAS can move the horizontal stab to guarantee sufficient handling capability using elevator alone. In its original report, Boeing said that MCAS could move the horizontal stabilizer a maximum of 0.6 degrees. However, after the Lion Air crash, it told airlines that MCAS could actually move it 2.5 degrees, or half the physical maximum. Boeing reportedly increased the limit because flight tests showed that a more powerful movement was needed at high AoA rather than at high Mach.
(3) A modification to the activation and resynchronisation schedule. MCAS will be limited to operate only for one cycle per high AoA event, rather than multiple. At present it will operate for 10s, pause for 5s and repeat for as often as it senses the high AoA condition is present. Furthermore the logic for MCAS to command a nose up stab trim to return to trim following pilot electric trim intervention or exceeding the forward column cutout switch, will also now be improved.

Furthermore there is expected be:

* Changes to the on board displays. This includes making the AoA Disagree Alert standard on all aircraft and offering a free option to have the AoA display on the PFD;
* FCOM explanation of MCAS and an updated Speed Trim Fail Non-Normal Checklist and revised QRH.
* Type conversion training to cover speed trim and MCAS, existing crew procedures and related software changes.
* Enhancements to the maintenance documentation, including specifically to the Airplane Maintenance Manual (AMM) and Interactive Fault Isolation Manual (iFIM)

On 16 Apr 2019 the MCAS flight test program concluded after 120 flights / 203 hours flight test time. The Boeing CEO, Dennis Muilenburg flew on a 737 Max demonstration flight, where he “saw first-hand this software in its final form, operating as designed across a range of flight conditions”.

Thanks. Those seem to be some huge changes.

1. Instead of moving the stabilizer for 10 seconds / pause 5 seconds until the stab runs out of travel they are limiting it to ONE correction only for a high AOA event.

2. “The system will have split vane monitor and Mid Value Select (MVS) input. This will both enhance detection of erroneous AoA vane behaviour and the MVS signal selection will pick the average of ADIRU L & R and the previous MVS output.” This sounds like they are averaging the two AOA vane inputs and then averaging that with the ADIRU L & R averages. Is that right?

It’s surprising that they depended only on one mechanical exterior AOA vane for MCAS input in the first place.

Well, losing an AoA sensor would then change the handling of the MAX.

...which would be wind attack at some angle that is not parallel to the ground.

It would not matter whether the aircraft is parallel with the ground or not.

There is one more parameter that is necessary...Aircraft Gross Weight. I am not sure if Aircraft CG would be required. I believe that CG would not impact the calculations significantly.

So,after the aircraft is loaded, weight & balance information is calculated by the load crew and given to the pilots to calculate Takeoff & Landing Data. This info, along with fuel weight, which is shown on the fuel gauges, would then be used to calculate AoA.

The 3 most important parameters are Airspeed (IAS), Aircraft Gross Weight, and G.

I believe this could be achieved by developing look-up tables using flight test data.