Re: Boeing 737 and 737 MAX software

[tranngocduong@gmail.com]

On Thursday, April 18, 2019 at 8:50:27 PM UTC+7, Simon Wright wrote:
> 
> > In the crashed Ethiopian 302 aircraft, ~30 seconds before impact, the
> > nose points 40 deg. toward ground, and the AoA indicates 75 deg. As 40
> > + 75 = 115 > 90 deg., this would imply negative airspeed.
> 
> I think that if it implied anything it would be that the speed over the
> ground was negative and the aircraft was descending extremely
> rapidly. Neither of which is impossible.

The 115 deg. is the [elevation angle of] airspeed (i.e. speed of the aircraft relative to the air), not groundspeed (ie. relative to the ground). That's because the AoA is the angle between the aircraft and the airspeed. 

It is true that even airspeed can be negative. Negative airspeed is not exceptional for acrobatic and fighter aircraft. For the 737 however, that would be an exceptionally unusual situation. 

Unusual enough to believe that a sensor, most likely the AoA sensor, failed. Enough to disable all AoA sensor-based flight control algorithms, such as the MCAS. Enough to trigger an alternative algorithm, raise an alert, or completely disengage the flight control computer.

> 
> But, as Maciej hs been saying, what makes you think that this
> caluclation was ever performed?

I don't think so. I think that the software must perform many redundant calculations similar to this one. Sensors and flight computers are redundant, just like hydraulic systems and control surfaces. This calculation demonstrates that redundant calculation can help detect possible sensor failure even without a second AoA sensor.