[SystemSafety] Another Bloomberg article

[Peter Bishop]

Quote from Bloomberg article:  "Delays in Boeing Max return began with
near crash in simulator"

https://www.bloomberg.com/news/articles/2019-11-08/delays-in-boeing-max-return-began-with-near-crash-in-simulator?fbclid=IwAR1R6_y2DtwhHi5wq9kaOCRLMEcwE2cxIeAu_-aivpR2k_swaZZxYTQr5ok

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By contrast, the 737 Max had two separate computers. One operated the
flight systems and another was available if the first one failed, with
the roles switching on each flight. But they interacted only minimally.

Boeing decided to make the two systems monitor each other so that each
computer can halt an erroneous action by the other. This change is an
important modernization that brings the plane more in line with the
latest safety technology but raised highly complex software and hardware
issues..

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I don't think this change is a good idea at all.

Who trusts whom when two computers think each other is wrong?
How do you address the fact that each computer is asynchronous with
slightly different sensor values and timing which might cause the two
computers to diverge at decision thresholds?

Either:

1) Change the hardware to improve fault detection within each computer
(i.e. it fails silent if it gets scrambled) e.g. each computer is
upgraded to a lockstep processor pair with hardware cross-comparison.

2) Change the system to a TMR design so failure of a single processor
can be tolerated.

The first option would be the easiest from a software perspective.

Peter Bishop

-- 

Peter Bishop
Chief Scientist
Adelard LLP
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