[SystemSafety] "FAA chief '100% confident' of 737 MAX safety as flights to resume"

[Peter Bernard Ladkin]

On 2020-11-22 13:49 , Olwen Morgan wrote:
> 
> Aerodynamic instability, unstable slope ... schmunstable schlope. ... It's still instablity.
It is wise to use technical terminology correctly. Let me explain in this case why it is important. 
It is important because the same word is used in aeronautics for very different physical 
characteristics that have little to do with one another.

BTW, "Unstable slope" is a nonsensical phrase. Slope is more or less stable or it is not. When it is 
not, you say "not stable".

The point about aerodynamic stability, and why it must be assured on commercial aircraft, is that if 
you take your hands off the controls and shut off all the automatics, the aircraft will fly as 
trimmed. That is rightly held to be very important for safe civil flight.

Here on the MAX, you have a system which automatically cuts in and changes the trim. If you have a 
weird situation and can't figure it out, relax stick pressure, cut out the automatics, and ........ 
the aircraft can nose-dive into the ground. Because MCAS has re-trimmed it.

This has not changed. (It could be fixed aerodynamically by installing a slightly bigger HS, but 
that means more drag and thus higher fuel usage. The old "A vs. B" debate has gone on now for some 
30 years, and I suspect this will give it new life. It will in any case be moot if the passengers 
don't return, as they didn't with MD.)

The fact that control engineers use the term "stable slope" to mean that the AoA/elevator angle 
curve has a more or less steady slope and doesn't level off or attain a cusp is a perfectly 
reasonable use of the word "stable", as mathematicians might use it. And has per se little or 
nothing to do with aerodynamic stability.

A third different use of the term is in control system stability. That becomes important in aircraft 
which are aerodynamically unstable (so, military aircraft) and which need control systems to fly 
them (that is, to present their pilots with the customary sort of human-control options).

Similarly, that you are a fairly stable complainant about the MAX has nothing to do either with that 
aircraft's aerodynamic stability or with the slope of the AoA/elevator angle curve or with the 
stability of any control system.

PBL

Prof. Peter Bernard Ladkin, Bielefeld, Germany
ClaireTheWhiteRabbit RIP
Tel+msg +49 (0)521 880 7319  www.rvs-bi.de





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