[SystemSafety] System safety: software vs. hardware includingaircraft hardware

[John Howard]

While I did learned a lot from this, I also have to agree that it seems to be misleading.

Example: “An airplane approaching an aerodynamic stall cannot, under any circumstances, have a tendency to go further into the stall. This is called “dynamic instability” and the only airplanes that exhibit that characteristic (fighter jets) are also fitted with ejection seats.”

With the prevalence of ‘by-wire’ flight control this statement seems to be an over simplification. Here is a bit of reading from Boeing that suggests the trend is to push the limits of dynamic stability to improve fuel efficiency. (using the 777 and MD-11 as examples)
https://www.boeing.com/commercial/aeromagazine/aero_02/textonly/fo01txt.html

(Also, I believe the fact that fighter jets have ejection seats has very little to do with their dynamic instability, and far more to do with the nature of their mission.)

-John Howard

From: Peter Bernard Ladkin
Sent: Monday, March 25, 2019 1:12 PM
To: systemsafety at lists.techfak.uni-bielefeld.de
Subject: Re: [SystemSafety] System safety: software vs. hardware includingaircraft hardware



On 2019-03-25 15:24 , E. Douglas Jensen wrote:
> For those who haven’t seen this, it is well put.
> 
> https://drive.google.com/file/d/1249KS8xtIDKb5SxgpeFI6AD-PSC6nFA5/view?fbclid=IwAR0XjHLmehzfLmgxTDVNrHsfy-4ycdEVWfN-rC4-l8FCTzQf-D9NH-AOs60
It is also, as far as I can tell, misleading.

I haven't heard any valid suggestion that this had anything to do with engine power and pitch-up on
power application. The software was intended to counter the phenomenon that, in a wind-up turn at
high AoA, the stick forces became lighter with increasing AoA (due apparently to the lift generated
by the nacelles). The airworthiness requirements say they should become heavier.

A puzzle which no one yet seems to have solved is that the elevator forces should be able to
overcome stabiliser trim at any setting, so why was the crew unable to arrest the rapid descent?

PBL

Prof. Peter Bernard Ladkin, Bielefeld, Germany
MoreInCommon
Je suis Charlie
Tel+msg +49 (0)521 880 7319  www.rvs-bi.de






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