Re: Boeing and Dreamliner

["Robert I. Eachus" <rieachus@attbi.com>]

Hyman Rosen wrote:

> Ada made the Ariane 5 crash!

No, stupid management decisions made Ariane 5 crash.  This is one of 
those stories where the truth really needs to catch up to the rumor. 
Some brilliant management type had the idea that reusing the flight 
control software from the Arianne 4 on Ariane 5 would save lots of money 
on testing and verification.  As a result, and for political reasons, 
there was no Ariane 5 contractor who even got to see the Ariane 4 source 
code.

And awful lot of analysis has been written looking at the chain of 
events that caused the Ariane 5 failure.  But what is usually missed 
happened way downstream of the unhandled exception.  The engines were 
commanded to deflect beyond what the stack could take, and the Arianne 5 
broke up.  How could THAT happen?  The reuse included the flight 
dynamics profile for the Arianne 4!  Since the Arianne 4 had smaller 
moments and a larger tolerance for guidance inputs, ANY significant 
correction sent to the engines, such as hitting wind shear, would have 
pushed the software into a regime where errors would build up. 
Eventually the commanded input would exceed the stack's structural 
limits and destroy everything.

There was some indication that such errors had occured during the 39 
seconds of flight, but all had been small enough errors to be damped out 
by other deviations.

Or to vastly simplify, the Arianne 5 would have been better off with no 
guidance system than with an Arianne 4 guidance system.  In that case, 
there would have been some small chance that it would actually perform 
as expected.  Or if the profile for the first flight had been slightly 
different so that the error that did occur would have been pushed from 
39+ seconds to 40, and thus safe, the launch would have failed anyway. 
There was no way the actual guidance system could achieve a working orbit.