Re: Boeing and Dreamliner

[aek@vib.usr.pu.ru (Alexander Kopilovitch)]

Robert I. Eachus wrote:

>... but the best thing to do 
>is to actually read the technical report on what happened. (I found a 
>copy at http://www.dcs.ed.ac.uk/home/pxs/Book/ariane5rep.html)

I read this report yesterday, and now I'd like to make several comments.

1) The report is quite good - it mentions many significant facts, gives some
necessary explanations, points at true cause of failure - the absence of
testing, and concludes with right recommendations.

2) Yes, this report is "politically correct" (which seems quite natural),
therefore it requires specific skills for reading (I can testify that 20
years of experience reading main Soviet newspaper Pravda ("Truth") are very
helpful -;) , and at least vague preliminary view on the subject (which was
provided within this thread).

3) Besides the things you already decribed (in full accordance with the report),
I discovered in this report one very interesting detail, which attracted my
special attention. Let's go to the quotes. First yours:

>... it doesn't take a 
>genius to tell you what that ten second period of "pressure surges" in 
>the flight control hydraulics was.  But of course the politics of the 
>situation kept the fact that oscillations were building up in the stack 
>from being an obvious part of the official report--and from being left 
>out of it:
>
>"One anomaly which was brought to the particular attention of the Board 
>was the gradual development, starting at Ho + 22 seconds, of variations 
>in the hydraulic pressure of the actuators of the main engine nozzle. 
>These variations had a frequency of approximately 10 Hz.
>
>"There are some preliminary explanations as to the cause of these 
>variations, which are now under investigation.
>
>"After consideration, the Board has formed the opinion that this 
>anomaly, while significant, has no bearing on the failure of Ariane 501."
>
>Don't you just love that attempted slight of hand?

I might just love that slight of hand, but during rereading the report I saw
the following:

"These nozzle deflections were commanded by the On-Board Computer (OBC)
software on the basis of data transmitted by the active Inertial Reference
System (SRI 2). Part of these data at that time did not contain proper flight
data, but showed a diagnostic bit pattern of the computer of the SRI 2, which
was interpreted as flight data."

"The reason why the active SRI 2 did not send correct attitude data was that
the unit had declared a failure due to a software exception."

This "showed a diagnostic bit pattern of the computer of the SRI 2, which
was interpreted as flight data" striked me. See, there was not just wrong SRI
behaviour, there was communication protocol error! OBC misinterprets SRI data,
it takes diagnostic "postmortem dump" as normal flight data! Certainly I have
no clue which part - SRI or OBC is guilty, but anyway it is highly unlikely
that this protocol error was caused by previous errors related to the flight
parameters.

Now the report's passage about variations of hydraulic pressure does not seem
"slight of hand" any more (if I understand this expression properly).
First, there was protocol error, so the "bugs" weren't localized in SRI.
Second, possibly there were other inconsistensies in SRI software. and those
additional inconsistencies might influence that "diagnostic pattern". which
was transmitted to OBC.

Well, I think that the report writers couldn't go further. They said enough...
perhaps even more than enough. Yes, an unexperienced reader has very little
chance to acquire proper view on the subject from this report without some
external help. So, it may be good thing indeed to provide some sort of FAQ
for the subject (as was already proposed in this thread). Something like that:

Q. Was Ada language somehow related to Ariane 5 crash in 1996?

A. Yes, at least some components of the Ariane 5 software was written
in Ada language.

Q. Did that software cause the crash?

A. Yes and No. They simply put the software written for previous model --
Ariane 4 (where it worked well) -- to new Ariane 5, and did not bother
themselves with testing it on the new rocket before the launch. So, when
the Ariane 4 software appeared (in the flight) incompatible with new Ariane 5
they became very surprised -- and blamed the software.

etc. (with references to the report).



Alexander Kopilovitch                      aek@vib.usr.pu.ru
Saint-Petersburg
Russia