Archives For Fault tolerance

Ariane 501 Launch

In 1996 the European Space Agency lost their brand new Ariane 5 launcher on it’s first flight. Here’s a recently updated annotated version of that report. I’d also note that the software that faulted was written using Ada a ‘strongly typed’ language, which does point to a few small problems with the use of such languages.

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New Battery boxes (Image source: Boeing)

The end of the matter…well almost

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No, not the alternative name for this blog. 🙂

I’ve just given the post Pitch ladders and unusual attitude a solid rewrite adding some new material and looking a little more deeply at some of the underlying safety myths.

JAL JA829J Fire (Image Source: Stephan Savoia AP Photo)

Boeing’s Dreamliner program runs into trouble with lithium ion batteries

Lithium batteries performance in providing lightweight, low volume power storage has made them a ubiquitous part of modern consumer life. And high power density also makes them attractive in applications, such as aerospace, where weight and space are at a premium. Unfortunately lithium batteries are also very unforgiving if operated outside their safe operating envelope and can fail in a spectacularly energetic fashion called a thermal runaway (1), as occurred in the recent JAL and ANA 787 incidents.

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Resilience and common cause considered in the wake of hurricane Sandy

One of the fairly obvious lessons from Hurricane Sandy is the vulnerability of underground infrastructure such as subways, road tunnels and below grade service equipment to flooding events.

The New York City subway system is 108 years old, but it has never faced a disaster as devastating as what we experienced last night”

NYC transport director Joseph Lhota

Yet despite the obviousness of the risk we still insist on placing such services and infrastructure below grade level. Considering actual rises in mean sea level, e.g a 1 foot increase at Battery Park NYC since 1900, and those projected to occur this century perhaps now is the time to recompute the likelihood and risk of storm surges overtopping defensive barriers.

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The following is an extract from Kevin Driscoll’s Murphy Was an Optimist presentation at SAFECOMP 2010. Here Kevin does the maths to show how a lack of exposure to failures over a small sample size of operating hours leads to a normalcy bias amongst designers and a rejection of proposed failure modes as ‘not credible’. The reason I find it of especial interest is that it gives, at least in part, an empirical argument to why designers find it difficult to anticipate the system accidents of Charles Perrow’s Normal Accident Theory. Kevin’s argument also supports John Downer’s (2010) concept of Epistemic accidents. John defines epistemic accidents as those that occur because of an erroneous technological assumption, even though there were good reasons to hold that assumption before the accident. Kevin’s argument illustrates that engineers as technological actors must make decisions in which their knowledge is inherently limited and so their design choices will exhibit bounded rationality.

In effect the higher the dependability of a system the greater the mismatch between designer experience and system operational hours and therefore the tighter the bounds on the rationality of design choices and their underpinning assumptions. The tighter the bounds the greater the effect of congnitive biases will have, e.g. such as falling prey to the Normalcy Bias. Of course there are other reasons for such bounded rationality, see Logic, Mathematics and Science are Not Enough for a discussion of these.

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This post is part of the Airbus aircraft family and system safety thread.

I’m currently reading Richard de Crespigny’s book on flight QF 32. In he writes that he felt at one point that he was being over whelmed by the number and complexity of ECAM messages. At that moment he recalled remembering a quote from Gene Kranz, NASA’s flight director, of Apollo 13 fame, “Hold it Gentlemen, Hold it! I don’t care about what went wrong. I need to know what is still working on that space craft.”.

The crew of QF32 are not alone in experiencing the overwhelming flood of data that a modern control system can produce in a crisis situation. Their experience is similar to that of the operators of the Three Mile island nuclear plant who faced a daunting 100+ near simultaneous alarms, or more recently the experiences of QF 72.

The take home point for designers is that, if you’ve carefully constructed a fault monitoring and management system you also need to consider the situation where the damage to the system is so severe that the needs of the operator invert and they need to know ‘what they’ve still got’, rather that what they don’t have.

The term ‘never give up design strategy’ is bandied around in the fault tolerance community, the above lesson should form at least a part of any such strategy.