Screwtape(Image source: end time info)

A short (and possibly evil) treatise on SILs from our guest blogger

May I introduce myself? The name’s Screwtape, some of you might have heard of me from that short and nasty book by C.S. Lewis. All lies of course, and I would know, about lies that is… baboom tish! Anyway the world has moved on and I’m sure that you’d be completely unsurprised to hear that I’ve branched out into software consulting now. I do find the software industry one that is oh so over-ripe for the plucking of immortal souls, ah but I digress. Your good host has asked me here today to render a few words on the question of risk based safety integrity levels and how to turn such pesky ideals, akin in many ways to those other notions of christian virtue, to your own ends. Continue Reading…

AirAsia QZ8501 CVR (Image source - AP Photo-Achmad Ibrahim)

Stall warning and Alternate law

According to an investigator from Indonesia’s National Transportation Safety Committee (NTSC) several alarms, including the stall warning, could be going off on the Cockpit Voice Recorder’s tape.

Now why is that so significant?

Because one should not hear a stall warning when the aircraft’s so called Normal protection laws are running and preventing the aircraft from being flown outside the flight envelope*. The most likely explanation for hearing the stall warning is that the aircraft had entered what’s called Alternate law and (obviously) was entering or in a stall. For those that don’t know much about the Airbus, Alternate law is a fallback mode that kicks in after the loss of onboard equipment. In alternate the aircraft’s normal ‘alpha protection’ laws are removed (along with others) and it becomes physically possible for a pilot (or the environment**) to stall the aircraft, hence the presence of a stall warning function in Alternate law.

While we need to wait for the investigators to complete their full investigation, it now looks increasingly likely that the pilots had lost a significant amount of the protection offered by Airbus automation, right at the moment when they needed it the most. The parallel of this disaster with the loss of Air France AF447 are disturbing to say the least.

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

Notes

*An alternative explanation to the stall is that the external environment was so extreme that the stall warning was triggered even while the aircraft was in Normal law, however as far as I am aware ‘stall warning’ is a mode dependent function.

**Plenty of aircraft have entered and been flown safely in Alternate law (for various reason) so just entering Alternate Law is not necessarily a problem. However when the circumstance require the crew to respond to abnormal situations without relying on the normal protections provided, it may prove to be a hazardous regime to operate in.

Aviation is in itself not inherently dangerous. But to an even greater degree than the sea, it is terribly unforgiving of any carelessness, incapacity or neglect.

 

Captain A. G. Lamplugh, British Aviation Insurance Group, London, 1930s.

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Sharks (Image source: Darren Pateman)

Practical risk management, or why I love living in Australia

We’re into the ninth day of closed beaches here with two large great whites spotted ‘patrolling our shores’, whatever that means. Of course in Australia closed doesn’t actually mean the beaches are padlocked, not yet anyway. We just put a sign up and people can make their own minds up as to whether they wish to run the risk of being bitten. In my books a sensible approach to the issue, one that balances societal responsibility with personal freedom. I mean it’s not like they’re as dangerous as bicycles Continue Reading…

A short digression on who vs whom, neatly illustrating why writing requirements in natural english can be so damn difficult… I also love the idea of spider fastballs :)