Archives For Safety

The practice of safety engineering in various high consequence industries.

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.

I was cleaning out my (metaphorical) sock drawer and came across this rough guide to the workings of the Australian Defence standard on software safety DEF(AUST) 5679. The guide was written around 2006 for Issue 1 of the standard, although many of the issues it discussed persisted into Issue 2, which hit the streets in 2008.

DEF (AUST) 5679 is an interesting standard, one can see that the authors, Tony Cant amongst them, put a lot of thought into the methodology behind the standard, unfortunately it’s suffered from a failure to achieve large scale adoption and usage.

So here’s my thoughts at the time on how to actually use the standard to best advantage, I also threw in some concepts on how to deal with xOTS components within the DEF (AUST) 5679 framework.

Enjoy :)

Indonesian AirNav radar screenshot (Image source: Aviation Herald)

So what did happen? 

While the media ‘knows’ that the aircraft climbed steeply before rapidly descending, we should remember that this supposition relies on the self reported altitude and speed of the aircraft. So we should be cautious about presuming that what we see on a radar screen is actually what happened to the aircraft. There are of course also disturbing similarities to the circumstances in which Air France AF447 was lost, yet at this moment all they are are similarities. One things for sure though, there’ll be little sleep in Toulouse until the FDRs are recovered.

Boeing 787-8 N787BA cockpit (Image source: Alex Beltyukov CC BY-SA 3.0)

The Dreamliner and the Network

Big complicated technologies are rarely (perhaps never) developed by one organisation. Instead they’re a patchwork quilt of individual systems which are developed by domain experts, with the whole being stitched together by a single authority/agency. This practice is nothing new, it’s been around since the earliest days of the cybernetic era, it’s a classic tool that organisations and engineers use to deal with industrial scale design tasks (1). But what is different is that we no longer design systems, and systems of systems, as loose federations of entities. We now think of and design our systems as networks, and thus our system of systems have become a ‘network of networks’ that exhibit much greater degrees of interdependence.

Continue Reading…

787 Battery after fire (Image source: NTSB)

The NTSB have released their final report on the Boeing 787 Dreamliner Li-Ion battery fires. The report makes interesting reading, but for me the most telling point is summarised in conclusion seven, which I quote below.

Conclusion 7. Boeing’s electrical power system safety assessment did not consider the most severe effects of a cell internal short circuit and include requirements to mitigate related risks, and the review of the assessment by Boeing authorized representatives and Federal Aviation Administration certification engineers did not reveal this deficiency.

NTSB/AIR-14/01  (p78 )

In other words Boeing got themselves into a position with their safety assessment where their ‘assumed worst case’ was much less worse case than the reality. This failure to imagine the worst ensured that when they aggressively weight optimised the battery design instead of thermally optimising it, the risks they were actually running were unwittingly so much higher.

The first principal is that you must not fool yourself, and that you are the easiest person to fool

Richard P. Feynman

I’m also thinking that the behaviour of Boeing is consistent with what McDermid et al, calls probative blindness. That is, the safety activities that were conducted were intended to comply with regulatory requirements rather than actually determine what hazards existed and their risk.

… there is a high level of corporate confidence in the safety of the [Nimrod aircraft]. However, the lack of structured evidence to support this confidence clearly requires rectifying, in order to meet forthcoming legislation and to achieve compliance.

Nimrod Safety Management Plan 2002 (1)

As the quote from the Nimrod program deftly illustrates, often (2) safety analyses are conducted simply to confirm what we already ‘know’ that the system is safe, non-probative if you will. In these circumstances the objective is compliance with the regulations rather than to generate evidence that our system is unsafe. In such circumstances doing more or better safety analysis is unlikely to prevent an accident because the evidence will not cause beliefs to change, belief it seems is a powerful thing.

The Boeing battery saga also illustrates how much regulators like the FAA actually rely on the technical competence of those being regulated, and how fragile that regulatory relationship is when it comes to dealing with the safety of emerging technologies.

Notes

1. As quoted in Probative Blindness: How Safety Activity can fail to Update Beliefs about Safety, A J Rae*, J A McDermid, R D Alexander, M Nicholson (IET SSCS Conference 2014).

2. Actually in aerospace I’d assert that it’s normal practice to carry out hazard analyses simply to comply with a regulatory requirement. As far as the organisation commissioning them is concerned the results are going to tell them what they know already, that the system is safe.