Re: [SystemSafety] UC The Accident to SpaceShip2

From: Corkery Tony < >
Date: Wed, 5 Aug 2015 14:08:27 +0000

I too found the implied censure a little harsh. The specific criticism was that they had not properly considered the effect of human errors; pointing to AC 437.55-1 where it does indeed cover the areas involved in the crash. However, the response from the engineers is that they had considered it but concluded that the pilot wouldn't do that action so didn't include any further safe guards.

Engineers conduct their activities in the environment of regulation, guidance and their experience and whilst 437.55-1 is explicit in that requirement, I suspect the engineers involved would have been experienced in the civil aviation world where the AMC for 1309 states under 'Operational and maintenance considerations' that for flight crew and maintenance tasks -

'These tasks, which are related to compliance, should be appropriate and reasonable. Quantitative assessments of the probabilities of flight crew and maintenance errors are not considered feasible. Reasonable tasks are those for which full credit can be taken because the flight crew or ground crew can realistically be anticipated to perform them correctly when they are required or scheduled. For the purposes of quantitative analysis,* a probability of one* can be assumed for flight crew and maintenance tasks that have been evaluated and found to be reasonable...'

So, having considered the task of operating the feather control at the correct time to be appropriate and reasonable, why (in the context of this guidance), would they consider any additional mitigation when they probably believed it was reasonable (and accepted practice) to assume a probability of 1 for compliance under internationally recognised guidance.

This guidance has often given me concerns, in respect of maintenance tasks. It is acknowledged that maintenance is a major contributor to accidents, yet application of this principle could mean that there is no real pressure to design systems to minimise maintenance intervention or spend effort making it simpler from a safety perspective, as you can define what you want, deem it appropriate and reasonable and claim a probability of 1 that it will be done correctly.

Seems to be conflicting advice in existence?

Tony Corkery

-----Original Message-----

Sent: 03 August 2015 13:15
To: The System Safety List
Subject: [SystemSafety] The Accident to SpaceShip2


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The NTSB held its public hearing on July 28th. All infos, including presentations from the hearing, available at and the NTSB's provisional executive summary, findings and safety recommendations at

The NTSB is big on the HazAn not having dealt adequately with HF aspects, including that the accident showed there was a critical system (the feather actuation/stow/lock/mechanism) with a single point of failure, namely human error.

However, I strongly disagree with the "summary" of Alister Macintyre, who wrote about it in the Risks Forum He speaks about "cut[ting] corners", and writes as if he thinks various people did things wrong. I don't see much evidence for that at all (although it is possible that some might come with the full report). I see people trying to get a job done, to bring a highly innovative piece of critical engineering - pioneering is an apt word - to fruition. And in this largely novel environment, needing to improve their HazAn. The HazAn is likely substantial intellectual property. Without evidence, it's on the verge of insulting to suggest anyone or any group involved with this project was slacking.

Compare. Lithium-ion primary and auxiliary batteries on the Boeing 787 is also new technology. An FMEA was done that suggested the worst that could happen to the environment during thermal runaway of one or more cells was development of smoke. That FMEA remained unchanged even after a thermal-runaway event during testing burnt down the test facility. And the NTSB visited the fabricating factory where it observed that hazard mitigation, namely certain quality control measures, was not as effective as was thought . Boeing has a lot more at stake - maybe the entire company again, who knows? - in getting it right than the backers of Scaled Composites. And they still didn't get the HazAn right.

When the technology is new, HazAn is a tricky business. No one wants to get it wrong. But they do. And they will. Which is why some of us are working on ways to get it done better.

I say more at

PBL Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany Je suis Charlie Tel+msg +49 (0)521 880 7319

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systemsafety_at_xxxxxx Received on Wed Aug 05 2015 - 16:09:24 CEST

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