Archives For Technology

The technological aspects of engineering for high consequence systems.

So here’s a question for the safety engineers at Airbus. Why display unreliable airspeed data if it truly is that unreliable?

In slightly longer form. If (for example) air data is so unreliable that your automation needs to automatically drop out of it’s primary mode, and your QRH procedure is then to manually fly pitch and thrust (1) then why not also automatically present a display page that only provides the data that pilots can trust and is needed to execute the QRH procedure (2)? Not doing so smacks of ‘awkward automation’ where the engineers automate the easy tasks but leave the hard tasks to the human, usually with comments in the flight manual to the effect that, “as it’s way too difficult to cover all failure scenarios in the software it’s over to you brave aviator” (3). This response is however something of a cop out as what is needed is not a canned response to such events but rather a flexible decision and situational awareness (SA) toolset that can assist the aircrew in responding to unprecedented events (see for example both QF72 and AF447) that inherently demand sense-making as a precursor to decision making (4). Some suggestions follow:

  1. Redesign the attitude display with articulated pitch ladders, or a Malcom’s horizon to improve situational awareness.
  2. Provide a fallback AoA source using an AoA estimator.
  3. Provide actual direct access to flight data parameters such as mach number and AoA to support troubleshooting (5).
  4. Provide an ability to ‘turn off’ coupling within calculated air data to allow rougher but more robust processing to continue.
  5. Use non-aristotlean logic to better model the trustworthiness of air data.
  6. Provide the current master/slave hierarchy status amongst voting channels to aircrew.
  7. Provide an obvious and intuitive way to  to remove a faulted channel allowing flight under reversionary laws (7).
  8. Inform aircrew as to the specific protection mode activation and the reasons (i.e. flight data) triggering that activation (8).

As aviation systems get deeper and more complex this need to support aircrew in such events will not diminish, in fact it is likely to increase if the past history of automation is any guide to the future.


1. The BEA report on the AF447 disaster surveyed Airbus pilots for their response to unreliable airspeed and found that in most cases aircrew, rather sensibly, put their hands in their laps as the aircraft was already in a safe state and waited for the icing induced condition to clear.

2. Although the Airbus Back Up Speed Display (BUSS) does use angle-of-attack data to provide a speed range and GPS height data to replace barometric altitude it has problems at high altitude where mach number rather than speed becomes significant and the stall threshold changes with mach number (which it doesn’t not know). As a result it’s use is 9as per Airbus manuals) below 250 FL.

3. What system designers do, in the abstract, is decompose and allocate system level behaviors to system components. Of course once you do that you then need to ensure that the component can do the job, and has the necessary support. Except ‘apparently’ if the component in question is a human and therefore considered to be outside’ your system.

4. Another way of looking at the problem is that the automation is the other crew member in the cockpit. Such tools allow the human and automation to ‘discuss’ the emerging situation in a meaningful (and low bandwidth) way so as to develop a shared understanding of the situation (6).

5. For example in the Airbus design although AoA and Mach number are calculated by the ADR and transmitted to the PRIM fourteen times a second they are not directly available to aircrew.

6. Yet another way of looking at the problem is that the principles of ecological design needs to be applied to the aircrew task of dealing with contingency situations.

7. For example in the Airbus design the current procedure is to reach up above the Captain’s side of the overhead instrument panel, and deselect two ADRs…which ones and the criterion to choose which ones are not however detailed by the manufacturer.

8. As the QF72 accident showed, where erroneous flight data triggers a protection law it is important to indicate what the flight protection laws are responding to.

Strowger pre-selection

The NBN, an example of degraded societal resilience?

Back in the day the old Strowger telephone exchanges were incredibly tough electro-mechanical beasts, and great fun to play with as well. As an example of their toughness there’s the tale of how during the Chilean ‘big one’ a Strowger unit was buried in the rubble of it’s exchange building but kept happily clunking away for a couple of days until the battery wore down. Early Australian exchanges were Strowger’s, my father actually worked on them, and to power their DC lines they used to run huge battery pairs that alternated between service and charging. That built in brute strength redundancy also minimised the effect of unreliable mains power on network services, remember back in the day power wasn’t that reliable. Fast forward to 1989 when we had the Newcastle (NSW) earthquake and lo our local exchange only stayed up for a couple of hours until it’s batteries died.

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Why writing a safety case might (actually) be a good idea

Frequent readers of my blog would probably realise that I’m a little sceptical of safety cases, as Scrooge remarked to Morely’s ghost, “There’s more of gravy than of grave about you, whatever you are!” So to for safety cases, oft more gravy than gravitas about them in my opinion, regardless of what their proponents might think.

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It is a common requirement to either load or update applications over the air after a distributed system has been deployed. For embedded systems that are mass market this is in fact a fundamental necessity. Of course once you do have an ability to load remotely there’s a back door that you have to be concerned about, and if the software is part of a vehicle’s control system or an insulin pump controller the consequences of leaving that door unsecured can be dire. To do this securely requires us to tackle the insecurities of the communications protocol head on.

One strategy is to insert a protocol ‘security layer’ between the stack and the application. The security layer then mediate between the application and the Stack to enforce the system’s overall security policy. For example the layer could confirm:

  • that the software update originated from an authenticated source,
  • that the update had not been modified,
  • that the update itself had been authorised, and
  • that the resources required by the downloaded software conform to any onboard safety or security policy.

There are also obvious economy of mechanism advantages when dealing with protocols like the TCP/IP monster. Who after all wants to mess around with the entirety of the TCP/IP stack, given that Richard Stevens took three volumes to define the damn thing? Similarly who wants to go through the entire process again when going from IP5 to IP6? 🙂

Defence in depth

One of the oft stated mantra’s of both system safety and cyber-security is that a defence in depth is required if you’re really serious about either topic. But what does that even mean? How deep? And depth of what exactly? Jello? Cacti? While such a statement has a reassuring gravitas, in practice it’s void of meaning unless you can point to an exemplar design and say there, that is what a defence in depth looks like. Continue Reading…

Here’s a companion tutorial to the one on integrity level partitioning. This addresses more general software hazards and how to deal with them. Again you can find a more permanent link on my publications page. Enjoy 🙂

Unreliable airspeed events pose a significant challenge (and safety risk) because such situations throw onto aircrew the most difficult (and error prone) of human cognitive tasks, that of ‘understanding’ a novel situation. This results in a double whammy for unreliable airspeed incidents. That is the likelihood of an error in ‘understanding’ is far greater than any other error type, and having made that sort of error it’s highly likely that it’s going to be a fatal one. Continue Reading…