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[PJ2]

vovachan;

Like a car airbag - G forces

How would that have worked in the AF447 accident? (I am assuming for a moment that the BEA Interim Report has it right and that the aircraft, intact, struck the sea in a relatively flat attitude with little forward speed, but fairly high vertical speed. Regardless, the question still applies to such a scenario.)

Regarding deployment based upon 'g' loads and air bag deployment sensing and times, let's take a look at some numbers.

I don't know the design metrics for airbag systems in cars but I think I can offer some reasonable parameters which others with the actual engineering knowledge can modify if my estimates are way off.

I theorize that vehicle airbag deployment design would reasonably contemplate 'g' forces which would spike over time periods of less than one half a second which obtain in collisions of the order of 15fps to possibly 200fps. As an aside, I suspect that for most vehicles, a collision at 200fps would not likely be survivable given the compromising of the passenger compartment but we're talking about an airbag system response time as it may apply to deploying a DFDR and CVR.

Takeoff and approach speeds of most transport category aircraft are in the neighbourhood of 130 to 180kts or between 200 and 300 fps. As another aside, nerve impulse speeds can reach about 160fps so the brain would not likely perceive any unfolding event which occurred at higher speeds.

Speeds below 10,000ft MSL are restricted to 250kts with exceptions to this speed limit on departure; that is about 420fps. That's 250kts IAS, so the TAS could be as high as 300kts, or 500fps. Above 10,000ft or FL100, climb, cruise and descent speeds are in the neighbourhood of 450kts TAS, give or take, which is about 750fps.

At the above-described transport speeds, even on the approach, most aircraft travel their entire fuselage length in slightly less than a second, and at climb, cruise and descent speeds, in about a fifth of a second.

By now you will see where I am going with this... The technical requirements to deploy a DFDR and CVR in the time frames described would require substantial design and manufacturing engineering not to mention certification and regulatory standards which must be tested and enforced. If we think about the "how to", the deploying mechanism must sever both recorders cleanly from their mounts and harnesses and fire them away from a structure which would literally be in the middle of an accident, in the time described, (sensing, arming, firing all in, say, a tenth of a second, so as to clear the failing structures). There is the weight penalty to consider as well - fuel burn increases by about 4% of the added zero fuel weight per flight hour - over the life of the airframe, that is a substantial amount of increased burn for what may be little practical return.

I return to my point made in post #36, which offers the notion that the expense and resources required to do this likely do not meet the risk-reward "thought process" let alone a formal analysis. The number of safety initiatives which must compete for industry and government-limited resources would place this kind of solution well down the list I should think. This isn't to say it isn't doable or isn't worth examining but the realities of doing this must be taken into account.

I'm not saying it isn't worth it. What I'm outlining here are some of the considerations which must be addressed when contemplating such a system. I will have missed some things because I am not an engineer or a mathematician but I do know about airplanes and accidents and flight data analysis solutions and off the top of my head these would be some but not all of the considerations.

The other deployment parameters which would continually assess the "health" of the entire system, (airplane, airplane systems, environment, 'g' loadings) and which could possibly be compared to algorithms built to determine when the aircraft is likely in distress, could form an alternative to the notion of "airbags and 'g' forces". Again, this "solution" must compete for limited resources.

Of course, no two accidents are the same; how do we design a system which is "sure it's time to deploy the recorders", without missing something critical?

Let's examine a practical case: - Investigators could only guess what happened in the last six minutes of SW111 because the fire on board destroyed the main electrical distribution system. The batteries likely continued to supply power however. The blunt reality today is, we dont' even have flight data and voice recorders that continue to work when the aircraft's main electrical system fails. Even getting that fixed, so that the recorders continue to function during serious electrical disruptions providing valuable last-second flight data which may hold the key to the entire accident, is a monumental political task and THAT solution is, and should be, already many positions ahead of "deploying recorders".

PJ2