[SystemSafety] QF 32 and the Airbus A380

[Bernd Sieker]

On 26.09.2012 02:46, Les Chambers wrote:
> Hi All
>
> Richard De Crespigny's excellent book (QF32) on the engine explosion in
> the Qantas A380 out of Singapore is a gripping read. Richard was the
> captain and commander.  The 400 passengers and crew on this aircraft owe
> him and his excellent team their lives.
>
> He provides some technical details of the A380 that are particularly
> interesting. For example, according to Richard, the A380 has 250,000
> sensors, 22 subsystems and three operating "laws". He also says that the
> airframe was designed with the avionics system in mind. That is the
> airframe is less rigid, and therefore lighter, as the flight control
> system is designed to avoid stressful in-flight situations. As a
> professional sceptic I've got to ask, "Is this true?".

Less rigid may well be right, but that only means that it flexes more, 
which can help alleviate airframe stresses.

Unlike previous Airbus airliners, the manufacturer has been successfull 
so far in preventing public distribution of the type's operating 
manuals, but:

All transport category aircraft have to be designed for the same load 
factors. These are +2.5 G and -1 G with high-lift devices retracted 
("clean"), and +2.0 and 0 with flaps/slats extended (design limit load).

The airframe has to be proven physically not to break at loads below 
150% of the design limit load. This is the famous "wing ultimate load 
test", where the wings are bent upwards until the break. The A380 is 
certified to the same standards as any other transport aircraft, 
although it failed the physical test just below 150% design load 
(http://www.flightglobal.com/news/articles/airbus-a380-test-wing-breaks-just-below-ultimate-load-target-204716/). 
The manufacturer strengthened the structure 
(http://www.flightglobal.com/news/articles/airbus-to-reinforce-part-of-a380-wing-after-march-static-test-rupture-206797/) 
and in the end got away with it without having to re-do the destructive 
test.

The "three laws" to which the author refers are the different operating 
modes of the EFCS (Electrical Flight Control System), and are almost 
certainly the same as in all other Airbus fly-by-wire airliners. The 
first and second variety (called "normal law" and "alternate law", 
respectively) limit the load on the airframe to the maximum design load 
(+2.5/2.0 and -1.0/0), the third one is called "direct law" and deflects 
the control surfaces directly proportional to the stick deflection, and 
it is in this mode that it becomes possible to overstress the aircraft 
structure.

The wing appears more "floppy" than on most other aircraft, mostly 
because it is very long and has a heavy engine suspended pretty far 
outboard; but that does not mean that it is any less strong than that of 
other aircraft.

Other wide-body aircraft, except the 747, only have two underslung 
engines and do not have as much weight outboard, so do not need to flex 
as much. The old 747-1/2/3/400 wings are of a much older design and may 
well be overengineered to a degree that is not necessary today to 
fullfil certification criteria, and may thus be more rigid.

The B777 wing was shown to break at 154% 
(http://www.youtube.com/watch?v=Ai2HmvAXcU0), which is a near-perfect 
result in terms of fulfilling the specification, while not being much 
heavier than necessary.

I expect the 747-8 wings to be almost as wobbly as the A380's. In fact 
Boeing had flutter problems with them and was forced to introduce an 
active flutter-damping system to alleviate the problem 
(http://www.flightglobal.com/news/articles/faa-and-boeing-agree-on-747-8-oams-special-condition-354600/).

> Could anyone
> recommend some light reading on the design of the A380. The story of the
> A380 development project would also be extremely interesting.

http://www.smartcockpit.com/aircraft-ressources/A380_Briefing_For_Pilots_Part%202.html

... is the only substantial thing from Airbus I can find on the A380, 
but it is not really about development, design and structure.

I haven't read the following, but this also sounds like something to read:
   "A380 Weight Savings Using Numerical Structural Optimization" in 
proceedings of 20TH AAAF Colloquium, Material for Aerospace Applications

Probably not exactly "light reading", though.

This may also give some ideas about new design methods for the A380:
 
http://www.altairhyperworks.co.uk/html/en-GB/Campaign/pages/papers/Aero/Paper6%20Airbus.pdf

And here's something a bit "lighter":
 
http://ultraligero.net/Cursos/diseno/Design_Of_Aircraft_Structures_Under_Special_Consideration_NDT.pdf


Cheers,

Bernd