That A 380 incident

The G\’s got a Q&A about what it all means. Here\’s the real worry:

Aviation experts believe that as well as the engine losing its protective cowling, sections of turbine blade sheared off. A photograph taken by a passenger from inside the plane shows a small hole in the left wing, possibly caused by flying debris.

Good discussion here.

Turbine blades shearing off is really something that shouldn\’t happen. They\’re an extremely strong alloy (single crystal nickel rhenium alloys, jet engines being the major consumer of the metal rhenium….why, yes, I have occasionally provided ammonium perrhenate, the precursor material for rhenium metal, to the Rolls Royce supply chain) and they are, as you might expect in a jet engine, spinning at extremely high speed.

If they start to shear off while doing so they\’ll make one hell of a mess of the wing: which, as you may or may not know, is where the fuel tanks are. One or two maybe causes what we\’ve seen. But one going unbalances the engine, making more shearing off more likely of others, leading, possibly, to a catastrophic failure.

Yes, I too think that firing hot sharp metal through fuel tanks is probably not a good idea.

I\’m sure there will be a lot of people at Rolls Royce crossing fingers and hoping that this is an exceptional event, not some problem inherent in these rather new engines.

But now to the chicken gun! In order to test that a bird strike does not cause such a catastrophic failure, traditionally new jet engine designs have had a chicken fired at them from a cannon. The engine is spun up to full power (ie, take off levels) and then said chicken fired from said cannon right at it, straight into the spinning blades.

If it munches the chicken and carries on, all well and good. If it explodes in a shower of turbine blades, well, back to the drawing board you go.

British Rail, when designing the 125s, decided that there were sufficient cuttings etc, where there might be low flying birds, that they\’d better test the new trains with the chicken gun. So, it was flown over from the States (Lockheed owned it I think? Maybe Northrop?) and set up. They procured a chicken (no, of course not, already dead one, from the supermarket) and fired.

Straight through the armoured window, the steel back of the drivers\’ seat and embedded itself in the back wall of the cabin.

This wasn\’t, to be polite about it, quite what the BR engineers were expecting. So, big report written up, detailing everything they had done, distance, gunpowder charge, tensile strength of window and so on, sent off to Lockheed (or Northrop?).

And back came the response:

Dear Sirs,

In order to use the \”chicken gun\” please note these operational steps.

1) First, defrost your chicken.

9 comments on “That A 380 incident

  1. I’ve heard a similar tale of the chicken gun going from a British aeroplane maker to an American aeroplane maker with the same punchline.

    Snopes has a bit more detail.

    I thought the engines were engineered with blade detachments in mind ( for reasons of bird strike or failure of the blade) so are built to contain jet blades that have become detached? The issue isn’t necessarily why a blade broke off but why it wasn’t contained.

  2. This was more severe than a blade seperating (which the engine should contain) – this was the large heavy turbine disk (to which the turbine blades attach) bursting – no engine can contain such large high energy pieces of metal detaching – it is like a shell from a tank which takes a lot of stopping.

  3. Ha!

    On the fabulous Mythbusters program on Quest (Freeview, Channel 38) they fired both frozen and thawed chickens at an aeroplane windscreen and, surprise surprise, it made no difference at all to the amount of damage.

    When the chicken is travelling at (effectively) hundreds of miles an hour, it makes no difference whether it is frozen or not, it depends on mass x velocity (temperature has no effect on mass, AFAIAA).

    It’s like the fact that liquid water is effectively as hard as concrete if you hit it faster than about thirty miles an hour (or else how do water skis work? Why does it hurt when you do a belly flop?).

  4. Seriously though, my understanding is that the A380 engine (the Trent 900) isn’t very different from the engine that Rolls Royce sells for the 777 (The Trent 800 – there are a lot of these in service), and these have been gradually developed from the RB-211 that Rolls first developed for the Lockheed Tristar in the 1970s. So new technology this isn’t.

  5. On the fabulous Mythbusters program on Quest (Freeview, Channel 38) they fired both frozen and thawed chickens at an aeroplane windscreen and, surprise surprise, it made no difference at all to the amount of damage

    I think they revisited that myth and built a new cannon, and found frozen chickens do make a difference.

  6. When the chicken is travelling at (effectively) hundreds of miles an hour, it makes no difference whether it is frozen or not, it depends on mass x velocity (temperature has no effect on mass, AFAIAA).

    The kinetic energy is a factor of mass and velocity (1/2 x m x [v squared]), but the impact force is a factor of mass and acceleration (F=MA). A frozen bird would in theory generate a greater impact force due to its hardness increasing the acceleration (deceleration), whereas a soft chicken would experience less deceleration. This is why hard objects hurt more than soft ones, even if their mass and speed are identical.

  7. “It’s like the fact that liquid water is effectively as hard as concrete if you hit it faster than about thirty miles an hour”

    So cliff divers can land on concrete as well?

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