What made an Airbus rudder snap in mid-air?

[Ivan Seeking]

...At 35,000 feet above the Caribbean, Air Transat flight 961 was heading home to Quebec with 270 passengers and crew. At 3.45 pm last Sunday, the pilot noticed something very unusual. His Airbus A310's rudder - a structure 28 feet high - had fallen off and tumbled into the sea. In the world of aviation, the shock waves have yet to subside. [continued]

http://observer.guardian.co.uk/international/story/0,6903,1436374,00.html

As you can imagine, there is great interest from Boeing on this one.

 

[physicsCU]

Hmm, the only thing I can think of is extreme metal fatigue.

Probably a problem at the plant where the metal was smelted. And if that part is composite, bad technique in putting it together.

But honestly, that is really baffling as to how a 28 foot high vertical stabilizer could break off.

Was the pilot able to land the plane, or did it crash?

It seems like rudders are a problem still. I remember a few years ago when B-737 rudders would stick to one side. One crashed into a mountain in Colorado Springs because of it. Another, the pilot was able to wrestle the plane back. That pilot has two big brass ones for successfully fighting a 737.

 

[enigma]

why is it that the articles wait until the third paragraph before they come to the most likely explanation:

There have been other non-fatal incidents. One came in 2002 when a FedEx A300 freight pilot complained about strange 'uncommanded inputs' - rudder movements which the plane was making without his moving his control pedals. In FedEx's own test on the rudder on the ground, engineers claimed its 'acuators' - the hydraulic system which causes the rudder to move - tore a large hole around its hinges, in exactly the spot where the rudders of both flight 961 and flight 587 parted company from the rest of the aircraft.

I guess the general populace doesn't think like an engineer...

 

[Q_Goest]

Yikes... that's scarey! Looks like the article mentions the composite material as being a prime suspect, and that similar incidents to this aircraft and similar aircraft have occurred in the past.

There's a couple of pictures of the broken tail on this link: http://www.airliners.net/discussions/general_aviation/read.main/1991424

 

[Ivan Seeking]

I like this comment from your link

My dad said Boeings are better 'cause their tails stay on...

 

[hitssquad]

This article suggests pilot dogma is to blame:

--
[...] the NTSB has determined that the pilot flying the Airbus caused the vertical fin to break off by rapidly moving the rudder from side to side. The Airbus was flying at a speed below Va and so, according to pilot dogma, should have been immune from a structural failure. But we pilots were wrong.
--
http://www.flyingmag.com/article.asp?section_id=12&article_id=527

 

[russ_watters]

The entire point of a flight control system is to improve the handling and safety of the plane. The plane should not allow the pilot to do something that could snap off the rudder/stabilizer. This is a design flaw in the flight control system, and since this is happening frighteningly often, a flaw in the engineering of the tail as well.

Airbus needs to stop playing the liability avoidance blame game and fix this. It'll hurt them badly if they don't - what other airliner have you ever heard of that had a problem like this?

edit: another thing the article mentions that is certainly worthy of investigation is fatigue of composites. Composites are hard and strong, but they are brittle. Trust me: I have a set of graphite shaft golf clubs with a design flaw that concentrates the shock of striking the ball at the end of the hosel (where the shaft meets the club head). They break often. It is possible that the fatigue-ing of these airplane parts has not been adequately tested.

 

[Ivan Seeking]

Airbus may already be in deep but are only now realizing it. They are heavily invested in composite materials. Apparently the possiblity of a fundamental problem here is what peaks expectations among some Boeing people.

 

[hitssquad]

another thing the article mentions that is certainly worthy of investigation is fatigue of composites. Composites are hard and strong, but they are brittle.

This may be an issue for bicycle frames and forks, as well:


--
Also, I am well aware of how brittle carbon frames are. A carbon fork will "snap" instantly under an impact that would, at most, slightly bend a steel fork. (And, the steel fork can often be repaired). When carbon suffers a "fatigue" failure, it fails instantly, not over a period of months, as does a steel frame. Not a problem for a "Pro" with a new bike waiting for him on top of the team van. A more serious problem for average folks. A "instant" frame failure can cause serious injury to your body, as well as to your bank account.

You may remember when Lance had a crash at the TdF near the top of a mountain stage. He may have been going only 10 mph at the time of impact. The crash was minor, and he hopped back on his bike. He had trouble getting the chain to mesh with his cogs...the right chainstay had cracked. A steel frame would have had, perhaps, a small dent in the same sort of crash. An easy repair.

I understand the reasons a "Pro" rider might prefer carbon. Lighter weight during a mountain stage. Free for the first bike. A free replacement in a few months (I have not heard of a single "Pro" riding a carbon frame that is more than one year old). But, for a "Joe Average" rider like me, a steel frame is still the best way to go.
--
http://www.bikeforums.net/archive/index.php/t-71040

 

[physicsCU]

It seems like both Boeing and Airbus are having problems with rudders.

The B-737 had a hydraulic issue that opened a valve on its own, causing a severe turn.

But the Airbus sounds like a composite failure. That is what sucks about composites, they show no failure signs until they break.

Also, why would the tail break off if the pilot is moving it back and forth very fast? Airbus has fly-by-wire controls, so the electric motors would break, that is plausible, but not the whole structure breaking off.

 

[PerennialII]

Out of curiosity, does anyone have an idea about the fatigue crack propagation rates & the failure respective fracture toughness in the types of composites they use in these constructs ? Was wondering about the ratio of growth to rates in steels and how small the critical crack sizes actually are...

 

[FredGarvin]

It seems like both Boeing and Airbus are having problems with rudders.

The B-737 had a hydraulic issue that opened a valve on its own, causing a severe turn.

But the Airbus sounds like a composite failure. That is what sucks about composites, they show no failure signs until they break.

Also, why would the tail break off if the pilot is moving it back and forth very fast? Airbus has fly-by-wire controls, so the electric motors would break, that is plausible, but not the whole structure breaking off.

The Boeing issue was with severe thermal shock on the rudder's power control unit (a very cold actuator and very hot hydraulic fluid). It is a good story to read because it shows that sometimes you really have to look long and hard and get lucky in failure analysis. One thing that Boeing did on subsequent fixes, which I believe Airbus has yet to do, it to redesign the PCU's with limiters which limit the rudder travel at lower speeds.

Here's a link to an executive summary for the A300 that crashed after takeoff from JFK (which was not far from where I was living at the time). I know the airline pilot's association does not fully agree with putting some of the blame on the first officer, but it does support design flaws.

http://www.ntsb.gov/publictn/2004/AAR0404.htm

 

[Astronuc]

Out of curiosity, does anyone have an idea about the fatigue crack propagation rates & the failure respective fracture toughness in the types of composites they use in these constructs ? Was wondering about the ratio of growth to rates in steels and how small the critical crack sizes actually are...

That's not an easy question. The critical flaw size depends on the local stress intensity conditions as related to the local fracture toughness - i.e. the basic definition. Obviously, as the local stress increases the critical flaw size decreases.

Composites are way more complicated. Most mechanics codes for structural systems do well with continuum mechanics (constitutive) models, and these are general suitable for metals. Composites have a fine microstructure that requires signficantly finer meshing. Also, the local stress intensity distribution is highly non-linear.

Complicating this the interfacial behavior between fiber and filler/bonding agent.

It would appear that Airbus's testing is inadequate. Ideally, the DBL's bound the limiting in-field conditions. Fatiguing, however is very complicated because of the variability of high and low stress-intensity. I believe the methodology used in this area still needs significant improvement - as appears to be the case from the anecdotes of broken golf clubs, broken bicycle frames, and lost tail rudders.

Clearly, Airbus needs to focus on the stress intensity field in the hinge regions of the rudder - high local stress concentrations - and likely beyond their DBL.

 

[PerennialII]

Yeah, the different possible failure modes never really "help" with composites, and fracture mechanics of composite structures isn't that straightforward either. It's reasonable to assume that considering the different possible failure mechanisms the "scatter" in structural performance, and as such in safe life, is quite a bit wider than with our safe - bet metallic structures ... got to delve a bit to get a hang of it.

Would think that with these sorts of structures experimental research would be exercised ... a lot, a whole lot more than we're used to with more damage tolerant materials. Wouldn't want to be responsible for the quality control of these things either .

 

[russ_watters]

Also, why would the tail break off if the pilot is moving it back and forth very fast? Airbus has fly-by-wire controls, so the electric motors would break, that is plausible, but not the whole structure breaking off.

The electric motors are stronger than the structure of the rudder, apparently. This, again, goes back to my comment: the plane should not be letting the pilots do something that could break it.

Regarding fatigue, for months before my golf clubs break, you can hear the individual fibers snapping if you bend the shaft. But overall, the club performs normally right up until it fails completely (I guess with enough fibers severed, the rest go all at once on one impact) - sending the club head up to 50 yards.

 

[FredGarvin]

Anyone who plays hockey will tell you the same thing happens with composite shafts. There's no real degredation in how they perform. All of a sudden, half of your stick goes flying across the rink. I personally do not like them.

 

[physicsCU]

The electric motors are stronger than the structure of the rudder, apparently. This, again, goes back to my comment: the plane should not be letting the pilots do something that could break it.

Sorry, I meant the motors would fail, not physically break apart. And I agree, the fly-by-wire system should have not allowed that movement. I can understand a hydraulic plane breaking like that, but the computer should have stopped it.

And yes, all Airbus planes are fly-by-wire.

 

[hydro]

Hmm, the only thing I can think of is extreme metal fatigue.

Probably a problem at the plant where the metal was smelted. And if that part is composite, bad technique in putting it together.

But honestly, that is really baffling as to how a 28 foot high vertical stabilizer could break off.

Was the pilot able to land the plane, or did it crash?

It seems like rudders are a problem still. I remember a few years ago when B-737 rudders would stick to one side. One crashed into a mountain in Colorado Springs because of it. Another, the pilot was able to wrestle the plane back. That pilot has two big brass ones for successfully fighting a 737.

It could not be metal fatigue as it was made of Composite materials,carbon fiber,resins etc.IMO composites tend to keep on curing over time to the point where they lose their flexibility and become brittle.Having worked on Aircraft composite control surfaces as a A&P Mechanic a "New" composite control surface is flexible as it should be.Check the flexibility by bending it a few years later by and the flexibility is not the same.Subject the control surface to the same bending and it will surely crack.I believe this is the reason the Vertical Stabilizers "Rudders" on those airplanes separated at their attach points.The B737 rudder problems were traced to a flawed design in the Rudder hydraulic actuator.Most airplanes have two rudder actuators the B737 was designed with only one.When coming down from high altitude the internal parts were very cold and the coefficient of expansion & contraction of these internals were in conflict causing a uncommanded movement in tne rudder causing several B737's to roll inverted when on approach for landing causing many lost lives when crashing.hydro

 

[Phrak]

How does a pilot notice a falling object low and to his six? Anyway,

what model of 310 was it? Airbus is big on reducing mass in every quarter--from avionics to toilet seats.

I might suggest, to compliment previous speculations, that the 310 may have been under-enjineered by manajerial pressure as a direct result of VPs slinging-about words, concepts, and demands of which they are inherently incapable of comprehending.

Imagine if you will, some CEO saying to himself, "we could trim the weight off of every subsytem by 30%, and we will."--then pressuring his betters to tell him why it can't done. Not some subsystems. All subsystems. Get the picture?

Add this to the inherent nature of engineers to project that they are capable of more than they are, and the rest of nature will oblige.

http://www.flightglobal.com/pdfarchive/view/1985/1985%20-%200008.html"

"The A310-300 weight reduction programme will exceed its 1200kg target"

The weight reduction program is ongoing to this day. Fuel is a big chunk of commerical air transport costs. Reduction in fuel costs over Boeing per passenger-mile is a big smelling point.

http://www.flightglobal.com/articles/2008/07/09/225122/tomorrow-can-wait-as-airbus-and-boeing-leave-next-generation-narrowbody-development-on-the-back.html

Don't underestimate the green energy folks who salivate over instilling fears of atmospheric carbon. Less fuel; less carbon. This works it's magic on the corporate public image. The eco-friendly folks want their day. They will have it. They will kill to get it.

http://www.tc.gc.ca/programs/environment/ecofreight/casestudies/AirTransat-eng.htm"

This is psychology guys, not aerospace.

 

[Cyrus]

Tail smale.