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09 June 2009

AF447: A Bad Omen

When we first heard about the AF447 accident, reports said it was an Airbus, in an area of heavy turbulence. There was one thought that ran through my head: Did the tail snap off?

Remember American Airlines 587? This was the Airbus 300 that, shortly after “9/11” crashed on departure and not far from Kennedy Airport in New York City. The accident was attributed to “wake turbulence” from a heavy-747 that had departed ahead of the Airbus. But something very disturbing came out of that investigation.

The Airbus is a “fly-by-wire” airplane. In other words, there are no mechanical linkages between the pilot’s joystick controller and the actual ailerons, elevator or rudder. No pushrods, no cables, no nuthin. Just electrical wires that run through a computer.

This computer is programmed so that the pilots cannot break the airplane. It will not allow the pilots to make a control input so fast or severe that it would overstress the things. On the surface, you might say, “This is good!” Yeah, well…but there’s always an exception, right? And there is one here, too. It’s the rudder.

Investigators discovered that below certain speeds, the Airbus computer allows the pilot much more rudder authority than would be expected or assumed. In fact, without knowing it, an Airbus pilot could push on his rudder pedal with sufficient force to snap the rudder clean off. Which is exactly what happened with American 586.

Okay, to clarify, not the rudder but the fin. Most people would probably call the whole vertical tailpiece the “rudder.” But it’s not. The rudder is just the part that wiggles back and forth to actually yaw the nose of the plane (or keep the nose from yawing, as the case may be). It is attached to what we call the “vertical fin,” which is the big immovable part. As the rudder wags back and forth, it imparts it’s aerodynamic loads on the fin, which is attached to the fuselage.

On the Airbus, the vertical fin is composite, not metal like on your basic Boeing airplane. The whole fin slides into a slot and is bonded to the fuselage, much like a big model airplane. I kid you not. In the crash of AA587, the whole damn fin snapped clean off as the pilot tap-danced on the pedals in response to the rockin’ and rollin’ his plane was doing as it reacted to the wake turbulence of the 747 ahead.

As it fin tore away, the aircraft yawed violently from side to side, generating enough force to snap the engines off and send them flying in two different directions. (This is why reporters were confused at first. There were three “crash sites” in Rockaway Beach, Queens. Turned out that one was the airplane and two were just fires created by the engines slamming into buildings on the ground, away from the main wreckage.)


The vertical fin of AA587 as it was lifted out of Jamaica Bay.

Ever since this trait of the Airbus line came to light, I’ve never really liked riding in them (and I haven’t). I know it’s silly and irrational. I mean, how often do tails just snap off of airplanes in flight? I’m sure that Airbus has modified the algorithms in their computers so that it’s more difficult if not impossible for a pilot to stomp on the rudder pedal hard enough to damage the plane. Right?

But then I saw this picture.


What you're looking at there is the entire vertical fin *and* rudder from AF447. It does not look like it hit the water with the rest of the airplane. Rather, it looks like it snapped clean off and fell separately.

The question will be: Did it happen as a result of the crash? Or was it part of the events that caused the crash?

I had a bad feeling about this before, and it's not getting any better.

5 comments:

Anonymous said...

Yeah, bolts (nuts and bolts)hold the vert. stabilizer on the boeings. On the U.S.A. aviation blogs, the A330
drivers interpret the automated reports sequence. From their stuff, one could envisage a complete 'ripping apart' of the entire airframe starting with the larger exposed parts. Way too early to tell. One of the data words designates failure of rudder extension limitation. By that time any 'fly by wire' system would be probably 'haywire'. There seems (judging by the very large debris area) the conflict of two enormous forces:- the speed of the wind and the speed of the plane. A bomb probably wouldn't do that. With bombs and prompt explosions, fairly finely divide material precipitates directly below. As you would know, at 400 plus knots, as soon as any structural failure occurs, everything else would be ripped apart. The discovery of open oxygen masks probably indicates a gradual (few minutes)
of disintegration. After all, the flight was in one of the worst flying areas on this planet - the dreaded 'pot of black'. This investigation will create much intrigue 'cause it's beyond the absolute flight envelope. At the moment, looks like she really broke up. From the limited published picture of the vert. stabiliser, it has sheared (not broken off the fuselage?) - that would take a lot of force to do that!!

Anonymous said...

Broken wing means tailspin?
The A330 has close inboard engines and 'extended narrow' (small front to back ratio) wings with 'winglets'.
Tailspin would go supersonic enough to twist vertical stabiliser. Rudder would break off long before.

Hal Johnson said...

An Army flight school buddy of mine, recently retired from AA, refers to that aircraft as a "Scarebus."

Bob Barbanes said...

Hal, the "old school" pilots have never liked the Airbus, but those who fly them come to love them I suppose. Ever met a pilot who disliked the ship he was assigned to fly? Me neither.

Anonymous #1, whether the fin "sheared off" or "broke off" is irrelevant. It came off, is all we know. Now, at what point in the crash sequence *did* it come off? Everyone is trying to interpret what the ACARS messages mean. At this point, no one knows.

Anonymous #2, your scenario of the break up is probably just as valid as anyone else's. Trouble is, no one knows.

And we may never.

Anonymous said...

Anonymous #1 says here for Bob Barnes' interest - excuse my lack of precision. Shearing in this sense means did not snap clean - was twisted. Language can be difficult in materials science. Laminates and composites, properly made, are much 'stronger' than single metal alloy sheets and they will 'fail' in a different way, often revealing in detail the forces they were subjected to. The great thing about your blog site is you encourage speculation. Maybe I should have said torsion (now we are really getting technical!). Just seems in several other airline disasters the 'fin' remnants have held very accurate pointers and clues as to what happened. I this disaster, because of the deep and rugged undersea terrain, it maybe difficult to gather enough other crucial parts to gain a complete and plausible scenario. It's certainly shaping up as a definitive and possibly record breaking salvage effort.