What made an Airbus rudder snap in mid-air?
At 35,000 feet above the Caribbean, Air Transat flight 961 was heading home to Quebec with 270 passengers and crew. At 3.45pm last Sunday, the pilot noticed something very unusual. His Airbus A310’s rudder — a structure over 8m high — had fallen off and tumbled into the sea. In the world of aviation, the shock waves have yet to subside.
Mercifully, the crew was able to turn the plane around, and by steering it with their wing and tail flaps managed to land at their point of departure in Varadero, Cuba, without loss of life. But as Canadian investigators try to discover what caused this near catastrophe, the specialist internet bulletin boards used by pilots, accident investigators and engineers are buzzing.
One former Airbus pilot, who now flies Boeings for a major United States airline, told The Observer: “This just isn’t supposed to happen. No one I know has ever seen an airliner’s rudder disintegrate like that. It raises worrying questions about the materials and build of the aircraft, and about its maintenance and inspection regime. We have to ask as things stand, would evidence of this type of deterioration ever be noticed before an incident like this in the air?”
He and his colleagues also believe that what happened may shed new light on a previous disaster. In November 2001, 265 people died when American Airlines flight 587, an Airbus A300 model which is almost identical to the A310, crashed shortly after take-off from JFK airport in New York. According to the official report into the crash, the immediate cause was the loss of the plane’s rudder and tailfin, though this was blamed on an error by the pilots.
Must be good pilots to get the ship down without a rudder. Reminds me of a comment an airline employee made to me once, that you did not need a guy in the cockpit with twenty thousand hours very often, but when you did need him, you needed him really bad.
And note this gem later in the story:
He said that while flight 961 had come down safely, to land a plane without a rudder in a crosswind or turbulence could be impossible. The rudder was all the more important on a plane such as an A310, because its wing design meant that it was “aerodynamically unstable” and needed the rudder for stability.
One Comment
A general rule of thumb, only the latest generation of fighter aircraft are designed to be aerodynamically unstable, so I have a MAJOR problem with that last statement. Okay, now why would you want a plane to be designed unstable, and what are the penalties?
An unstable plane will constantly “wobble” in all axes. This will be damped out by the flight control system (watch video of the F-117 - which is unstable, or the SR-71 which is, but only around two axes). A fighter needs the ability to change flight path abruptly. An unstable aircraft will do this a lot easier than a stable one (inertia is a very big factor in aircraft). Designing something to be unstable means you are constantly correcting the flight path (hundreds of times a second). This means there are constant changes to the airflow over the fuselage and wings, which equates to drag, which equates to fuel used to overcome drag.
A stable aircraft overcomes this as you can design it to a range of weights and speeds for maximum aerodynamic efficiency. Boeing has done this with the Dreamliner. At nearly transonic speeds this design process requires a great deal of number crunching. Most drag is generated by the wings, and there are well-known tricks to overcome that. The Dreamliner pays far more attention to the fuselage and where the wing joins the fuselage than any aircraft since the F4U. The result is greater fuel efficiency. By the way, it also requires a higher standard of construction than you get with your run of the mill airliner. This rudder incident suggests that Airbus is not there.
The way you fix most flying problems with an aircraft is with power (and hence fuel). An unloaded airliner handles like an aerobatic craft. Only when you add 2-400 passengers and all of their luggage do you have problems.
Now what could cause a rudder failure - the most obvious thing is materials weakness (flight conditions is second). Now they run these aircraft through a large number of tests before they are certified to carry people. But experience with fighter aircraft has taught designers that you can induce fatigue in control surfaces through repeated usage. Now it may be that the Airbus *is* borderline unstable in certain configurations. If so, then a lot of rudder work is needed to keep it flying. Constantly using the rudder could lead to fatigue and failure. The NTSB would need to look at the rudder and find out.
What of flight conditions? They are using a grade of aluminum that handles a wide range of temperatures, corrosion, pressures and humidities (all of these cause the skin to expand and contract). I would probably rule out flight conditions.
What of something else? Well, an Alaska Airlines flight crashed in the Pacific due to a rudder failure. This was traced to a worm gear used to move the rudder - it jammed, and when the flight control system forced it to move anyway the rudder failed, the tail fin buckled and came off, and the aircraft crashed. There was no evidence of this in the crash of an Airbus in New York. But could there have been a maintenance problem? The NTSB didn’t think so, and it’s their job to find out.
Just my 2 cents worth.