Air France Tragedy

Her is a web posting by Miles O'Brien that gives a lot of details to explain how the Air France flight got into trouble and crashed. If you are like me, this is all new stuff, and scary. I've bolded the parts that worry me the most:

The doomed Airbus A-330-200 was flying ever so close to its maximum altitude – in a zone pilots call the “Coffin Corner”. It refers to the edge of so-called “flight envelope” of an aircraft. At this altitude, the air is much thinner and that significantly narrows the swath of speed at which the airplane can safely operate.

Because there are relatively few air molecules passing over the wings, they need to be moving faster to generate enough lift to keep the plane at altitude. They will stop flying (stall) at a much higher speed (true airspeed) than they would on approach to an airport at sea level.

At the other end of the safe speed spectrum is the sound barrier. The wings on an airliner like the A-330 are not designed to break the speed of sound. Venture toward Chuck Yeager country and an airliner will begin buffeting. And as altitude increases, the buffet speed (the sound barrier) decreases (once again the dearth of air molecules is to blame).

So you see the squeeze play as a plane flies toward the Coffin Corner: the margin between the between the high and low speed limits gets thinner and thinner (along with the air).

Matter of fact, given its estimated weight, altitude and the outside air temperature (which also affects air density), AF 447 was flying through the eye of a speed needle only about 25 knots (28 mph) wide.

...

So while you are napping, eating or watching a movie on that flight to LAX, you should know the plane you are flying is cruising along at the ratty edge of its capabilities. Why? Money. The higher an airliner flies, the better gas mileage it gets.

But rest easy, white-knucklers; flying in the “Corner” is routine and safe - so long as the weather is benign, the air is smooth and the sensors, avionics, computers and autopilot are all doing their job.

But of course that was not the case for Air France 447.

Go read the whole post for all the details. It is fascinating stuff that I had not heard of elsewhere.

His next blog entry goes into the equipment failures. The Airbus is a "fly by wire" machine which overrules the pilot and had a vulnerability. It uses an airspeed indicator to decide a lot about the control of the plane. The Airbus had triple redundancy on this part, but they all were susceptible to one external event: freezing over. Here's a key bit of that blog entry:

Instead, electrical wires transmit the pilot’s commands to hydraulic actuators that move the aero surfaces. Between the pilot and those surfaces is a bank of computers that are actually flying the plane. The computers are programmed with some strict rules (in fact, Airbus calls them “Laws”) designed to assess the human commands from the flight deck – and veto them if they would put the plane in harm’s way. Point the nose too high or too low – or bank to steeply and the computer will correct your bad airmanship. Who’s in charge here?

Pilots like to call their autopilots “George” (old phonetic shorthand for “gyro”, which makes the AP work) – on an FBW airplane, “HAL” might be more apt.

But what happens when the silicon co-pilot gives up the ghost? It gets very ugly - very quickly. Just before Air France 447 went down, it transmitted a four-minute spurt of text data reporting 5 failures and 19 warnings via its Aircraft Communications Addressing and Reporting System (ACARS). The data is cryptic and we will only know the full scenario if searchers find the black boxes, but we know the autopilot disengaged, the flight control computer failed, warning flags appeared over the primary flight data screens used by the captain and first officer and the rudder moved beyond its limits.

ll of it is consistent with a flight control system that was getting some bad information about how fast the airplane was moving through the air. The device that performs this task is called a pitot tube. Pointed in the direction of flight, it measures the relative pressure of air as it flows in. For pilots this is a crucial device – (like an EKG for a heart surgeon, I suppose).

If you don’t know your airspeed, you can easily stall or overspeed the plane. That’s why the A-330 has three pitot tubes. They tend to be ice collectors on an airplane flying through precipitation. If they glaze over, or get clogged with crystals, they won’t work – so that is why they are heated. Even so, A-330 pitot tubes were icing up and failing in flight so Airbus issued a “service bulletin” recommending airlines replace them with a newer model that has a more powerful heater. It was not considered urgent – and so the pitot tubes on the doomed plane had not been removed and replaced.

...

Now here is a key point to remember: as systems fail in an Airbus, the laws that the computers live by change from “normal”, to “alternate”, to “abnormal alternate” to “direct”. At each stage the computers surrender more authority to the humans – until finally silicon surrenders and the carbon pilots are on their own – with no help at all from HAL – at just the point they need him most.

Again, go read the blog for more details. Fascinating stuff.