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What Happened to Flight 447? A Designer's Worst Dilemma

What Happened to Flight 447? A Designer's Worst Dilemma

Engineering design decisions are viewed by the unknowing as precise, no-compromise conclusions. If that were so all of us would feel better about our design choices. But in the real world, decisions are always made under the watchful eye of the accountants. So compromises are made and we live with the consequences. Except when people die. Then the binary nature of designing complex devices hits home: good decision, good result. Bad decision, bad result. Maybe really bad.

The designers at Airbus must be going through a lot of soul searching right now as they sift through the incredible three-minute burst of telemetry data that originated from Flight 447 on May 31. If you placed yourself in their shoes, you know you'd be hoping against hope that whatever happened was not the result of a bad decision you made. "228 people are dead. The airplane came apart. What if I screwed up?"

But maybe the basic aircraft design â€“ at least the part involving Fly-By-Wire automation â€“ wasn't the proximate cause of the crash. The latest speculation coming out of France can be inferred from a directive published by Airbus suggesting inspections of the heated pitot tubes on A330-200 aircraft. The timing of the directive would seem to indicate a connection to the loss of Flight 447; however, Airbus cautions it has made no determination of cause yet.

The pitot tube is a device that measures ram air pressure caused by the aircraft moving through the atmosphere. A correlation exists between the amount of air pressure induced at the entrance to the tube and a reference point defined by the properties of still air (static air pressure) existing under the same air density conditions. The pitot tube contains redundant heating elements to negate the effects of ice accumulation. This feature exists on all transport category airplanes and on most General Aviation (GA) airplanes certified for flight into known icing conditions. Airbus is suggesting that pitot tubes be inspected to insure they are working properly, which means they're capable of providing correct airspeed information and the heating elements are functioning properly.

Flying an airplane under Instrument Flight Rules, in cloud, with turbulence and ice requires both skill and fully functional equipment.

The need for accurate air-speed indication is so important that air carriers and many GA operators turn on the pitot heat anytime they leave the earth. They even use it on hot, clear days so it's always up to temperature just in case. There's another reason for that mandate: Airliners and GA airplanes have been lost because the pilot forgot to turn on the pitot heat when he or she encountered icing conditions and became confused by the resulting erroneous air-speed readings.

I experienced an in-flight failure of the pitot tube heating elements once when flying westbound over New York where the mountains are beginning to rise up just before you get into Pennsylvania.

In cloud and picking up ice, the air-speed indicator crept lower and lower until it was pointing at zero, a situation that was obviously wrong since I was still airborne. The procedure when this happens in the airplane I was flying is to advise Air Traffic Control of your situation, request an immediate lower altitude and then fly attitude and power settings known to prevent a stall or an overspeed as you descend. In my case, although there were ice and clouds, there was no turbulence and I broke out of the clouds at Minimum Vectoring Altitude, which meant I could have gone no lower without perhaps hitting something. The air was warmer there and as soon as the ice melted, things returned to normal. For the pilots on Flight 447, with so many systems failing and flying in severe turbulence, the outcome was pretty much guaranteed to be different than my experience.

Whether the loss of pitot tube heat was the central reason for the crash is open to speculation. And the best evidence, the flight recorder, lies at the bottom of the ocean. But here's the thing: If a pitot tube heater failed and brought this airplane down, isn't that something that indicates another problem? Most airliners have several of these devices. One model of the Airbus I'm familiar with has four pitot tubes. So what was going on with the others on Flight 447? Were they also covered with ice? What did the computers that actually fly the Airbus make of all this? Or did the failure of all four electrical systems make the whole issue moot since available electrical energy, after the collapse of the four main buses, would have been limited to that supplied by a ram air-driven turbine generator – assuming the pilots had time to deploy it â€“ and some batteries.

As for me, I'm not qualified to solve this problem. I've never flown an Airbus, even in a simulator. But virtually all aircraft accidents are eventually determined to be caused by a chain of events. And the one thing I will speculate about is this one will be too. Provided we ever know the whole story.

Contributing Editor John Loughmiller is an Electronics Engineer specializing in Single Channel Per Carrier communications systems and control logic system design for automated communications devices. He's also a 4,500 hour commercial pilot, flight instructor, aircraft owner and is a Lead Safety Team Representative for the Federal Aviation Administration.     

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