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, decisionsare always made under the watchful eye of the accountants. So compromises aremade and we live with the consequences. Except when people die. Then the binarynature of designing complex devices hits home: good decision, good result. Bad decision, bad result. Maybe really bad.

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

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

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

Flying an airplane underInstrument Flight Rules, in cloud, with turbulence and ice requires both skilland fully functional equipment.

The need for accurateair-speed indication is so important that air carriers and many GA operatorsturn 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 anotherreason for that mandate: Airliners and GA airplanes have been lost because thepilot forgot to turn on the pitot heat when he or she encountered icingconditions and became confused by the resulting erroneous air-speed readings.

I experienced an in-flightfailure of the pitot tube heating elements once when flying westbound over New York where the mountains are beginning to rise upjust 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, asituation that was obviously wrong since I was still airborne. The procedurewhen this happens in the airplane I was flying is to advise Air Traffic Controlof your situation, request an immediate lower altitude and then fly attitudeand power settings known to prevent a stall or an overspeed as you descend. Inmy case, although there were ice and clouds, there was no turbulence and I brokeout of the clouds at Minimum Vectoring Altitude, which meant I could have goneno lower without perhaps hitting something. The air was warmer there and assoon as the ice melted, things returned to normal. For the pilots on Flight447, with so many systems failing and flying in severe turbulence, the outcomewas pretty much guaranteed to be different than my experience.

Whether the loss of pitottube heat was the central reason for the crash is open to speculation. And thebest evidence, the flight recorder, lies at the bottom of the ocean. Buthere's the thing: If a pitot tube heaterfailed and brought this airplane down, isn't that something that indicatesanother problem? Most airliners haveseveral 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 alsocovered with ice? What did the computers that actually fly the Airbus make ofall this? Or did the failure of all four electrical systems make the wholeissue moot since available electrical energy, after the collapse of the fourmain buses, would have been limited to that supplied by a ram air-driventurbine generator – assuming the pilots had time to deploy it â€“ and somebatteries.

As for me, I'm not qualifiedto solve this problem. I've never flown an Airbus, even in a simulator. Butvirtually all aircraft accidents are eventually determined to be caused by achain of events. And the one thing I willspeculate about is this one will be too. Provided we ever know the wholestory.

ContributingEditor John Loughmiller is an Electronics Engineer specializing in SingleChannel Per Carrier communications systems and control logic system design forautomated communications devices. He's also a 4,500 hour commercial pilot,flight instructor, aircraft owner and is a Lead Safety Team Representative forthe FederalAviation Administration.