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 â assuming the pilots had time to deploy it â and some
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
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