A Blue Screen of Actual Death

A Past Flight May Offer Clues to Air France 447 – Yahoo! News

A Boeing executive once recalled a conversation with Herb Kelleher, then the Chairman of Southwest Airlines. Kelleher, he said, told him, “You want me to buy a fly-by-wire plane. Well, I’d have to go into one of my passenger cabins and ask my customers.. ‘Would you pay an extra $10 for this to be a fly-by-wire plane?’ And then I would need to explain to him what ‘fly-by-wire’ means. After that, my passenger would want $10 back from me to fly in a plane like that.”

The thing that pilots don’t like about fly-by-wire, and Airbus planes in particular, is that the pilot really isn’t in control of the plane. The article talks about “wrestling control from the computer” but on an Airbus, what they are really doing is convincing the computer to give them control.

One “feature” of Airbus systems is that the computer’s specification for a safe flight envelope overrides the pilot’s inputs. Simply put, if the pilot makes inputs that would put the plane into an attitude the computer holds as outside the safe envelope, the computer will not respond.

For example, if the computer perceives the angle of attack (nose up) would be too steep for the airspeed – threatening a stall condition (when the plane stops flying), the computer would override a pilot’s input for nose-up… even if the autopilot were not engaged and the pilot was “in control.”

Pilots, in general, do not like this because there may be emergency conditions – which no one can forsee – when they might need to make this kind of input in order to save the aircraft.

The case in the article brings up another condition. The computer really does not “know” what the plane is doing. It only knows what its electronic inputs are telling it. Those inputs, in turn, should be fed from the various sensors in and on the aircraft. But if those sensors fail, or if there is an intervening problem that feeds erroneous information to the computer, it will respond.

In this case, the computer likely believed it was in a dangerous nose-up condition, and was trying to level off… when in reality, it was responding to bad information about the angle of attack, resulting in the plane going into a steep dive.

What happened to AF 447? Unless the flight recorders can be recovered and read, we may never know. But incidents like the one in the article are disturbing. We are putting software and technology that is too complex for us to fully test and understand into safety-critical roles.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s