Faulty Pitot tubes probably would have made doomed Air France A330 fly too fast

Written by John Dodge, Contributor

It's frightening if not ironic to think an invention from the early 1700s could have triggered a chain of events that brought down Air France Flight 447.

Pitot tubes that today measure airspeed were invented centuries ago by French engineer Henri Pitot (pronounced Pee-toe) to measure fluid flow. Now, a major segment of the Pitot tube Wikipedia page is headlined "Air France Flight 447." Thus, 447 joins immortalized flight numbers such as 109, 587, 11 and others.

One might assume a plane as sophisticated as the A330-200 which I flew across the Atlantic a week before the May 31st crash would be smart enough to compensate for faulty airspeed readings. After all, the principal under which Pitot tubes measures airspeed is stunningly simple - the same as "sticking your hand out the car window," according to MIT professor of aeronautics and astronautics John Hansman.

Hansman who holds among other degrees a Ph.D in meteorology is uniquely qualified to discuss about what might have happened to 447. He has specialized in cockpit automation and aviation weather hazards which likely played principal roles in the demise of ill-fated flight.

Given what investigators know in the absence of the flight data recorders, he theorizes the plane could have been traveling too fast possibly as the result of ice-clogged Pitot tubes. Indeed, Air France on Saturday acknowledged acknowledged "malfunctions" with Pitot tubes on A320s, but said problems had not been observed on A330s or A340s.

A lack of speed probably didn't lead to the catastrophe than ended 228 lives, Hansman said.

"I believe the A330 is equipped with an angle-of-attack sensor for minimum airspeed so failed airspeed sensors should not have gotten it into a low-speed problem."

The plane was believed to be flying at around 35,000 feet where temperatures can plummet to -50 Celsius. Ice cystals can be a problem but tend not to stick to plane surfaces above 40,000 feet. However, towering thunderstorm clouds produce "supercooled water" whose temperature is below freezing but turns to ice when it comes in contact with a plane's surfaces.  For pilots and passengers alike, such clouds also generate frightful up and down drafts.

"Pitot tubes tend to collect ice and are heated which apparently was not sufficient to deal with very severe icing conditions. If the tubes were blocked, the airspeed would indicate low," he explained. Those readings would tell the computers running the plane to throttle up the engines.

If all the A330's Pitot tubes were blocked (trying to confirm that there are four on an A330), the faulty readings could be accepted by the craft's computers as accurate. "If the Pitot tubes ice up, you do not have an easy way to determine you have a problem. If you have three bad and one good, the readings from three bad ones can be accepted as good," he explained.

The situation can degrade quickly when a plane is going too fast through thunderstorms. And the pilot is unlikely to be aware the plane is going too fast except perhaps by the sharpness of the bumps amplified by speed.

"It would be impossible to tell until they were going way too fast. When the computer increases the engines, they don't [physically] move the [cockpit] throttles. They were in the vicinity of thunderstorms and were expecting to get bounced around. There's also not a lot of noise from the engines or mechanical feel from the controls," he said. TimesOnline has a great piece on how difficult it is to control such a big plane under such circumstances.

As the craft's speed increases,  there's a tendency for its nose to point down.

"If you're going too fast, you start get shock waves on the wings which can cause the airplane to go nose down. That was significant hazard with the first generation of swept-wing jets. It's a condition known as Mach tuck.

Hansman discounted the notion that the A330 is so sophisticated that the pilot could not have taken over from the autopilot. Commercial pilot and electrical engineer John Loughmiller wrote an excellent piece on the subject as it relates to Flight 447.

But pilot control may have quickly become a moot point as the plane lost both pressurization and electrical power. Still, after the complete failure of the fivefold redundant electrical in an A330, the pilot can mechanically adjust the plane's elevators and rudder which control the up and down and lateral motions, respectively.

But the process is comparatively slow. "By the time you get to that point in an event, things have gone pretty bad," he said. TimesOnline has a great piece on how difficult it is to control such a big plane under adverse circumstances.

It's too early in the investigation to pinpoint what modifications, if any, have to be made to A330. Airbus appeared to have an aggressive Pitot tube replacement program already in place since April 27 for all its passenger jetliners.

"We have to make sure there are safeguards so this doesn't happen again. It's not clear at this point if it's procedural or something in the flight control logic." While the Pitot tube theory tops the list of culprits, other causes such as lightening, structural breaches, mechanical failure and pilot error will all be scrutinized before a final conclusion is reached about the tragic end of Flight 447.

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This post was originally published on Smartplanet.com

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