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June 11, 2009

Flight 447: The Pilots' Deadly Scenario

Flight447_dt
Brazil's Air Force official Henry Munhoz shows a photo of a piece of the Air France 447 flight being recovered from the Atlantic Ocean, Monday, June 8, 2009.
Photo: AP Photo/Roberto Candia

by Clive Irving

Whatever happened to Air France Flight 447, it happened in a way that denied its chief pilot, 58-year-old Marc Dubois, the chance even to send a Mayday distress call. But some of his colleagues are speaking for him. Bloomberg News reports today that a spokesman for Air France's biggest pilots' union, Eric Derivry, is citing as a likely cause the faulty air-speed readings that were indicated in the final burst of data sent from the airliner before it disappeared.

"These pilots were confronted with serious technical problems and erroneous indications of speed," said Derivry. Under those conditions, said the head of another pilots' union, Bruno Sinatti, "piloting becomes very difficult, near impossible."

It is extremely unusual for pilots to speak out in such clear terms before investigators have anything approaching persuasive and definitive evidence for the cause of a crash. They are usually averse to rushing to judgment, and the most skeptical critics of unproven theories. The fact that the French pilots have become so outspoken shows a surprising degree of confidence on their part that faulty air-speed instruments were to blame.

Their view gets some support in the latest issue of Aviation Week. In a detailed investigation of the flight systems of the Airbus A330, the reporters quote a longtime A330 pilot saying that if the pilots realized that the airplane was flying at the wrong speed--either too fast or too slow--what is called a pitch change, in which the stable balance of flight is lost, "can be extremely difficult to recover from."


This critical moment would come within a well understood sequence of events. Flight 447 was at cruising altitude, 35,000 feet, and should have been flying at around 550 miles per hour. At that altitude the air is very thin, and that greatly affects the lowest safe speed. We know that Flight 447 was in an area of high-altitude turbulence from a violent storm system. That makes speed even more critical. For example, on approach to an airport at low level, in landing configuration, the A330 can slow to 160 miles per hour and remain perfectly stable. At 35,000 feet the minimum safe speed in turbulence jumps to over 300 miles per hour.

So this is a scenario: Flight 447 is getting faulty speed readings. (The type of storm involved is capable of being an ice maker, and ice is known to be a cause of problems with the A330's speed sensors.) The autopilot is on, as it would normally be on cruise. Even though the speed is falling, it does not automatically disconnect, as it is designed to do, until there is a serious anomaly. Just exactly where in the gap between 550 miles per hour and 300 miles per hour that would happen is unclear. But right then, with no warning and already in a critical situation, it is left to the pilots to regain control.

They have only a few seconds to understand what is happening. And they are already in the jeopardy described by the Air France pilots, needing great sensory skills of their own to recover a stable course. So stressed, in fact, that they do not send a Mayday call. Add the impact of storm turbulence and the scenario is grim indeed.

There is the other possibility that the airplane was flying too fast and hit a point at which its structure would begin to break up. Most experts discount this--the engines would have been very audibly under stress, and the crew would have been alerted. A quietly falling speed in which disaster comes by stealth is the greater threat.

Under pressure from its pilots, Air France is accelerating the replacement of the speed sensors on its A330s with upgraded versions. Airbus has said that this is a normal upgrading of equipment, and "a performance issue, not a safety issue." That sounds more legalistic than lucid.

Read Clive Irving's dispatches on Flight 447 in The Daily Beast:
The Myth of the Black Box (June 7)
The Secrets of Flight 447 (June 6)
Who Was Flying Flight 447? (June 5)
Last Words of Flight 447: From a Robot (June 3)

Comments

Many reports I have read suggest the Pitout tubes were iced up/inoperable and he pilots did not know the speed of the aircraft and made an incorrect adjustment. Since the navigation system surely relies (at least partially) on GPS, I would have thought there would be a system in place or it would be possible to calculate the speed from the GPS - even if all external instruments had failed?

A GPS or any other external system one might come up with can only provide groundspeed and has no idea what an airplane's airspeed is, and airspeed is all that matters in this situation.

If a 550-knot airplane had a 50-knot tailwind, its groundspeed would be 600 knots, which is what a GPS would report. And a GPS has no way of knowing what a tailwind (or headwind) component is in order to make an arithmetic calculation and correction

Nobody has yet come up with a better system than the one Henri Pitot invented in the early 1700s, which is why even Mach-2 fighters have Pitot tubes.

No, it's not a misspelling of "pilot tube," and yes, it should be capitalized.

Stephan Wilkinson

Stephan Wilkinson

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