Doomed Flight AF 447 Questions Raised about Airbus Automated Control System

It took just three-and-a-half minutes for Air France flight AF 447 to plunge 11,000 meters into the Atlantic two years ago. An initial analysis of the plane's data recorder hints at errors made by the pilots. But questions have also been raised about the A330's automated control systems.

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The initial AF 447 crash report raises just as many questions as it answers.
AFP

The initial AF 447 crash report raises just as many questions as it answers.


Everything pointed to a routine flight, two years ago. The Airbus A330 was flying along at its cruising altitude high above the Atlantic Ocean and had just passed an area of light turbulence. The captain of the flight, Marc Dubois, left the cockpit for a bit of rest.

Co-pilot Pierre-Cédric Bonin, whose wife was a passenger onboard the aircraft, told the cabin crew that "in two minutes we should enter an area where it'll move about a bit more than at the moment."

The co-pilot's exact words are part of the interim report on the crash of Air France flight AF 447, released by the French aviation accident investigation agency BEA on Friday. The report, dry and almost entirely free of commentary, provides insight into the final three-and-a-half minutes of the flight, before it plunged into the Atlantic killing all 228 people on board.

Based on an initial analysis of the flight data recorders, the anticipation prior to the report's release was high. Both the data recorder and the voice recorder belonging to the Airbus, which crashed on June 1, 2009, were found on the ocean floor at a depth of 4,000 meters (13,125 feet) in early May.

'A Mysterious Crash'

The crash of the A330 had made millions of airline passengers uneasy. How, many wondered, was it possible for a passenger jet to simply be lost as it traversed the ocean? It was reminiscent of ships disappearing without a trace on the high seas in bygone centuries. Would the data recorders finally solve the mystery?

It was only much later, after hours of radio silence and well after the plane was scheduled to have completed its crossing of the Atlantic, that planes were dispatched to search for the missing Airbus.

Even experienced accident investigators were caught completely off guard by the calamity. "This is a mysterious crash," said Peter Goelz, former head of the National Transportation Safety Board (NTSB) in Washington. He said it was in the same category as air disasters such as that on the island of Tenerife in 1977, with 583 deaths, the deadliest in the history of air travel.

On Wednesday of this week, families of the victims -- from 32 countries -- are set to gather in Paris and Rio de Janeiro to mark the second anniversary of the crash. The meeting is taking place at a time when the veil that has covered the accident in mystery may slowly be lifting. The four-page BEA report provides answers to several of the most pressing questions the crash left behind -- and raises just as many additional issues.

The drama began at 2:10 a.m. and 5 seconds GMT: Without warning, the autopilot and the auto-thrust disengaged. The report is silent as to why. But crash investigators have an explanation: The three speed gauges on the outside of the aircraft, known as pitot sensors, had become iced up.

Extremely Dicey Situation

Suddenly, the routine flight turned into a nightmare. "I have the controls," co-pilot Bonin told his colleagues. At that point, the aircraft pitched to the right and Bonin quickly moved to correct and to pull up the plane's nose. His colleague informed him: "We've lost the speeds."

It must have been clear to both of them that they were suddenly in an extremely dicey situation. At the plane's cruising altitude of 11,000 meters (36,000 feet), maintaining a precise speed is critical. Just 15 kilometers per hour (9.3 mph) faster or slower and the plane can stall. With the margin of error so small, pilots call this altitude "coffin corner."

And it wasn't long before the cockpit was pierced with the feared warning: "Stall! Stall!" The warning comes from a synthetic voice and is accompanied by signal loud enough that the business class passengers behind the cockpit must have been able to hear it.

In such moments, however, the noise from outside the aircraft is one that heralds disaster. The sound of the wind rushing past disappears and only the high-pitched whine of the two turbines is audible inside the cabin. Aviation experts know that the noise means that the plane's wings are no longer providing enough lift.

It is at this moment in the sequence of events where expert opinions diverge when it comes to the answer to the most fundamental of all questions: Who was to blame for the deaths of the 228 people on board the flight? An animated debate has erupted between the plane's manufacturer Airbus and other experts. The interim report is short on clear answers.

A Serious Error

The pilots reacted to the stall warnings with maximum thrust -- just as was called for in the training manuals. But they also pulled the nose of the aircraft up. It is an intuitive thing to do, but aeronautically it is a serious error.

Increased thrust can result in an aircraft's nose rising on its own and manufacturers themselves have recognized the problem. In a Flight Operations Telex dated May 12, 2010, Airbus removed the maximum thrust instruction from its flight manuals.

But why would co-pilot Bonin pull up instead of pushing the nose down? It wasn't long before the plane's angle to the onrushing air became dangerously high.

An explanation for the A330's rising nose, however, could also be provided by a line in the BEA report referring to the trimmable horizontal stabilizer. Situated at the tail of the aircraft next to the flaps controlling the aircraft's pitch, known as the elevator, the horizontal stabilizer likewise helps control the plane's horizontal stability.

According to the BEA's interim report, the horizontal stabilizer moved from three degrees to 13 degrees, almost the maximum. In doing so, it forced the plane into an increasingly steep climb. It "remained in the latter position until the end of the flight," the report notes.

Airbus considers the reading not to be out of the ordinary and refers to the co-pilot's efforts to pull the nose up.

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