The message arrived at 2:14 a.m. GMT. It was encoded, no longer than a text message, and it was a portent of doom: 213100206ADVISORY.
Last week, engineers at Air France decoded the transmission to mean: "drop in cabin pressure." The message of the plane's impending demise was sent by the computer system on board the Airbus A330. Then the communication system went silent. Two hundred twenty-eight people died in the crash, but by Monday -- after a long search -- only 17 bodies had been found in a deep part of the Atlantic.
The circumstances of the crash of flight AF 447, from Rio de Janeiro to Paris, seem almost sinister. An aircraft disappears, far from the nearest radar system, and no physical traces are found for days -- except a computer-generated obituary, probably transmitted at the very moment of the catastrophe, thanks to a technology used in most modern jets called ACARS, or Aircraft Communications Addressing and Reporting System.
In their last radio message, the pilots had reported flying through black, electrically charged thunderclouds. What happened after that remained a mystery, at least until investigators last week managed to piece together a vague explanation based on the last 24 ACARS transmissions, pinged by satellite to the control center in Paris.
According to one message, 221002006AUTO FLT AP OFF, the autopilot system was switched off in the turbulence of the tropical storm. The first of the indicator panels in the cockpit failed a short time later.
Captain Marc Dubois, a highly experienced pilot with more than 11,000 flight hours under his belt, was confronted with a fatal situation. On a pitch-black night, in the midst of a thunderstorm, with no view of the horizon or any other points of reference he could have used for orientation, the instruments he needed to show him the position of the aircraft began to fail.
It is particularly difficult for a pilot, faced with such a situation, to no longer know what his air speed is. But this was precisely what one of the messages ACARS sent to the control center in Paris indicated: 341040006NAV ADR DISAGREE -- an error message feared by pilots.
Several sensors mounted on the fuselage below the cockpit measure the flight speed of the aircraft based on air flow. However, these sensors reported different flight speeds, presumably because at least one of them had iced over in the storm.
Last Thursday, aircraft manufacturer Airbus explained, in an "Accident Information Telex" it sent to all airlines operating Airbus models, how pilots faced with such a situation can determine their correct speed. Last Friday evening, Air France announced plans to accelerate the replacement of air-speed sensors on its Airbus planes.
But how did the 58-year-old captain and his two copilots react?
If search teams fail to recover the flight recorder, which consists of two metal devices that record flight data and cockpit conversations, this question may never be answered. "It would be a real shame for aviation," says Robert Francis, the former vice chairman of the National Transportation Safety Board, the agency that investigates aviation accidents in the United States. "If we want to avoid dramas like this in the future, we have to know what went wrong," says the safety expert. For this reason, Francis wants to see all important flight data transmitted via satellite in the future, using ACARS technology. "This crash demonstrates how valuable this technology could be," he says.
Significant upgrades to aircraft would not even be required, according to Francis. All that is needed, he says, is to reprogram the software in the communication system, turning it into a sort of online black box. Krishna Kavi, an engineer and professor at the University of North Texas in Denton, presented the US Federal Aviation Administration (FAA) with a similar system 10 years ago. "The cost is low," he says. For the 256 parameters recorded by a black box, Kavi came up with a volume of data requiring transmission of four to eight kilobits per second. "This is a fraction of what mobile wireless devices transmit today," says Kavi.
Airlines already use ACARS for a number of purposes. The system notifies mechanics on the ground of unusual vibrations in an aircraft's engines, so they can prepare any necessary repairs. Flight crews also use ACARS to send messages to their airlines about delays, or passengers with potential connection problems.
Lufthansa has used ACARS to transform one of its intercontinental aircraft into a flying weather station. Once every 30 minutes, the onboard system reports the temperature, wind speed and atmospheric pressure to a central office in Germany.
This very task was being performed by the ACARS system on board a jumbo jet traveling from São Paulo to Frankfurt during the night of the Air France crash. The data it collected show how quickly the disaster must have unfolded. "Our aircraft was flying through the area of bad weather only half an hour ahead of the Airbus, and the automatic weather data showed nothing unusual," explains Lufthansa spokesman Michael Lamberty.
After the 2001 terrorist attacks, a satellite telephone provider called Iridium offered the FAA the use of its radio frequencies for the real-time transmission of flight recorder data. According to the company, "the Iridium system could have an important impact on flight safety."
But transmission of flight data is expensive. It takes up satellite bandwidth. Former NTSB official Francis is familiar with these problems. But he argues that a constant flow of data during flight would not be absolutely necessary. "We would already gain a lot if the system would only transmit data the minute the aircraft entered an unusual situation," he says.
Experts with Germany's Federal Bureau of Aircraft Accidents Investigation in Braunschweig and the European Aviation Safety Agency in Cologne consider it "technically feasible" to report flight data on every flight to a central office via an online system. But pilots are the ones raising objections. "It would be tantamount to the full-scale monitoring of pilots," says Jörg Handwerg of Cockpit, a German pilots' association.
Safety expert Francis knows his proposal affects the personal rights of pilots. For this reason, he says, data would have to be encoded to prevent unauthorized individuals from listening in on radio communications. He also advises against transmitting conversations in the cockpit. "People's last words before they die are part of the private sphere and should not be broadcast around," says Francis, who became known more than 10 years ago as a result of his investigation of the explosion of a TWA jumbo jet near New York.
On the other hand, he says, nothing highlights the need for improved radio transmission of data than last week's desperate search for wreckage from the downed Air France jet. According to Francis, ACARS should always transmit an aircraft's position data, thus enabling rescue teams to search more effectively in an emergency.
"This crash demonstrates, in a drastic way, that we must improve our monitoring systems," he says. In a world in which satellites perform monitoring and navigation tasks, says Francis, it should not be possible for aircraft to simply disappear.