Our proposed ground-based monitoring system would aggregate data in just this way. Investigators could thus examine information from a crashed aircraft for symptomatic patterns, to infer more precisely what had happened to it.
There is nothing new about this methodology. Analysts have used it for years to diagnose computer viruses, malware, and cyberattacks. Manufacturers and the governmental bodies that regulate them also employ it to identify failures in the design or manufacture of automobiles before issuing a recall. It is strange, then, that those responsible for air travel—the first and arguably the most thoroughly researched field in industrial safety—should have put off taking this step for so long.
The data collected by a flight data recorder vary according to whether the aircraft is in the takeoff, landing, or cruising phase. The U.S. Federal Aviation Administration specifies 88 parameters that must be recorded. One typical parameter is variation in altitude relative to a base altitude. Other such parameters are time aloft, airspeed, vertical acceleration, heading with respect to magnetic north, fuel flow, positions of various flight-surface controllers, and engine data. Most parameters are recorded at the rate of four 12-bit samples per second; others, less frequently. An airline may collect additional information for its own use as well.
Back in 2000, my then student Mohamed Aborizka and I figured out the communication requirements for transmitting flight recorder data continuously to a monitoring system on the ground. The airplane would transmit directly to the ground where possible, but when flying high or over water, it would have to resort to transmission via networks of satellites, some high up in geosynchronous orbit, others much lower down. In this way, it would cover even the polar regions. We favor satellites transmitting in the global Ku-band (that is, microwaves at 12 to 18 gigahertz), because they can avoid the interference with physical obstacles that plague terrestrial microwave systems. Also, satellites transmitting in this band can send signals strong enough to allow a receiver to use a very small dish. However, because satellite-borne bandwidth is a limited resource, we proposed economizing on the bandwidth by streaming only flight data, not the cockpit voice recording. The voice recording would go into an onboard recorder, as it does today. In fact, to ensure against the loss of communication to the ground station, we suggested that the current black box technology might continue, as a backup.
Most aircraft already shunt some information to ground stations. The data, which come at regular intervals, have to do with the flight path and airspeed, as well as information that maintenance crews need to service the plane when it lands. This system mostly uses VHF frequency-shift keying, which can handle just 16 bits per second, now popular in ships at sea.
The messages now sent to ground stations generally contain 220 bytes at a time in a package called a block, although some messages may span several blocks. We're talking about a paltry transmission rate—less than 2 kilobytes per second per aircraft. However, because several thousand airplanes may be in flight at a time, the combined data may come to perhaps 6 megabytes per second. But today such a volume is hardly prohibitive: A single WiMax connection can download 1 or 2 MB/s, and one of the new 4G phone systems might go as high as 10 MB/s. Solutions to these transmission problems, and the somewhat harder one of mining the vast archive of data, lie within our grasp.
One major problem does remain: how to get around the lack of a uniform communication medium. The world, after all, is covered by many different wireless systems—some designed for cities, some for rural areas, others for use over the ocean.
To stay in touch with every aircraft, a glass-box system would have to switch among all these communication channels. For example, an aircraft flying over land, at low altitude, can access high bandwidths by tapping into cellphone networks using VHF and UHF, which typically reach no farther than about 200 kilometers. When flying high or over water, satellite communication systems, which have lower carrying capacity, would have to be used instead.
This juggling act is child's play for software-defined radio, which switches among frequencies and communication protocols to achieve high reliability in widely varying conditions and circumstances. Such systems do tend to be expensive, having been designed to operate on a vast number of frequencies. But a glass-box system wouldn't need so many frequencies, which would simplify it considerably.
Today the best satellite-delivered bandwidth operates on the Ku-band and uses the protocols known as MPLS VLAN (multiprotocol label switching virtual local area network). These channels allow specific data to flow to secure Internet Protocol servers on the ground.
It may be necessary to vary the amount of data transmitted according to the status of a flight. For example, more data need to be transmitted during takeoff and landing, when several parameters change rapidly, than during cruising. Similarly, whenever the ground-based monitoring system notices something unusual, it requests additional data to clear things up. To handle this fast-shifting demand for data, a glass-box system must incorporate dynamic scheduling, doling out more or less channel bandwidth to different aircraft.