Understanding Air France 447 Read Online Free PDF Page B

the next reporting point, and some additional data. This system had its genesis in the late 1930’s supporting flying boats (large seaplane airliners) across the Atlantic using Morse code. The HF communications now are all by voice but the medium still has limitations as a result of the physics of radio wave propagation.
    HF radio uses a frequency range that can bounce off the ionosphere to provide very long range communications. However, with that capability come drawbacks. HF radio can be greatly affected by the sun, upper atmospheric conditions, sunspots, and other “space weather.” Communication can often be difficult, and sometimes nearly impossible. Call quality can range from good to non-existent. Pilots are assigned two frequencies in case the first one is not workable in the aircraft’s location. Because of the difficulty, pilots often talk to a radio operator whose job it is to relay messages between the aircraft and controllers, so that the controllers can spend their time concentrating on managing the air traffic. The process often involves slow clear talking, repeated transmissions, and an occasional message relay from other aircraft to complete the transfer of information.
    The lack of real-time position information (like radar) limits the capacity of the ATC system in remote regions. Unlike a radar environment, where airplanes can be accurately controlled within a few miles of each other, separation in oceanic areas relying on position reports is often 80 miles in-trail and 50 or more miles laterally.
    The same radio operator can cover huge areas of oceanic airspace with many aircraft, and the same frequency may be in use by pilots and radio operators on both sides of the ocean. Radio frequency congestion is often such that it can take several minutes to get a word in to make a report or request.
    The transmissions on HF are often noisy and the frequencies congested; therefore, pilots do not normally monitor it full time. Instead they use a Selective Call system (SELCAL) that allows the radio operator to transmit an aircraft’s unique SELCAL code. When the radio receiver recognizes its SELCAL signal it sounds a call signal in the cockpit. The call notifies the pilots to contact the radio operator for a message.
    It just so happened that the night of May 31, 2009 HF communications were particularly poor. When AF447 did not check in with Dakar control, the controller attributed it to the poor state of communications that night and nobody realized the flight was in trouble until hours later.
    The most modern airliners, including Air France’s A330s, also use two forms of digital communications for tracking and message relay between the flight and controllers: ADS and CPDLC. Each of these technologies use cheaper and faster VHF radios when within line of site of a ground based station, but switch to satellite communications to relay the messages when out of VHF range. The satellite link takes slightly longer as the connection must be established each time, but it is often quicker than the HF system, and the messages are not subject to poor reception issues.
    ADS (Automatic Dependent Surveillance) is a position reporting system. There are several varieties of this system, but the mode in question is known as ADS-C or ADS Contract. This mode requires a logon from the flight deck with the ATC facility. The controllers on the ground set parameters that tell the logged-on system on board the airplane when to make automatic position reports. The reports include the airplane’s position, time over the last reporting point, estimate for the next reporting point, altitude, speed, and a handful of other data automatically gathered from the flight management system. These transmissions are completely transparent to the crew. They have no indications of what is sent or when. Their only interaction with the ADS system is to log on and log off, though the log-off is automatic.
    Typically, the system is set to make