The use of radio equipment and avionics, in general, has increased markedly for all types of aircraft during the past thirty years. One of the reasons for this increase is the Federal Aviation Administration’s (FAA) requirement, that all aircraft operating in high-traffic areas be equipped with a two-way radio for communication with air traffic controllers and tower operators. The development of solid-state electronic technology has made it possible to install highly complex and sophisticated systems for communication, navigation, and automatic flight control in all types of aircraft.
Radio communications for aircraft are primarily for the purpose of air traffic control; however, commercial aircraft also utilize a range of high frequencies for communicating with ground stations and other aircraft for business and operational purposes. Communications for air traffic control are in the VHF band in the range between 118 and 135.975 MHz.
Communication systems for airliners involve interphone systems in the aircraft, passenger address systems, aircraft-to-aircraft communications, ground station communications, and air traffic control communications.
The interphone communications for one type of airliner are arranged in two separate systems which may operate together or independently. One system is used primarily for flight interphone operation, and the other, the service interphone system is used for two-way communication between stations while the airplane is on the ground.
The flight interphone system allows crew members in the airplane during flight operation to communicate with each other by any one of the communication radio systems and also to monitor some of the navigation systems which produce audible signals. Both the pilot and copilot are provided with loudspeakers at their stations so that they may monitor incoming signals without using headsets.
The flight interphone system is independent of the service interphone system and is operated by the use of control panels at the pilot’s, copilot’s, navigators, engineer’s, and radio-rack stations. A handset jack in the external power receptacle box is provided so that groundcrew members may communicate with anyone inside the airplane; the jack is connected to the pilot’s interphone control panel.
By using this system, maintenance personnel may communicate with each other at widely separated points on the airplane and also communicate with flight crew members during preparations for flight. The attendant’s interphone stations and the control-stand interphone stations are equipped with telephone-type handsets. All other service interphone stations are equipped with a handset jack so that a handset or a combined handset and headset may be plugged in and operated at any time.
HIGH-FREQUENCY COMMUNICATION SYSTEM
The high-frequency (HF) communication system on a large airplane operates in the frequency range of 2.0 to 25 MHz. The HF range is actually a middle-frequency range inasmuch as it starts just above the standard broadcast band, which ends at approximately 1700 kHz. The HF system on an airplane is used to provide two-way voice communication with ground stations or with other aircraft. The output is also directed to Selcal decoders for air traffic control signaling.
The HF radio control panel is located where it is easily accessible to the pilot or copilot; it includes a frequency
selector switch, a volume control, and a press-to-talk button for the Secal system (The word Selcal is derived from the term selective calling). The volume control includes a switch for turning the equipment on and off.
The frequency selector switch is tuned as desired to select a particular communication channel. When the channel is selected, the number of the channel appears in a window directly above the switch knob. The volume control knob is located in the center of the frequency-selection dial. When the knob is turned to its maximum counterclockwise position. it turns the HF communications equipment off.
On the Boeing 707 airplane, the principal HF antenna consists of a probe extending forward from the upper tip of the vertical stabilizer. On other aircraft, a number of different designs are installed, including antennas within various components of the aircraft that are covered by plastic-type shields. The cover may be of fiberglass or similar material that will allow electromagnetic waves to reach the antenna. The probe antenna is used for both receiving and transmitting and is matched to the transmission line at any frequency by means of a tuning unit.
VHF COMMUNICATION SYSTEMS
VHF communication systems are employed largely for air traffic control. These systems are installed in all types of aircraft so the pilot may be given information and directions and may request information from control towers and flight service stations. On the approach to any airport with two-way radio facilities, the pilot of an aircraft calls the tower and requests information and landing instructions. In airline operations, the flight of an aircraft is continuously monitored by flight controllers, and the crew is given instructions as necessary to maintain conditions of safe flight. The VHF communication system operates in the frequency range of 118 to 135.975 MHz. The nature of radio-wave propagation at these frequencies is such that communication is limited to line-of-sight distances. The advantage of VHF communication, however, is that these signals are not often distorted or rendered unintelligible by static and other types of interference.