Aircraft communications

Aircraft Communications

By Jim Sparks

October 1998

It has been about forty years since the Federal Aviation Administration required all aircraft operating in high traffic areas to be equipped with two-way communication radios. In this time period, technology has advanced radically in the area of solid state electronics. In fact, what is "new technology" today may be "obsolete" tomorrow. The type of aircraft communications that most frequently comes to mind is that which takes place between Pilot and Controller, and uses a "Very High Frequency" (VHF). Several other means are available and they include "High Frequency" (HF), "Satellite Communications" (SAT COM) — even some telephone systems can be used in flight.

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Audio Panel

VHF communication systems are the most widely used for maintaining contact between ground and aircraft. This employs "Line Of Sight" transmission, which translates to a range of about thirty miles for an aircraft operating at 1,000 feet above the ground, or about 135 miles with an aircraft operating at 10,000 feet.

Adjustments to the frequency range have been made over the years as the capacity of ground-based radio stations tends to double every sixteen years. Initially, the "Radio Frequency" (RF) range was from 118 to 132 MHz. Channel spacing was set at 200 kHz intervals. In 1958, the first expansion occurred and was accomplished by reducing the channel spacing to 100 kHz. This change doubled the number of available channels. The following year, the upper end of the usable frequency range was extended from 132 MHz to 136 MHz. In 1964, the next change in channel spacing was made and decreased the 100 kHz to 50 kHz, and was again repeated in 1974 when spacing went to 25 kHz. Extension of the frequency range occurred again in 1979 when it went up to 137 MHz. Military communications can use the frequencies from 137 to 151.975 with 25 kHz spacing. Several areas in Europe have already established a need for additional channels.

By reducing the channel spacing to 8.33 kHz, the ability to triple the number of usable frequencies is realized. This change will most probably not be implemented in the United States; however, "N" registered aircraft wishing to operate in countries where 8.33 is in effect will need to comply or obtain a waiver. The equipment required for this voice link includes a transceiver or transmitter/receiver, antenna, microphone, audio panel, and speaker. In many cases, a headset can be used in place of a hand microphone and loud speaker.

The transceiver is where most of the action is. This device has eight separate functions to perform.

First of all, as a transmitter, a "carrier wave" has to be generated at a specific frequency and wavelength. Secondly, this carrier wave requires amplification so it has the ability to travel through the rest of the transmitter. Once the carrier is established, the next step is to couple the information to be sent on the wave. This step is called modulation. After the intelligence is added, the signal goes to the final power amplifier where the signal strength is determined and the modulated wave then leaves the transmitter.

The "receiver" section of the transceiver works almost in reverse of the transmitter. Once the signal has been received, it is directed into the tuner. It is here that only the selected frequency is recognized and passed further on. Signal strength at this point is quite weak and requires some amplification before being directed to the demodulator. In this area, the intelligence that was added to the carrier wave in the transmitter is removed. This intelligence is in a rather weak state and requires amplification before it can be used by any of the aircraft audio equipment such as headphones or speakers.

In 1985, the International Civil Aviation Organization (ICAO) made recommendations for improvements to allow immunity to specific levels of VHF FM broadcasts. The purpose was to reduce interference of commercial broadcast stations and aircraft radios. These recommendations are outlined in ICAO Annex 10.

Most VHF Communication
Transceivers will require a fairly simple change in order to comply with the recommended criteria. In some cases, adding a discrete filter to the RF input to the first stage of signal amplifier, and a minor circuit change to compensate for signal loss is all that is required. It is recommended that these modifications be accomplished by a qualified and authorized shop.

The ICAO set January 1, 1995 as the date where after no manufacturer could sell non-compliant equipment, and by January 1, 1998, all aircraft should be in compliance. It is important to understand that member states have the ultimate authority to set the date for FM Immunity to be implemented, and in most cases, more relaxed schedules have been adopted.

Transceivers can be considered "computers" — that is to say they receive and process information from a variety of sensors and provide an appropriate output. Some of the sensors needed for a communication radio to operate include a frequency selector, microphone, antenna coupler, and an antenna. The antenna could also be considered a load during the transmission phase of operation as a speaker or headphone could be during reception.

Flight line troubleshooting can be greatly enhanced by understanding the inputs and outputs of a transceiver. For example, in most commercial aircraft, the transmitter/receiver is connected to an audio panel. This provides selector switches to connect the output of the receiver to the audio amplifier to make the signal strong enough to be heard through the flight deck speakers. The fact that there are usually separate panels for both crew members, provides the ability to connect the output of the transceiver to either amplifier. So, if the audio signal is lost, an initial determination can be made simply by actuating switches if the problem is in the transceiver or audio amplifier.

Another means of analysis is to see if the signal can be heard on the headset. In many cases, headphones receive their input directly from the pre-amplified output of the receiver. So, if the headsets work and the cockpit speakers don't, the problem may be the audio amplifier and not the transceiver.

This same process applies to the transmitter. Most audio panels have selector switches for different audio inputs such as: pilot microphone, co-pilot microphone, and "boom mike" (microphone incorporated in a headset). Many audio panels also provide the ability to couple each microphone to a public address (PA) system. By coupling a suspect microphone to the PA system and attempting to broadcast, is one way to confirm a fault. If the mike will not supply the communication radio or the public address system, the microphone is most probably at fault. Many aircraft provide a quick disconnect plug for microphones as well as for headsets. Swapping these components from one audio panel to another is a common practice in troubleshooting. However, swapping a component with an internal short to another properly operating system could cause additional failures.

Before condemning a microphone or headset, it is usually a good idea to confirm the integrity of the quick disconnect electrical connector.

The microphone is a primary input to the Communication Transceiver and its job is to convert sound into an electrical signal. Sound is caused by a disturbance in air molecules and produces waves with frequencies of 20 to 20,000 cycles per second. As a person speaks, air molecules are compressed and refracted, and this causes the sound wave to travel from the point of origin at sea level and 20 degrees C, at a speed of 1,120 feet per second. Sound is then reproduced as these waves bounce against an eardrum, and then produce a vibration that the brain deciphers and makes intelligible. A microphone serves the same purpose as an eardrum. In a dynamic microphone, a flexible, corrugated diaphragm is attached to a movable coil. The coil will then interact with a small, fixed permanent magnet. As the sound waves contact the diaphragm, relative motion is produced between the coil and magnet. The result is a voltage that corresponds to the sound wave.

Crystal and ceramic microphones operate in a similar manner. Both use a piezo-electric principle — the sensor element vibrates at a specific frequency while an external force (in this case, a sound wave), is applied to the sensor at the rate the vibration changes. This produces a changing frequency, which is then delivered to the transmitter or audio amplifier. Each type of microphone has its own advantages and disadvantages. For example, a dynamic microphone typically has a rugged construction, high output levels, lightweight, wide frequency response, and low hum pickup.

A ceramic or crystal microphone is relatively inexpensive, provides a good frequency response and has a high output level. On the down side, these devices are more susceptible to mechanical shock damage and in some cases, may deteriorate in high temperature and humidity conditions.

Speakers work just in reverse of microphones. The final output from a receiver or audio amplifier is directed to a fixed coil. An iron-cored diaphragm is installed in conjunction with a paper cone. As the current through the coil changes, rapid movement is produced in the iron diaphragm, which in turn causes the paper cone to move a significant amount of air molecules — resulting in sound.

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