Turbine Pilot

The turbine set is never out of touch

The good thing about jets is that they go fast and far. The bad thing about jets, if you need to keep in touch with one, is that they go fast and far. VHF voice communication is limited to line-of-sight distances. UHF radios can work over thousands of miles, but voice quality can sound like muffled shouting from the bottom of a mine shaft. And some things just don't lend themselves to voice communications — engine-monitoring reports, for instance — that might contain dozens of fields of alphanumeric data per engine. Fortunately, there is help: ACARS and SELCAL.

Short for Aircraft Communications Addressing and Reporting System, ACARS is a digital data system that allows two-way communications between participating aircraft and central ground stations. Operated by ARINC, ACARS incorporates a network of more than 600 dedicated ground-based VHF stations, as well as the Inmarsat satellite network and some additional UHF stations. In total, they allow for airborne communication from almost anywhere in the world.

On-board equipment includes an ACARS management unit (MU) and a control unit (CU). The CU contains an alphanumeric keypad and a display screen. Most ACARS installations include a printer. A crewmember merely has to key in a message and hit the Send key. The MU auto-tunes the correct area frequency and transmits the message on that frequency. Some preformatted messages are sent or received automatically without any crew involvement whatsoever. Depending on the installation, these can include aircraft out, off, on, and in times (called OOOI reports), engine-monitoring reports, and ATC predeparture clearances.

Air-to-ground data sent through ACARS is received by the appropriate remote station, then sent by landline or satellite transmission to ARINC's Annapolis, Maryland, headquarters. There it is processed by ARINC's computers and relayed to the recipient, be that a dispatch office or an ATC facility. Return messaging works in reverse.

ACARS is used to send maintenance updates, weight and balance information, position reports, weather data, and a host of other information. It currently handles most commercial air/ground datalink traffic in the world — more than 10 million messages per month and growing, according to ARINC. All of this saves time, relieves frequency congestion, enhances safety, and saves operators money, too, by facilitating more timely and accurate information exchange.

Airlines account for the bulk of high- volume ACARS message traffic, but nearly 1,500 corporate aircraft are also ACARS-equipped. A typical GA installation such as the Orbital Sciences Magellan CNS-12 costs around $20,000. ARINC's low-message-volume corporate subscriptions start at $125 per month, adequate for many small to medium flight departments, and go up in price from there.

Interestingly, a whole new hobby for radio enthusiasts is evolving — the monitoring, downloading, and decoding of ACARS transmissions using scanners, a PC, and appropriate decoding software.

SELCAL, or Selective Calling, is another tool for communicating with aircraft. It is especially useful for flights that are out of radar contact in oceanic or other remote airspace. Each aircraft's SELCAL installation is assigned a unique four-letter code. Ground-based facilities equipped to do so can interrogate the SELCAL unit by transmitting the four letters for that particular aircraft, coded in the form of audio tones. If the on-board decoder recognizes the signal, it sounds a distinctive chime in the cockpit, alerting the crew to establish voice contact on the frequency in use. A number of commercial companies provide SELCAL relay services.

Usual procedure on long oceanic flights is for the crew to request a functional check of the SELCAL system from the service provider early in the flight. If it works, they no longer need to monitor the UHF frequency, other than when making required position reports. If ATC or a company dispatcher needs to communicate with the flight, they initiate a SELCAL contact through the provider. Normally the message is simply relayed to the flight, but a telephone patch can also be established that allows the crew to speak directly to the other party. Unlike ACARS, crews must manually tune the correct UHF frequency on the radio used by the SELCAL system. Often, just two or three UHF frequencies will provide SELCAL coverage for an entire long-range flight.