What is SELCAL?
Selcal is an acronym for SELECTIVE CALL. It's a technique that allows a ground radio operator to alert an aircrew that they wish to communicate with them. It works very similar to a paging device.
Selcal is an automatic recognition system that is operated by a two tone signal. The equipment is connected to the HF radios on aircraft and monitors for a call even when the squelch is turned up, and the pilots can hear nothing. This enables the pilots to have some aural peace when crossing the Atlantic or other oceans as HF radio can be very noisy. Selcals are made up of a four letter code and when heard have a distinctive bing-bong sound. As a flight enters the Oceanic FIR, a Selcal check is made the signal activates the on board Selcal receiver which alerts the pilots with a flashing warning light and an audible alarm.
Selcals are issued to airlines by the ARINC (Aeronautical Radio Inc) in USA, and with a total of only 10920 available codes, duplications are possible. This problem is overcome by allocating duplicate codes to aircraft operating in different parts of the world, so in theory they should never be working on the same frequency. If however, duplicate Selcals appear on the same frequency the problem is generally resolved by moving one of the flights to another frequency.
The Selcal is made up of two pair of tones, the first pair being transmitted for approximately 1 second, with the second pair transmitted for the same duration following a 0.2 second pause. The individual tone frequencies are designated by letters A - S excluding the letters "I" and "O". A typical Selcal code is AB-CD, which indicates that the frequencies designated by letters "A" and "B" would sent followed by the frequencies designated by letters "C" and "D". Duplicate letters are not permitted in either pair, since simultaneous transmission of two tones of the same frequency would not be distinguishable by the aircrafts Selcal decoder. Also, the same tone is nor permitted to be used in both the first and second pair. Each two-letter pair must have the first letter lower in the alphabet than the second, i.e. a code could be AB not BA.
Selcal codes have been in use on aircraft since around the late 1960's and are still in use and still being issued today.
Selcal codes are made up of 4 letters which consist of two two-letter pairs. A typical selcal code would be AB-CD or PQ-RS. Each letter represents a radio frequency :
A = 312.6 Hz
B = 346.7 Hz
C = 384.6 Hz
D = 426.6 Hz
E = 473.2 Hz
F = 524.8 Hz
G = 582.1 Hz
H = 645.7 Hz
J = 716.1 Hz
K = 794.3 Hz
L = 881.0 Hz
M = 977.2 Hz
P = 1083.9 Hz
Q = 1202.3 Hz
R = 1333.5 Hz
S = 1479.1 Hz
The letters 'I', 'N', 'O' and anything after the letter 'S' are not used.
Where and why are selcals used?
Selcals are mainly heard on HF radio frequencies.
HF radio frequencies are usually used by aircraft that are overflying large unpopulated areas like the oceans and deserts because VHF radio only has a maximum line-of-sight range of around 150 miles.
Because a time period of some 40 or 50 minutes can elapse between compulsory ATC reporting points when overflying oceans and deserts, it gets extremely tedious for the flight crews to constantly monitor the frequency in between times in case the ground controller may want to speak with them, hence where the selcal comes into play.
Once the selcal code has been tested with the ground controller, they can leave 'maintain a selcal watch', turn down the HF audio and remove their headsets.
How do SELCALs work?
The flight crew will call the ground controller on the pre-allocated HF frequency as they are entering their airspace and request a selcal check. The selcal code will be on a small placard on the flight deck somewhere, usually close to the aircraft's registration mark.
The flight crew will give the selcal verbally to the ground controller who in turn will enter the selcal letters into an encoder which converts the letters to the frequencies listed above. Once this is done, the ground controller presses the CALL button and the two tones (one for each pair of letters) are sent out in AM within a split second of each other, on the HF frequency that the ground controller is communicating with the flight crew on.
The tones sound like a 'bing-bong' on the airwaves and if the tones match those programmed into the selcal unit on the aircraft, a chime or gong will sound on the flight deck, alerting the flight crew that the selcal check was successful. If the chime or gong doesn't sound then the flight crew will ask the ground controller to send the tones again. If the selcal still refuses to work, the ground controller will inform the flight crew to 'maintain a listening watch' and they will have to keep their headsets on and endure the HF static, or try again in 10 minutes or so.
What aircraft are SELCAL-equipped?
All regular medium to long distance trans-ocean and trans-desert airliners are selcal-equipped plus thousands of other airliners which are used on short distance flights. Most military cargo and troop carrying jets are selcal-equipped, along with a good number of Lockheed Hercules props. All large business jets are selcal-equipped, along with a fair proportion of medium to small ones.
What happens on VATSIM?
As Microsoft Flight Simulator and X-Plane cannot emulate HF Traffic, VATSIM have acknowledged that users do complete many trans-Oceanic flights online so all approved Pilot Clients and ATC Clients have the ability to emulate SELCAL.