selcall

[alexban]

Hi
may I ask how does the selcall works? Is it necessary to have the HF station set on a specific freq or even ON? I didn't find it in the FCOM and i've been told you can be 'called' even if the station is off :hmm:

[mupepe]

you're correct, must be set to appropriate freq's and set to ON..

[alexban]

Any longhaul pilots around ?
As I've read the hf is scanning automatically on a range of frequencies ,so I was told that you'll get selcall ,even if you're not on a specific freq , is it right?
How about if you have the hf station off? Some guy said that selcall is based on a completely different station, which I doubt very much.
And kids , tech log forum it is supposed to be for technical questions and answers...go try cartoon network if you feel funny :ok:

[vapilot2004]

Selcal uses standard HF frequencies to contact specific flights by transmitting tones that are decoded in the aircraft. If the decoded tones match the aircraft's preset ID, a chime will sound and on newer aircraft an EICAS message will also appear. Usually a call indication on the audio panel will tell you which radio has the call.

HF frequencies will vary according to location, time of day and season. Selcal also can trigger via VHF when flying over (or near) land. Using the VHF system, a dispatcher can contact aircraft directly using a standard telephone. VHF frequency will depend on your location within the US and Canada. Not sure if VHF is used in Europe for Selcal.

Eventually Selcal will fall out of use as more and more operators have SATCOM systems fitted. As it stands now, Selcal is the cheapest and simplest (equipment-wise) of the two systems to own and operate, but SATCOM is generally more reliable.

Selcal requires some kind of an input (VHF or HF) so, yes the radios need to be on. :8

[Intruder]

've read the hf is scanning automatically on a range of frequencies ,so I was told that you'll get selcall ,even if you're not on a specific freq , is it right?
Nope. I know of no aircraft HF radios that scan.

Selcal is a set of tones that are transmitted on a specific active HF frequency after the aircraft checks in with the operator at the ground station. The receiver on the aircraft sounds the Selcal alter when that aircraft's tones are received. While the audio panel does not have to be set to listen to that frequency, the frequency must be tuned in the HF radio, and the receiver must be on.

[ICT_SLB]

SelCal (Selective Calling) uses two pairs of codes (originally Motorola tuned reed relays similar to the ones Ma Bell used to use to switch long distance calls). The 4 letter codes are assigned by your country's ICAO representative usually to a specific airframe but some operators such as OEMs may have common company code for test purposes.

The ground station transmits the code in two simultaneous bursts - say AB + CD - and, if you're tuned to that frequency and it's your code, you'll get a chime and possibly an EICAS message or light. Common installations have a total of up to five SelCal channels - 3 VHF & 2 HF is the most I've seen.

The main advantage is that, especially on HF on long hauls, you don't have to have the squelch so low to hear anything that the white noise gets to you although modern HFs are much better. BTW there are scanning (or at least frequency-hopping) military HFs.

[Flight Detent]

Just for Intruder....

Yes, there are HF radios that scan a preset range of frequencies, automatically looking for the best signal/noise ratio to communicate with, it's called ALE, Automatic Link Establishment.
If all the radios in the net have the same range selected, by prior arrangement, then they will all communicate on the same frequency, and that frequency will/can change as the signal/noise ratio changes.

Very neat really!

Cheers...FD...:cool:

[Intruder]

I haven't seen one on an airplane yet. Do you know any airlines that have them installed for HF voice?

[snips]

Maritime wise. Ships have DSC Digital Selective Calling which works very nicely. I find it amazing that we're stuck using SECAL when there are much better systems around (ALE/DSC):}

[paperdragon]

Sure VHF secall is used in Europe, we´re eqipped, and it works just fine!:8

[Dan Winterland]

The physics of how the system works is very Heath Robinson. A small transducer (speaker) is directed to the reeds. These resonate in sympathy to the chimes generated by the codes. When activated, they sound the alert. This is why you get spurios SELCALs when you are tuned to a commercial station which is playing music. The music produced tones which get the reeds vibrating. If the music has a sequence of notes approximating to your aircraft's call, the SELCAL will sound. One airframe I used to fly regularly used to have it's SELCAL triggered by the tune played by the BBC world Service prior to the 'pips'.

[ICT_SLB]

Dan,
I don't remember a speaker even in the old Motorola SelCal I did the install for many years ago - the signal through the reed caused it to switch IIRC. Modern units are microprocessor based (and have been for about 20 years) so the code setting for the individual aircraft is a just a matter of wiring up the correct program pins. BTW American, for one, uses VHF SelCal on its regionals - at least it's on their CRJ-700s. The unit we fit is all contained in the switch panel that selects which VHF (or HF if installed) you want to monitor.

[Dan Winterland]

The system I described was original equipment as fitted to the VC10. Don't know who made it, but I suspect being British it was probably Smiths.

The triggering from music happens to modern units too. It happened to me in an Airbus recently.

[blue_side_up]

Aviation Spectrum Resources Inc, of the US, now manages all worldwide SelCal code assignments on behalf of ICAO. Aircraft are issued with a 4-letter code that generally stays with them until the aircraft is retired. A shortage of possible code letter combinations means that codes are recycled (who says aviation isn't green?), and it's entirely possible that more than one aircraft worldwide could have the same SelCal letter code assignment simultaneously.
http://www.asri.aero/about_asri.html