SELCAL

Key takeaways

SELCAL belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SELCAL to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SELCAL from memory before moving on to harder problems.

Reference excerpt

In international aviation, SELCAL or SelCal is a selective-calling radio system that can alert an aircraft's crew that a ground radio station wishes to communicate with the aircraft. SELCAL uses a ground-based encoder and radio transmitter to broadcast an audio signal that is picked up by a decoder and radio receiver on an aircraft. The use of SELCAL allows an aircraft crew to be notified of incoming communications even when the aircraft's radio has been muted. Thus, crewmembers need not devote their attention to continuous radio listening.

Use SELCAL operates on the high frequency (HF) or very high frequency (VHF) radio frequency bands used for aircraft communications. HF radio often has extremely high levels of background noise and can be difficult or distracting to listen to for long periods of time. As a result, it is common practice for crews to keep the radio volume low unless the radio is immediately needed. A SELCAL notification activates a signal to the crew that they are about to receive a voice transmission, so that the crew has time to raise the volume. An individual aircraft has its own assigned SELCAL code. To initiate a SELCAL transmission, a ground station radio operator enters an aircraft's SELCAL code into a SELCAL encoder. The encoder converts the four-letter code into four designated audio tones. The radio operator's transmitter then broadcasts the audio tones on the aircraft's company radio frequency channel in sequence: the first pair of tones are transmitted simultaneously, lasting about one second; a silence of about 0.2 seconds; followed by the second pair of tones, lasting about one second. The code is received by any aircraft receiver monitoring the radio frequency on which the SELCAL code is broadcast. A SELCAL decoder is connected to each aircraft's radio receiver. When a SELCAL decoder on an aircraft receives a signal containing its own assigned SELCAL code, it alerts the aircraft's crew by sounding a chime, activating a light, or both. The crew next turns up the volume on the aircraft radio to hear the incoming voice transmission. Using ICAO radio protocol, they must verify with the transmitting operator that they are the intended message recipients. The crew then uses the received information. SELCAL details are defined in the latest version of ARINC Characteristic 714A.

Code registration

An individual aircraft is given a SELCAL code upon application to the SELCAL code registrar, Aviation Spectrum Resources, Inc. (ASRI). The code is technically assigned to the owner-operator of the aircraft rather than the aircraft itself; if an aircraft is sold, the new owners-operators must apply for a new code. The code is a sequence of four letters, written or transmitted as an ordered two sets of two letters each (e.g. AB-CD). The letters are chosen from a subset of the Latin script comprising A through S, excluding I, N and O. The letters within a given pair are written or transmitted in alphabetical order (e.g., AB-CD is an allowable distinct SELCAL code, as is CD-AB, but CD-BA is not). A given letter can be used only once in a SELCAL code; letters may not be repeated (e.g. AB-CD is allowable, but AA-BC and AB-BC are not). Each letter designates a specific audio tone frequency.

Limitations The rules for SELCAL code assignment, with sixteen available letters/tones, limit the number of possible allowable codes to 10,920. Additionally, SELCAL codes assigned previously use a subset of only twelve letters/tones. Therefore, more than one aircraft may be designated by the same code. To avoid confusion from two or more aircraft using the same SELCAL code, ASRI tries to assign code duplicates to aircraft that do not usually operate in the same region of the world or on the same HF radio frequencies. However, aircraft commonly move between different geographical regions and it is now routine for two aircraft with the same SELCAL code to be found flying in the same region. Therefore, air crew always verify both SELCAL and call sign (i.e., aircraft tail registration, or telephony designator and flight identification) to be sure their aircraft is the intended recipient.

SELCAL 32 The SELCAL 32 Expansion Project was started after October 2018 to increase the number of possible allowable codes, and was approved by ICAO for full implementation in November 2022. The SELCAL 32-tones were fully launched for use by airline operators in November 2023. SELCAL 32-tones are numerical-only codes, compared to the prior 12-tone and 16-tone codes being alphanumeric.

See also Aircraft Communication Addressing and Reporting System Airline teletype system Communications satellite

References

External links Online searchable selcal database from AirFrames.org About Selective Calling (SELCAL) from Aviation Spectrum Resources, Inc. SELCAL on SKYbrary IVAO: SELCAL Information What are SELCALs? from Selcal.co.uk SELCAL 32 Expansion Project from Aviation Spectrum Resources, Inc.

Worked examples

Example 1 — a first encounter with SELCAL

Start with the simplest possible case. Write down what SELCAL claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SELCAL before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SELCAL ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SELCAL

In research: SELCAL appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SELCAL in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SELCAL is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aviation communications, so understanding it makes those chapters shorter.
In everyday life: Look for SELCAL outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study SELCAL in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SELCAL means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SELCAL out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SELCAL in simple terms?

In international aviation, SELCAL or SelCal is a selective-calling radio system that can alert an aircraft's crew that a ground radio station wishes to communicate with the aircraft. SELCAL uses a ground-based encoder and radio transmitter to broadcast an audio signal that is picked up by a decoder…

Why does SELCAL matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SELCAL?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SELCAL.