ATC Reference

ICAO Phonetic Alphabet

NATO/ICAO standardised spelling alphabet for all aviation communications. Click a letter to highlight it, or use the converter to spell any word phonetically.

Letters A – Z

Numbers 0 – 9

ICAO ATC pronunciation — differs from everyday speech

Transponder & Squawk Codes

Discrete and special-purpose Mode A transponder codes used in ATC worldwide. SSR interrogates on 1030 MHz; aircraft reply on 1090 MHz. Codes are four octal digits (0–7), giving 4,096 unique codes. Click any row for expanded context.

Code	Meaning	Region	Status

Standard Radio Phraseology

ICAO standard callouts and responses organised by flight phase. Click Copy to copy the phrase to clipboard.

Airspace Classes & Flight Levels

Seven ICAO airspace classes (A–G) define what services ATC provides and who can fly there. Flight Levels, RVSM rules, and hemispherical assignment are covered below.

← Select an airspace class

Flight Levels Explained

What FL numbers mean, how altitude and pressure relate, RVSM separation rules, and a full guide to common cruise levels.

What is a Flight Level?

A Flight Level (FL) is a standard pressure altitude expressed in hundreds of feet, referenced to 1013.25 hPa (29.92 inHg) — the International Standard Atmosphere sea-level pressure. FL200 = 20,000 ft on standard pressure, regardless of actual air density or local QNH.

Transition Altitude & Level

Below the Transition Altitude (TA), pilots set local pressure (QNH) and read altitude in feet. Above the Transition Level (TL), all aircraft set 1013 hPa and read in FLs. The band between them is the Transition Layer. TA varies by country — FL180 in the USA, as low as 3,000 ft in parts of Europe.

RVSM Airspace

Between FL290 and FL410, Reduced Vertical Separation Minima (RVSM) applies: vertical separation is 1,000 ft instead of 2,000 ft, doubling capacity. Aircraft must be RVSM-approved with accurate auto-pilots and dual altimeters. Non-RVSM aircraft require special ATC coordination.

Hemispherical Rules

In ICAO states, flight direction determines available FLs. Eastbound (000°–179°): odd FLs — FL290, FL310, FL330… Westbound (180°–359°): even FLs — FL280, FL300, FL320… NAT tracks and some regions use different assignment schemes. Always follow the published local rules.

How to Read a Flight Level

Type any FL number — watch it decode live

FL

FL200

→

200 × 100

→

20,000 ft

              FL000FL150FL300FL450FL600
            

Altimeter set to 1013.25 hPa / 29.92 inHg — standard pressure (QNE)

Tap an example

Below Transition Altitude

Altimeter → local QNH
Read in feet (ft)
5,000 ft

Above Transition Level

Altimeter → 1013.25 hPa
Read as Flight Level
FL050

Altitude Bands at a Glance

Major zones from surface to cruise — click any band for details

Common Flight Levels Reference

Standard cruise and operating levels with typical users and notes

FL	Altitude (ft)	Zone	Typical Users & Notes

Hemispherical Level Assignment

ICAO odd/even rules by direction — RVSM and non-RVSM bands

ATC Facilities & Services

Each ATC facility handles a specific phase of flight. Understanding who does what — and their typical frequency ranges — is essential for every aviator.

Wake Turbulence Categories

ICAO wake turbulence categories define minimum separation requirements between aircraft. Select a category to see separation minima.

← Select a wake category

IFR Clearances — CRAFT

The CRAFT mnemonic helps pilots remember all elements of an IFR departure clearance. Hover each part of the sample clearance to identify its component.

Sample IFR Clearance — Interactive Breakdown

Move your cursor over any coloured part of the clearance below to see what it means

ATC Transmission

Move over a highlighted segment to see its meaning.

ATC Knowledge Quiz

Test yourself on phonetics, squawk codes, airspace classes, wake turbulence, the CRAFT mnemonic, and flight levels.

Select topics

Number of questions

🎙 Pronunciation Coach

Practise real ATC phraseology out loud. You get a random sentence each time — read it, and the coach checks every word and tells you what to fix.

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