Communication in Aviation: Phraseology, Radios, and Readback

Aviation communication protocols are the standardized rules that govern how pilots, air traffic controllers, and ground personnel exchange information. They cover the words used, the order those words come in, the technology that carries them, and the procedures that take over when normal communication breaks down. The framework is built from ICAO phraseology, a defined chain of ATC facilities, mandatory readbacks, radio and datalink systems, and specific emergency and failure procedures.

Standardized Language and Phraseology

The International Civil Aviation Organization sets the global standards for how pilots and controllers talk to each other. The goal is that the same words mean the same things anywhere in the world. ICAO phraseologies are designed to be “efficient, clear, concise, and unambiguous,” and controllers and pilots are expected to use them whenever they apply.¹

English is not the only language permitted on aviation frequencies. ICAO allows ground stations to use their local language for radiotelephony communications, but English must always be available as a common fallback. All pilots, controllers, and aeronautical station operators must demonstrate proficiency in the language used on the frequencies where they work.² English dominates international aviation in practice because it acts as the shared language when a pilot and controller do not speak the same native tongue.

The Phonetic Alphabet and Number Pronunciation

Radio static, engine noise, and accents can make ordinary letters and numbers hard to distinguish. ICAO addresses this with a phonetic alphabet that assigns a unique word to every letter: “Alfa” for A, “Bravo” for B, “Charlie” for C, and so on. Pilots use this system when identifying their aircraft on initial contact with any ATC facility.³ Numbers follow the same logic. The digit three is pronounced “tree,” five becomes “fife,” and nine is “niner,” all to prevent confusion between similar-sounding numbers.⁴

Response Words That Actually Mean Different Things

Aviation phraseology includes specific response words that each carry a distinct meaning, and mixing them up causes real problems. “Roger” means only that you received the transmission. It does not mean you understood it, agree with it, or plan to follow it. “Wilco” means “I will comply” and is used when you intend to carry out an instruction. “Affirm” is the aviation equivalent of “yes.” Responding “Roger” to a critical clearance when the controller needs confirmation of compliance leaves a dangerous gap in understanding.

Who Pilots Talk To on the Ground and in the Air

ATC facilities are structured around the phases of flight. A single trip from gate to gate involves handoffs between several controllers, each responsible for a defined piece of airspace or airport surface.

Clearance Delivery issues the route clearance before pushback, including the assigned departure procedure, initial altitude, and transponder code.
Ground Control handles all aircraft and vehicle movement on the taxiways, and must coordinate with Local Control before authorizing any aircraft to cross or use an active runway.⁵
Local Control, better known as the Tower, has primary responsibility for the active runways and the airspace immediately surrounding the airport, and clears aircraft for takeoff and landing.⁵
Approach and Departure Control manages arriving and departing traffic in the terminal airspace around the airport, sequencing aircraft between the airport environment and the en-route structure.
Center, or En-Route Control, handles aircraft at higher altitudes as they travel between terminal areas, often covering vast geographic regions.

At each handoff, the pilot establishes contact on a new frequency, confirms receipt of current airport information at terminal facilities, and acknowledges the new controller’s instructions.

ATIS: The Recorded Briefing

Before contacting Approach or Ground Control at a busy airport, pilots tune to the Automatic Terminal Information Service, a continuously looping recorded broadcast that covers current weather, active runways, and instrument approaches in use. Each update is assigned a phonetic letter code (for example, “Information Bravo”). Pilots listen to the broadcast, then tell the controller which code they have on initial contact. That single letter confirms the crew already knows the basics and frees the frequency for actual control instructions.⁶

How a Radio Call Is Structured

The FAA prescribes a five-part format for initial radio contact to keep transmissions brief and organized:⁷

The facility name being called, such as “Boston Tower.”
The aircraft identification, meaning the full callsign as filed in the flight plan.
The aircraft’s position, particularly on the airport surface (“at gate 14” or “holding short runway 27”).
The request or message type, such as “request taxi to runway 33 Left” or “ready for departure.”
“Over,” when required to indicate the transmission is complete and a response is expected.

After initial contact is established, subsequent calls from the same position can drop the facility name. Transmissions from the controller mirror the same structure, leading with the aircraft identification.⁸

Readback: Closing the Loop

Readback is where communication stops being one-directional. When a controller issues a clearance containing an altitude assignment, a heading vector, or a runway assignment, the pilot is expected to repeat those elements back verbatim. The controller then listens for accuracy and corrects any errors.⁹

Pilots read back altitudes, altitude restrictions, vectors, and runway assignments in the same order the controller gave them, and always include their aircraft identification so the controller knows the right crew received the clearance. This matters most on congested frequencies where aircraft with similar-sounding callsigns are operating at the same time.⁹ When a pilot just says “Roger” to a complex clearance, the controller has no way to verify the pilot actually heard what was transmitted.

Radio and Datalink Technology

VHF and HF Radio

Very High Frequency radio is the workhorse of aviation communication. Nearly all air-to-ground voice contact within line-of-sight range uses VHF, operating in the 117.975–137.0 MHz band.¹⁰ The limitation is physics. VHF signals travel in straight lines and cannot bend around the curve of the Earth, so range depends on altitude. An aircraft at cruise may reach a VHF station over 200 miles away, while an aircraft on the ground may only get 20 or 30.

Over oceans and remote areas where VHF coverage disappears, aircraft switch to High Frequency radio or satellite communications. HF signals can bounce off the ionosphere and follow the Earth’s surface, reaching across thousands of miles, though audio quality is significantly lower.¹⁰

ACARS and CPDLC

Digital datalinks supplement voice communication and reduce congestion on busy frequencies. The Aircraft Communications Addressing and Reporting System has been in use since 1978 and handles a wide range of messages between aircraft and ground stations, including ATC clearances (particularly pre-departure and oceanic clearances), airline operational messages like load sheets and weather updates, and technical data such as engine performance downloads. ACARS originally relied on VHF channels but has expanded to satellite and HF data links for global coverage.¹¹

Controller-Pilot Data Link Communications is a more structured system built specifically for ATC. CPDLC provides a standardized set of message types for route clearances, altitude assignments, speed instructions, and crossing constraints. In oceanic airspace, where VHF coverage is unavailable, CPDLC serves as the primary means of communication between pilots and controllers, with voice over HF or satellite as backup.12 Text-based clearances eliminate the “did I hear that right?” problem. The instruction sits on a screen for the crew to review, accept, and reference later.

ADS-B

Automatic Dependent Surveillance-Broadcast is a surveillance technology rather than a two-way communication system, but it has changed how aircraft convey their position. Aircraft equipped with ADS-B Out continuously broadcast their GPS-derived position, altitude, speed, and identification. ATC and other equipped aircraft can receive this data directly, providing more accurate position information than traditional radar. Since January 1, 2020, the FAA has required ADS-B Out equipment in most controlled airspace, including Class A, B, and C airspace and Class E airspace at and above 10,000 feet, with additional geographic requirements around Class B airports and over the Gulf of Mexico.¹³

Emergency Communication

Aviation recognizes two levels of emergency communication, and using the right one matters. A “Mayday” call signals a distress condition where the aircraft or its occupants are in grave and imminent danger. A “Pan-Pan” call signals an urgency condition that is serious but does not pose an immediate threat to life. Both signals are spoken three times at the start of the transmission so no one mistakes them for routine traffic.¹⁴

A Mayday call commands radio silence on the frequency. All other aircraft and ground stations must stop transmitting and yield the frequency to the aircraft in distress. A Pan-Pan call has priority over everything except a Mayday. After the initial signal, the pilot transmits identifying information including aircraft type and callsign, the nature of the emergency, current position and heading, fuel remaining in minutes, and the number of people on board.¹⁴

Emergency Transponder Codes

Alongside voice declarations, pilots communicate emergencies silently through transponder codes. Setting the transponder to 7700 signals a general emergency and triggers an alert on ATC radar displays. Code 7600 indicates a communications failure, telling controllers the aircraft has lost radio contact. Code 7500 signals unlawful interference, meaning a hijacking. These codes get critical information to ATC even when voice communication is impossible or compromised.¹⁴

The Emergency Frequency

The frequency 121.5 MHz is the international aeronautical emergency frequency. All aircraft operating in U.S. airspace are required to maintain a listening watch on it. ICAO extends this requirement to aircraft on long over-water flights and in areas where interception or other hazardous situations may occur. Ground-based aeronautical stations must also monitor 121.5 MHz continuously during their hours of operation.¹⁵ The shared frequency provides a last-resort channel when a pilot cannot reach anyone on the assigned frequency.

What Happens When the Radio Fails

Losing radio contact in flight, known as a NORDO situation, is one of the scenarios pilots train for repeatedly, because the entire ATC system depends on two-way communication. The procedures differ sharply depending on whether the pilot is flying visually or on instruments.

VFR Conditions

If radio failure occurs in visual meteorological conditions, or if a pilot flying on instruments encounters visual conditions after the failure, the rule is straightforward: continue the flight under visual flight rules and land as soon as practicable. “Practicable” does not mean the nearest patch of pavement. It means a suitable airport within a reasonable distance.¹⁶

IFR Conditions

When radio failure happens in instrument conditions and the pilot cannot transition to visual flight, the situation is more complex. The pilot follows a specific hierarchy for both route and altitude.¹⁶

For route, the pilot flies the route assigned in the last clearance received. If being radar-vectored at the time of failure, the pilot flies direct to the fix specified in the vector clearance. If no route was assigned, the pilot follows whatever route ATC had advised to expect. If none of those apply, the pilot reverts to the route filed in the flight plan.

For altitude, the pilot maintains the highest of three values: the altitude in the last clearance, the minimum altitude for IFR operations on that route segment, or the altitude ATC had advised to expect. Flying the highest applicable altitude keeps the aircraft above terrain and above other IFR traffic that ATC may be separating below. The pilot should also set the transponder to code 7600 to alert controllers to the radio failure.¹⁶

Light Gun Signals

When all else fails, controllers can communicate with aircraft visually using a focused light gun from the control tower. Each color and pattern carries a specific meaning:¹⁷

Steady green means cleared for takeoff on the ground, or cleared to land in flight.
Flashing green means cleared to taxi on the ground, or return for landing in flight.
Steady red means stop on the ground, or give way to other aircraft and continue circling in flight.
Flashing red means taxi clear of the runway on the ground, or airport unsafe, do not land in flight.
Flashing white means return to starting point on the airport (ground only).
Alternating red and green means exercise extreme caution, both on the ground and in flight.

Two Accidents That Shaped the Rules

The consequences of communication breakdowns are not theoretical. Two of the most studied accidents in aviation history trace directly to failures of language, phraseology, or assertiveness on the radio.

In 1977, two Boeing 747s collided on the runway at Tenerife in the Canary Islands, killing 583 people in the deadliest accident in aviation history. The captain of one aircraft began his takeoff roll without receiving a takeoff clearance, using the ambiguous phrase “we are now at takeoff,” which the controller interpreted as a position report rather than a declaration of action. Heavy fog, frequency congestion, and non-standard language all contributed. The accident drove sweeping changes to ATC phraseology, including stricter rules around the word “takeoff,” now reserved for the actual clearance to depart.

In 1990, Avianca Flight 052 ran out of fuel and crashed on Long Island after the crew failed to use the word “emergency” in any of their communications with New York ATC. The crew repeatedly told controllers they were “running low on fuel,” but never used the specific terminology that would have triggered priority handling. The NTSB found that “the flightcrew’s failure to use the word ’emergency’ was a significant factor in the controller’s failure to understand the critical nature of the flight’s fuel status.”¹⁸

Beyond the ATC Frequency

Crew Resource Management

Communication protocols do not stop at the radio. Inside the cockpit, Crew Resource Management training shapes how pilots talk to each other. The FAA defines CRM as “the effective use of all available resources: human resources, hardware, and information,” and it focuses on situation awareness, communication skills, teamwork, and decision-making. CRM is required for crews operating under Part 121 scheduled airlines and is built into training for Part 135 operators and fractional ownership programs.¹⁹

The core principle is that rank should never prevent a crew member from speaking up about a perceived error. Both pilots are expected to call out deviations, challenge assumptions, and confirm shared understanding of the aircraft’s state during every phase of flight.

Ground Operations

On the ground, the flight crew coordinates with personnel operating on entirely different systems. Ramp controllers, maintenance crews, fueling teams, and pushback drivers typically communicate via interphone (a direct aircraft-to-ground intercom connection), dedicated ground frequencies, or standardized hand and wand signals for marshalling. These ground communications happen outside the ATC system but are equally critical during pushback, engine start, and taxi in congested ramp areas.

Licensing and Proficiency Requirements

English Language Proficiency

Anyone applying for an FAA pilot certificate, mechanic certificate, air traffic control authorization, or remote pilot certificate must demonstrate the ability to read, speak, write, and understand English. The FAA benchmarks this against ICAO’s Operational Level 4 standard, which evaluates pronunciation, vocabulary, fluency, comprehension, and the ability to handle unexpected situations in conversation. Level 4 does not require flawless English. It requires that a pilot be understood by the aeronautical community, communicate effectively on work-related topics, and resolve misunderstandings through clarification. Applicants who cannot meet the standard due to a medical condition affecting hearing or speech may still qualify for a certificate with specific operational limitations.²⁰

FCC Radio Operator Permit

Pilots flying internationally face an additional licensing requirement. The FCC requires a commercial radio operator license for aircraft radio stations, except those operating only on VHF frequencies without making foreign flights. Any pilot flying outside the United States needs a Restricted Radiotelephone Operator Permit. Domestic-only VHF operations are exempt.²¹

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International Civil Aviation Organization. Manual of Radiotelephony
2
SKYbrary Aviation Safety. English Language Proficiency Requirements
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FAASafety.gov. Radio Communications Phraseology and Techniques
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SKYbrary Aviation Safety. ICAO Phonetic Alphabet
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Federal Aviation Administration. Airport Traffic Control – Terminal
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Federal Aviation Administration. Aeronautical Information Manual – Automatic Terminal Information Service (ATIS)
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Federal Aviation Administration. Aeronautical Information Manual – Radio Communications Phraseology and Techniques
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Federal Aviation Administration. Air Traffic Control – Radio and Interphone Communications
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SKYbrary Aviation Safety. Air-Ground Communication
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SKYbrary Aviation Safety. Aircraft Communications, Addressing and Reporting System>
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Federal Aviation Administration. Oceanic Controller Pilot Data Link Communications (CPDLC)
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eCFR. 14 CFR 91.225 – Automatic Dependent Surveillance-Broadcast (ADS-B) Out Equipment and Use
14
Federal Aviation Administration. Aeronautical Information Manual – Distress and Urgency Procedures
15
SKYbrary Aviation Safety. Guarding 121.5 MHz
16
eCFR. 14 CFR 91.185 – IFR Operations: Two-Way Radio Communications Failure
17
Federal Aviation Administration. Air Traffic Control – Visual Signals
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National Transportation Safety Board. Aircraft Accident Report – Avianca Flight 052
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Federal Aviation Administration. AC 120-51E – Crew Resource Management Training
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Federal Aviation Administration. AC 60-28B – FAA English Language Standard for an FAA Certificate
21
Federal Communications Commission. Commercial Radio Operator License Program