DALLAS — Every day, more than 200,000 aircraft take off or land at airports around the world. This number has been increasing as the years go by, so there has always been a need to create worldwide rules and methods to keep the skies safe from accidents.

One of the most delicate and crucial aspects to keep an eye on is the communication of messages between pilots and air traffic controllers.

Usually, aircraft are separated from each other to avoid collisions, so it is very important to develop a series of procedures for ATC communications that all pilots and air traffic controllers must follow in order to deliver any information as clearly and efficiently as possible.

Certain departments of the International Civil Aviation Organization (ICAO) or the Federal Aviation Administration (FAA) have been dedicating all of their lives exclusively to developing this norm.

Here are some examples of the rules and methods that have been implemented that show how important standardization in global aviation communications is.

Pronunciation

Even though the official language recognized and promoted by international aviation authorities for air traffic control communications is English and all pilots must demonstrate a broad knowledge of speaking and understanding it, ethnic and cultural identities profoundly impact the pronunciation of certain words and terms regarding aviation phraseology.

Dialects and accents are key aspects to consider in critical situations where a clear understanding between pilots and air traffic controllers is very important to avoid major incidents.

Because of that, the ICAO and NATO created the Radiotelephony Spelling Alphabet (RSA), which linked a unique and differentiated word to every one of the 26 letters of the Roman alphabet. That’s why you will never hear an aircraft communicate to the control tower that it is taxiing via the taxiway “A2”, but “ALPHA TWO” instead.

A specific rule for some numbers has also been implemented, where the pronunciation of some digits has been changed to avoid confusion with similar terms. The number nine, for example, is pronounced “NINER” to not get mistaken for “NEIN”, which means “no” in German.

ICAO also indicates a specific method of pronunciation for indicating altitudes and headings. These are two parameters that are typically transmitted to the traffic in the same transmission from ATC, mostly during the departure or approach phases of the flight.

To differentiate them, every heading transmitted is pronounced with three digits, where heading 90º is "ZERO-NINER-ZERO." In comparison, the altitude shall be pronounced either directly in feet or in “flight level” with two or more digits.

Similarly, an aircraft climbing to a height of 9000 feet communicates its level as “NINER THOUSAND FEET” or “FLIGHT LEVEL NINE-ZERO,” depending on the local procedures of the airport.

Further, any altitude that is rounded to the nearest hundred is pronounced as a whole number, not separately. In that case, an aircraft flying at 30000ft does so at “FLIGHT LEVEL THREE HUNDRED,” not “FLIGHT LEVEL THREE-ZERO-ZERO.”

Instruction Order

Air traffic controllers experience a high workload. As a result, the instructions they give to aircraft usually contain plenty of important information combined into one phrase that is then read back by the pilots. As such, it is a common practice acquired by ATC to rephrase any critical instruction and rearrange the information according to its importance, from the least to the most critical.

Data that has been given before or that can be double-checked always goes first. It is then followed by temporal and spatial orientation, ending finally with the specific instruction, which is the most important piece of information the pilot should understand and read back.

For example, in most European airports, the takeoff clearance for a flight starts with the callsign of the traffic, followed by a wind and SID check, the active runway for departure, and finally, the takeoff clearance: “AIR FRANCE ONE-EIGHT-ZERO-ZERO, CONFIRM SID NANDO TWO-ROMEO WITH WIND CALM; RUNWAY THREE SIX LEFT, CLEARED FOR TAKEOFF.”

In certain situations, an instruction given by an air traffic controller is influenced by events surrounding the aircraft. These are called “conditionals” and happen, for example, when a flight asks for pushback from its gate at the same time as another aircraft is taxiing behind it. In those cases, if ATC wants to clear that flight for pushback, it must be certain that it happens after the taxiing aircraft behind clears the space.

To do so, ATC adds the word “behind” at the beginning and end of the communication to specify the correct moment the flight should start pushing back: “AIR FRANCE ONE-EIGHT-ZERO-ZERO, BEHIND THE TAXIING TRAFFIC, PUSHBACK IS APPROVED, BEHIND.”

Frequency Time Saving

A very common phrase used in commercial aviation is "The frequency is more valuable than gold." This is because the frequency that space pilots and air traffic controllers use for communication, called frequency, is often saturated by constant incoming messages, mostly during the busiest hours of operations at a certain airport. This can lead to big issues when an aircraft suffers an emergency and requires the complete attention of ATC to express its situation and receive instructions.

In most cases, the frequency at which the emergency is developing adds an additional air traffic controller to operate exclusively with and for this traffic, but this is not an everyday situation. To prevent frequency saturation, the ANSPs have also developed two crucial rules that must be applied by air traffic controllers, but most importantly by pilots, in order to create a safe and efficient method of communication.

Firstly, any person communicating at this frequency must avoid, as much as possible, the use of conjunctions or prepositions when transmitting a message, as well as including any information that is irrelevant or can be told in future communications. For example, it is recommended for pilots to communicate "DESCEND FLIGHT LEVEL TWO-TWO-ZERO, AMERICAN ONE-ONE-FIVE" instead of "WE WILL DESCEND TO FLIGHT LEVEL TWO-TWO-ZERO, AMERICAN ONE-ONE-FIVE".

This is done to keep the frequency free of communications for as long as possible. In ATC services such as the delivery position, where such data is provided to pilots, this gesture is highly appreciated both by pilots and air traffic controllers.

Secondly, during peak hours, often the air traffic controller will instruct an aircraft leaving for a busier ATC dependency to "monitor next frequency", meaning that the pilot must switch to the next frequency but cannot make the first contact, even though it has been transferred. This is done to not interrupt any ATC operator during a moment of high workload, but to let him call the new incoming traffic once there is a free space for new communications.

This practice is seen especially in the approach (APP) phases of a flight, where the high concentration and precision required of the controller are contrasted with the high load of landing aircraft.

The Correct Use of Words

History has taught us that the correct use of words is very important to avoid accidents. This was seen after March 25, 1977, when the ground collision of two Boeing 747s at Tenerife-Los Rodeos Airport claimed the lives of 583 people. It is the deadliest commercial aviation disaster in history, and it happened partially due to bad word choice during communications.

While the Pan Am Boeing 747 was backtracking on the active runway for departure, the KLM Boeing 747 was already positioned at the runway threshold, ready to perform the takeoff.

As the KLM crew requested their clearance, the air traffic controller first informed them of the flight path they should have taken after the departure: “KLM8705, cleared to the Papa-beacon, climb to and maintain flight level nine-zero, right turn after takeoff."

However, at the same time that the ATC was communicating this information to the KLM flight, the pilots of the Pan Am Boeing 747 also communicated to the tower that they were still taxiing on the runway and that they had not cleared it yet.

Ironically, this critical information given by the Pan Am flight created an interference with the one given by Tenerife Tower, which blocked the message, and the KLM pilots possibly only ended up hearing “KLM8705, cleared to... takeoff," which was unluckily interpreted as a takeoff clearance while the other Boeing 747 had not cleared the runway.

After the investigation, authorities released a new rule, applicable worldwide, that no other instruction than a takeoff or landing clearance shall use the words “takeoff” or “landing” to avoid misunderstandings.

Instead, synonyms such as “departure” or “approach” are allowed for use when a controller or pilot wants to inform any intention relative to this phase of the flight, such as a flight level change or heading.

Featured image: Featured image: Adrian Nowakowski/Airways