What controllers and pilots are actually saying. 46 terms you will hear across the archive, explained in plain English.
The radar unit handling aircraft descending toward an airport, typically within about 40 miles. It sits between en-route control and the tower.
Automatic Terminal Information Service — a looped broadcast of weather, runway in use and field conditions. Crews listen before contacting a controller so the frequency is not spent reading it out.
Two aircraft transmitting at once, so neither is heard clearly. Controllers ask stations to repeat. On busy frequencies it happens constantly and is a genuine safety issue.
En-route control, handling aircraft at cruise between airports across large blocks of airspace. In Europe the equivalent is called Area Control.
The position that issues route clearances before an aircraft starts up. On busy fields it is a separate frequency to keep ground control free.
The specific phrase authorising a departure. It is deliberately unique — no other instruction uses the word "takeoff" — to prevent the kind of misunderstanding that caused the Tenerife disaster in 1977.
The altitude at which a crew must decide to land or go around. If the runway environment is not visible at that point, the approach is discontinued.
The formal act of telling ATC the aircraft needs priority handling. It unlocks assistance, clears the airspace, and brings emergency services to the runway. Crews are trained not to hesitate over it.
To abandon the planned destination and land somewhere else — for weather, a medical problem, a technical fault, or fuel. The alternate is normally chosen before departure.
A declaration that the aircraft will land below its required reserve. This is an emergency, and it gives the flight priority over all other traffic.
Altitude above the standard pressure datum, expressed in hundreds of feet. FL350 is roughly 35,000 feet. Used above a transition altitude so all aircraft share one reference.
The vertical half of an ILS, defining the descent path to the runway — typically three degrees. Losing it mid-approach usually means a go-around.
Abandoning a landing and climbing away to try again. It is a normal, trained manoeuvre — not a failure. Causes range from an unstable approach to another aircraft still on the runway to a sudden change in wind.
The controller responsible for aircraft and vehicles moving on taxiways, but not on runways. Aircraft switch from ground to tower before entering a runway.
A suffix on the callsign of an aircraft with a maximum takeoff weight above 300,000 pounds. It warns other traffic and controllers of the wake turbulence it leaves behind. "Super" is used for the largest aircraft.
An instruction to stop before a runway or taxiway and wait. "Hold short of runway two-seven" means stop at the marked line and do not cross. It requires a full readback — one of the few instructions where that is mandatory.
Instrument Landing System. Two radio beams — one for lateral alignment, one for descent angle — that let an aircraft find the runway in poor visibility. The most precise approach in common use.
Move onto the runway, align with the centreline, and wait for takeoff clearance. Used when the controller wants an aircraft ready to go but cannot yet release it. Called "position and hold" in the United States until 2010.
The lateral half of an ILS, giving left-right alignment with the runway centreline. Aircraft can fly a localizer-only approach when the vertical guidance is unavailable.
The international distress call, spoken three times. It means the aircraft is in grave and imminent danger and needs immediate assistance. It outranks everything else on the frequency — controllers will clear the airspace and stop other traffic. From the French m'aidez, "help me".
An intense, localised downdraft that spreads outward on hitting the ground, producing severe windshear. It has brought down airliners on approach and is now a core part of pilot training.
An advisory that the aircraft cannot accept significant delay without dipping into reserves. It is not yet an emergency, but it tells the controller that holding is no longer an option.
The published procedure an aircraft follows after abandoning an instrument approach — a specific climb, heading and altitude, designed to keep it clear of terrain and other traffic while it repositions.
The position on an instrument approach where the crew must either see the runway or abandon the approach. Continuing past it without visual reference is a serious violation.
One level below a mayday, also spoken three times. It signals urgency without immediate danger to life — a sick passenger, a failing system, a problem the crew is managing but wants priority for. Controllers give the flight preference but do not clear the sky.
The local pressure setting that makes an altimeter read height above sea level. Getting it wrong puts an aircraft at a different altitude than it believes — a factor in several controlled-flight-into-terrain accidents.
Repeating a clearance back to the controller so both parties confirm it was heard correctly. Altitudes, headings, runway assignments and hold-short instructions all require one. A missed or wrong readback is where a large share of incidents begin.
The command TCAS issues when a collision threat is imminent — "climb, climb" or "descend, descend". Following it takes precedence over any controller instruction, and crews report it as soon as the manoeuvre is complete.
"I have received your last transmission." It is not agreement and it is not compliance — a common misconception. A pilot who says "roger" to a clearance has confirmed hearing it, not accepting it.
Any aircraft, vehicle or person entering a runway without clearance. It is one of the most serious categories of ground event because the margins are measured in seconds, and it accounts for several of the worst accidents in aviation history.
A request to repeat the last transmission. Radio is noisy, accents vary and frequencies get crowded — asking twice is normal and far safer than guessing.
The number of people aboard, requested during an emergency. The phrasing is deliberate: it counts everyone, crew and passengers, living and otherwise, for rescue services.
To set a four-digit code on the transponder, the device that identifies an aircraft to radar. "Squawk 4271" means set that code so the controller can tell you apart from everyone else in the sky.
Unlawful interference — hijacking. Entering it triggers a response far beyond air traffic control. It is never used casually, and controllers will usually ask the crew to confirm it obliquely rather than directly.
Radio failure. The aircraft can still fly and navigate but cannot hear or transmit. Controllers switch to light signals and expect the crew to follow the last clearance received, then the published lost-communications procedure.
The transponder code for a general emergency. Setting it makes the aircraft light up on every radar screen within range, immediately, without the crew having to say anything. Often the first sign a controller gets that something is wrong.
"Wait, I will get back to you." A controller managing several aircraft uses it to hold a request without losing track of it. It is not a refusal.
Informal term for being blocked — one transmission overriding another. Heard often on congested frequencies in busy terminal airspace.
Traffic Collision Avoidance System. It watches nearby transponders and, if a collision is likely, issues a Resolution Advisory telling each crew to climb or descend. Crews must follow it even when it contradicts ATC.
The controller responsible for runways and the airspace immediately around the airport. Takeoff and landing clearances come from here.
A TCAS alert one level below a Resolution Advisory. It tells the crew where nearby traffic is so they can look for it, without commanding a manoeuvre.
A pilot declining an instruction they cannot safely carry out — too tight a turn, too steep a descent, a heading into weather. It requires no justification, and controllers are trained to accept it and rework the plan.
Headings issued by a controller to guide an aircraft rather than letting it navigate itself. "Vectors for the ILS" means the controller will steer the aircraft onto the approach.
The pair of rotating vortices trailing from a large aircraft's wingtips. They can roll a smaller aircraft uncontrollably, which is why controllers impose spacing between arrivals and departures by weight category.
"Will comply." Unlike roger, it commits the pilot to carrying out the instruction. Often heard as "roger, wilco", though strictly the two mean different things.
A sudden change in wind speed or direction, often near thunderstorms. It can rob an aircraft of lift at the worst possible moment — low, slow and close to the ground.
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