What are ATC clearances? ​

A clearance is an authorization from ATC for an aircraft to do something specific. Fly a heading. Climb to a flight level. Slow to 250 knots. Intercept the ILS. Hold at a fix.

Pilots can't just do what they want. In controlled airspace, every altitude change, every turn, every approach has to be cleared by the controller. The clearance is the fundamental unit of ATC communication.

Types of clearances ​

Altitude clearances ​

The most common clearance type. "Climb and maintain flight level three five zero" or "descend and maintain eight thousand."

The key word is "maintain" - the pilot climbs or descends to that altitude and stays there until given a new clearance. The controller uses altitude clearances to establish vertical separation, set up descents for arrival, or climb departures to their cruise altitude.

There's also "descend via" and "climb via," which clear the aircraft to follow the altitude restrictions on a published SID or STAR. One clearance replaces half a dozen individual altitude assignments.

Heading clearances ​

"Turn left heading two seven zero." The aircraft turns to fly a magnetic heading of 270 degrees.

Heading clearances (vectors) are the controller's steering wheel. They're used to create lateral separation, set up ILS intercepts, keep aircraft away from weather, or route traffic around each other. A heading clearance overrides whatever route the aircraft was flying.

Speed clearances ​

"Reduce speed to two one zero knots." Speed control is the most subtle tool controllers have. Slowing a trailing aircraft opens up spacing. Speeding up a lead aircraft does the same thing.

Below 10,000 feet, all aircraft must stay at or below 250 knots per FAA regulation. Controllers can assign slower speeds below that limit.

Speed clearances are especially useful on final approach sequences. Telling the lead aircraft to maintain 180 knots and the trailing aircraft to slow to 160 knots creates the exact spacing you need without turning either one.

Approach clearances ​

"Cleared ILS runway two seven left approach." This authorizes the pilot to fly the published instrument approach procedure. The aircraft intercepts the localizer, captures the glideslope, and follows them down to the runway.

An approach clearance is usually preceded by vectors to line the aircraft up. The controller has to make sure the aircraft is at the right heading, altitude, and distance before issuing it.

Hold clearances ​

"Hold at GREKI as published, expect further clearance at one five three zero." The aircraft flies a racetrack pattern at the specified fix, waiting until the controller is ready to bring it in.

Holds are the parking lot of the sky. When arrival traffic exceeds what the airport can handle, aircraft stack up in holding patterns at different altitudes.

Route clearances ​

"Cleared direct PARCH." The aircraft leaves its current route and flies a straight line to the named fix. Route clearances shortcut the airway system when traffic allows it.

Clearance structure ​

Real clearances follow a strict format:

1. Who - the aircraft callsign
2. What - the instruction
3. Limit - the endpoint or value
4. Conditions - any restrictions (speed, altitude at a fix, etc.)

"Delta fourteen twenty-three, descend and maintain flight level two four zero, cross GREKI at or above flight level two eight zero."

That's callsign, instruction, limit, and a condition at an intermediate fix. Clean, unambiguous, no room for confusion.

Pilot readback ​

Pilots read back every clearance to confirm they heard it correctly. If the readback is wrong, the controller corrects them. Altitude clearances, heading clearances, runway assignments, and hold short instructions all require mandatory readback.

This loop - clearance, readback, correction if needed - prevents miscommunication. It's one of the most important safety mechanisms in ATC.

In radarcontrol.io ​

All clearance types are available as text commands:

• c350 or c35000ft - altitude (climb/descend to FL350 or 35,000ft)
• h270 - heading (turn to 270)
• s210 - speed (reduce to 210 knots)
• i 27L - ILS approach clearance for runway 27L
• hold GREKI - hold at the GREKI fix
• d PARCH - direct to PARCH
• dv - descend via STAR
• cv - climb via SID

Commands chain together: DAL123 c60 s210 h270 descends to 6,000ft, slows to 210, and turns to heading 270 in one instruction.

Full details in the command reference.

Try issuing clearances in these airspaces:

• Atlanta Center (ZTL) - Use dv on the CHPPR1 STAR into Atlanta. Play now.
• New York TRACON (N90) - Vector arrivals onto the ILS at JFK and Newark.
• Southern California TRACON (SCT) - Manage speed and altitude across seven airports.

Related: What is a SID and STAR? | What is an ILS approach? | How arrivals are sequenced

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