An ATC clearance allows an aircraft to proceed under specified traffic conditions within controlled airspace for the purpose of providing separation between known aircraft. A major contributor to runway incursions is lack of communication with ATC and not understanding the instructions that they give. The primary way the pilot and ATC communicate is by voice. The safety and efficiency of taxi operations at airports with operating control towers depend on this communication loop. ATC uses standard phraseology and require readbacks and other responses from the pilot in order to verify that clearances and instructions are understood. In order to complete the communication loop, the controllers must also clearly understand the pilot’s readback and other responses. Pilots can help enhance the controller’s understanding by responding appropriately and using standard phraseology. Regulatory requirements, the AIM, approved flight training programs, and operational manuals provide information for pilots on standard ATC phraseology and communications requirements.


A flight filed for a short distance at a relatively low altitude in an area of low traffic density might receive a clearance as follows:

“Cessna 1230 Alpha, cleared to Doeville airport direct, cruise 5,000.”

The term “cruise” in this clearance means a pilot is authorized to fly at any altitude from the minimum IFR altitude up to and including 5,000 feet and may level off at any altitude within this block of airspace. A climb or descent within the block may be made at the pilot’s discretion. However, once a pilot reports leaving an altitude within the block, the pilot may not return to that altitude without further ATC clearance.

When ATC issues a cruise clearance in conjunction with an unpublished route, an appropriate crossing altitude is specified to ensure terrain clearance until the aircraft reaches a fix, point, or route where the altitude information is available. The crossing altitude ensures IFR obstruction clearance to the point at which the aircraft enters a segment of a published route or IAP.

Once a flight plan is filed, ATC issues the clearance with appropriate instructions, such as the following:”Cessna 1230 Alpha is cleared to Skyline airport via the Crossville 055 radial, Victor 18, maintain 5,000. Clearance void if not off by 1330.”

Or a more complex clearance, such as:

“Cessna 1230 Alpha is cleared to Wichita Midcontinent airport via Victor 77, left turn after takeoff, proceed direct to the Oklahoma City VORTAC. Hold west on the Oklahoma City 277 radial, climb to 5,000 in holding pattern before proceeding on course. Maintain 5,000 to CASHION intersection. Climb to and maintain 7,000. Departure control frequency will be 121.05, Squawk 0412.”

Clearance delivery may issue the following “abbreviated clearance” which includes a departure procedure (DP):

“Cessna 1230 Alpha, cleared to La Guardia as filed, RINGOES 8 departure Phillipsburg transition, maintain 8,000. Departure control frequency will be 120.4, Squawk 0700.”

This clearance may be readily copied in shorthand as follows:

“CAF RNGO8 PSB M80 DPC 120.4 SQ 0700.”

The information contained in this DP clearance is abbreviated using clearance shorthand. The pilot should know the locations of the specified navigation facilities, together with the route and point-to-point time, before accepting the clearance.

The DP enables a pilot to study and understand the details of a departure before filing an IFR flight plan. It provides the information necessary to set up communication and navigation equipment and be ready for departure before requesting an IFR clearance.

Once the clearance is accepted, a pilot is required to comply with ATC instructions. A clearance different from that issued may be requested if the pilot considers another course of action more practicable or if aircraft equipment limitations or other considerations make acceptance of the clearance inadvisable.

A pilot should also request clarification or amendment, as appropriate, any time a clearance is not fully understood or considered unacceptable for safety of flight. The pilot is responsible for requesting an amended clearance if ATC issues a clearance that would cause a pilot to deviate from a rule or regulation or would place the aircraft in jeopardy.

Clearance Separations

ATC provides the pilot on an IFR clearance with separation from other IFR traffic. This separation is provided:

  1. Vertically—by assignment of different altitudes.
  2. Longitudinally—by controlling time separation between aircraft on the same course.
  3. Laterally—by assignment of different flightpaths.
  4. By radar—including all of the above.

ATC does not provide separation for an aircraft operating:

1. Outside controlled airspace.
2. On an IFR clearance:
  • With “VFR-On-Top” authorized instead of a specific assigned altitude.
  • Specifying climb or descent in “VFR conditions.”
  • At any time in VFR conditions, since uncontrolled VFR flights may be operating in the same airspace.

In addition to heading and altitude assignments, ATC occasionally issues speed adjustments to maintain the required separations. For example:

“Cessna 30 Alpha, slow to 100 knots.”

A pilot who receives speed adjustments is expected to maintain that speed plus or minus 10 knots. If for any reason the pilot is not able to accept a speed restriction, the pilot should advise ATC.

At times, ATC may also employ visual separation techniques to keep aircraft safely separated. A pilot who obtains visual contact with another aircraft may be asked to maintain visual separation or to follow the aircraft. For example:

“Cessna 30 Alpha, maintain visual separation with that traffic, climb and maintain 7,000.”

The pilot’s acceptance of instructions to maintain visual separation or to follow another aircraft is an acknowledgment that the aircraft is maneuvered as necessary to maintain safe separation. It is also an acknowledgment that the pilot accepts the responsibility for wake turbulence avoidance.

In the absence of radar contact, ATC relies on position reports to assist in maintaining proper separation. Using the data transmitted by the pilot, the controller follows the progress of each flight. ATC must correlate the pilots’ reports to provide separation; therefore, the accuracy of each pilot’s report can affect the progress and safety of every other aircraft operating in the area on an IFR flight plan.