Readback. A procedure whereby the receiving station repeats a received message or an appropriate part thereof back to the transmitting station so as to obtain confirmation of correct reception.
Source: ICAO Annex 10 Vol II
An uncorrected erroneous read-back (known as a hear-back error) may lead to a deviation from the intended clearance and may not be detected until the controller observes the deviation on his/her situational display.
Less than required vertical or horizontal separation (and an AIRPROX) is often the result of hear-back errors.
The flight crew must read back to the air traffic controller safety-related parts of ATC clearances and instructions which are transmitted by voice. The following items must always be read back:
a) ATC route clearances;
b) clearances and instructions to enter, land on, take off from, hold short of, cross or backtrack on any runway; and
c) runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and, whether issued by the controller or contained in Automatic Terminal Information Service (ATIS) broadcasts, transition levels.
Other clearances or instructions, including conditional clearances, must be read back or acknowledged in a manner to clearly indicate that they have been understood and will be complied with.
The controller must listen to the read-back to ascertain that the clearance or instruction has been correctly acknowledged by the flight crew and shall take immediate action to correct any discrepancies revealed by the read-back. (ICAO Annex 11 Chapter 3 Para 3.7.3)
Aspects of read-back/hear-back
The pilot’s read-back must be complete and clear to ensure a complete and correct understanding by the controller. The action of reading back a clearance gives the controller an opportunity to confirm that the message has been correctly received, and if necessary, to correct any errors.
Read-back of a clearance should never be replaced by the use of terms such as “Roger”, "Wilco" or “Copied”. Likewise, a controller should not use similar terms to acknowledge a message requiring a definite answer (e.g. acknowledging a pilot’s statement that an altitude or speed restriction cannot be met).
Failure to correct faulty read-back
The absence of an acknowledgement or a correction following a clearance read-back is perceived by most flight crews as an implicit confirmation of the read-back. The absence of acknowledgement by the controller is usually the result of frequency congestion and the need for the controller to issue clearances to several aircraft in succession.
The bias of expectation of clearance in understanding a communication can affect pilots and controllers. The bias of expectation can lead to:
Transposing the numbers contained in a clearance (e.g. a flight level) to what was expected, based on experience or routine; and,
Shifting a clearance or instruction from one parameter to another (e.g. perceiving a clearance to maintain a 280 degree heading as a clearance to climb/descend and maintain flight level 280).
Failure to request confirmation or clarification
Misunderstandings may include half-heard words or guessed-at numbers. The potential for misunderstanding numbers increases when an ATC clearance contains more than two instructions.
Reluctance to seek confirmation may cause pilots to:
Accept an inadequate instruction (over-reliance on ATC); or,
Determine for themselves the most probable interpretation.
Failing to request clarification may cause flight crew to believe erroneously that they have received an expected clearance (e.g. clearance to climb to a requested level).
Failure to question instructions
Failing to question an instruction can cause a crew to accept an altitude clearance below the minimum safe altitude or a heading that places the aircraft on collision course with another.
Pilots must read back the safety related part of all communications.
ATCOs must listen carefully to the read-back and correct any factual error or any apparent misunderstanding.
If there is any doubt in the minds of flight crew as to the precise content of a clearance, or there is any doubt about how to comply (for example the identity or location of a waypoint) then they should request a repeat of either the clearance or a specific part of it that was not understood by the request 'Say again'.
16 December 2016, An EVA Air Boeing B777-35E (ER) (B16726) Performing Flight EVA015/BR15 from Los Angeles KLAX to Taipei RCTP was on departure when some confusions turned into a traffic conflict and a near terrain collision.