Definition

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

Effects

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.

Defences

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