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Read-back or Hear-back

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Category: Air Ground Communication Air Ground Communication
Content source: SKYbrary About SKYbrary
Content control: EUROCONTROL EUROCONTROL

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
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.

Expectations

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.

Solutions

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'.

Accidents and Incidents

The following events include the missing of an incorrect read back as a factor:

  • A320 / CRJ2, Sofia Bulgaria, 2007 (On 13 April 2007 in day VMC, an Air France A320 departing Sofia lined up contrary to an ATC Instruction to remain at the holding point and be ready immediate. The controller did not immediately notice and after subsequently giving a landing clearance for the same runway, was obliged to cancel it send the approaching aircraft around. An Investigation attributed the incursion to both the incorrect terminology used by TWR and the failure to challenge the incomplete clearance read back by the A320 crew.)
  • AT75 / B739, Medan Indonesia, 2017 (On 3 August 2017, a Boeing 737-900ER landing at Medan was in wing-to-wing collision as it touched down with an ATR 72-500 which had entered the same runway to depart at an intermediate point. Substantial damage was caused but both aircraft could be taxied clear. The Investigation concluded that the ATR 72 had entered the runway at an opposite direction without clearance after its incomplete readback had gone unchallenged by ATC. Controllers appeared not to have realized that a collision had occurred despite warnings of runway debris and the runway was not closed until other aircraft also reported debris.)
  • B734, Sharjah UAE, 2015 (On 24 September 2015, a Boeing 737-400 cleared for a night take-off from Sharjah took off from the parallel taxiway. The controller decided that since the taxiway was sterile and the aircraft speed was unknown, the safest option was to allow the take-off to continue. The Investigation noted that the taxiway used had until a year previously been the runway, becoming a parallel taxiway only when a new runway had been opened alongside it. It was noted that the controller had "lost visual watch" on the aircraft and regained it only once the aircraft was already at speed.)
  • B77W, en-route, northeast of Los Angeles USA, 2016 (On 16 December 2016, a Boeing 777-300 which had just departed from runway 07R at Los Angeles was radar vectored in Class ‘B’ airspace at up to 1600 feet below the applicable minimum radar vectoring altitude. The Investigation found that the area controller’s initial vectoring had been contrary to applicable procedures and their communication confusing and that they had failed to recover the situation before it became dangerous. As a result, as the crew were responding in night IMC to a resulting EGPWS ‘PULL UP’ Warning, the aircraft had passed within approximately 0.3 nm of obstructions at the same altitude.)
  • E190 / D328, Basel Mulhouse France, 2016 (On 7 March 2016, an Embraer 190 entered the departure runway at an intersection contrary to an ATC instruction to remain clear after neither a trainee controller nor their supervisor noticed the completely incorrect readback. An aircraft taking off in the opposite direction was able to rotate and fly over it before either controller noticed the conflict. The Investigation was told that the crew of the incursion aircraft had only looked towards the left before lining up and concluded that the event had highlighted the weakness of safety barriers based solely on the communications and vigilance of pilots and controllers.)

Related Articles

Further Reading

AGC Safety Letters:

EUROCONTROL Action Plan for Air-Ground Communications Safety: