Read-back or Hear-back

Read-back or Hear-back

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:

On 21 March 2022, an Airbus A320 level at FL 360 lost separation with another Airbus A320 which continued its descent beyond its cleared level. A predictive conflict alert prompted the controller to issue multiple calls confirming the clearance limit but with no response so when both aircraft tracks crossed at FL 360, lateral separation was reduced to 3.8 nm. It was concluded that the immediate cause of the conflict was the failure of the descending aircraft to respond to ATC alerting calls but that its origin was an undetected incorrect readback of the descent clearance.

On 24 October 2021, a Bombardier DHC8-400 inbound to Belagavi initially advised to expect a non-precision procedural approach to runway 08 was subsequently instructed and acknowledged clearance for an equivalent procedural approach to runway 26. An approach to runway 08 was then flown without ATC intervention or pilot error recognition but with no actual consequences. The error was attributed to pilot expectation bias and distraction and controller failure to order a go-around after eventually realising what was happening. The context which had facilitated the errors was considered to be procedure and performance inadequacy at both the aircraft operator and ATC.

On 20 October 2021, the flight crew of a Bombardier CRJ1000 making a LNAV/VNAV approach at Nantes using Baro-VNAV minima read back an incorrect QNH which was not noticed by the controller. The crew then flew the approach approximately 530 feet below the procedure vertical profile which led to the MSAW system being activated and advised to the flight. The crew response was delayed until the controller had twice repeated the correct QNH after which the error was recognised and the vertical profile corrected. The investigation noted that neither the operator’s procedures nor aircraft instruments allowed straightforward crew detection of their error.

On 23 May 2022, an Airbus A320 came extremely close to collision with terrain as the crew commenced a go around they did not obtain any visual reference during a RNP approach at Paris CDG for which they were using baro-VNAV reference to fly to VNAV/LNAV minima. The corresponding ILS was out of service. The Investigation has not yet completely established the context for the event but this has been confirmed to include the use of an incorrect QNH which resulted in the approach being continued significantly below the procedure MDA. Six Interim Safety Recommendations have been issued.

On 28 November 2020, a Boeing 737-300F taxiing for an early morning departure at Singapore Changi crossed an illuminated red stop bar in daylight and entered the active runway triggering an alert which enabled the controller to instruct the aircraft to immediately exit the runway and allow another aircraft already on approach to land. The Investigation found that the flight was the final one of a sequence of six carried out largely overnight as an extended duty predicated on an augmented crew. The context for the crew error was identified as a poorly managed operator subject to insufficient regulatory oversight.

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