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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

Description

Read-back is defined as 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. (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 radar 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 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.

Solution

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:

  • B763 / B763, Kansai Japan, 2007 (On 20 October 2007, at night, a Boeing 767-300 operated by Air Canada was taxiing for Runway 24L at Kansai International Airport for take-off. Meanwhile, another Boeing 767-300, operated by Japan Airlines, had been given landing clearance and was on approach to the same runway. After an incorrect readback, the Air Canada B767 entered the runway to line up. As a consequence of the runway incursion, the B767 on approach executed a go-around on the instructions of air traffic control.)
  • B738/A319 en-route, south east of Zurich Switzerland, 2013 (On 12 April 2013, a Ryanair Boeing 737-800 took a climb clearance intended for another Ryanair aircraft on the same frequency. The aircraft for which the clearance was intended did not respond and the controller did not notice that the clearance readback had come from a different aircraft. Once the wrong aircraft began to climb, from FL360 to FL380, a TCAS RA to descend occurred due to traffic just transferred to a different frequency and at FL370. That traffic received a TCAS RA to climb. STCA was activated at the ATS Unit controlling both Ryanair aircraft.)
  • E145, en-route, north east of Madrid Spain, 2011 (On 4 August 2011, a Luxair Embraer 145 flying a STAR into Madrid incorrectly read back a descent clearance to altitude 10,000 feet as being to 5,000 feet and the error was not detected by the controller. The aircraft was transferred to the next sector where the controller failed to notice that the incorrect clearance had been repeated. Shortly afterwards, the aircraft received a Hard EGPWS ‘Pull Up’ Warning and responded to it with no injury to the 47 occupants during the manoeuvre. The Investigation noted that an MSAW system was installed in the ACC concerned but was not active.)
  • SF34/AT72, Helsinki Finland, 2011 (On 29 December 2011 a Golden Air ATR 72 making a daylight approach to runway 22R at Helsinki and cleared to land observed a Saab 340 entering the runway and initiated a low go around shortly before ATC, who had observed the incursion, issued a go around instruction. The Investigation attributed the breach of clearance by the Latvian-operated Saab 340 primarily to poor CRM, a poor standard of R/T and inadequate English Language proficiency despite valid certification of the latter.)
  • 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.)


Related Articles

Further Reading

AGC Safety Letters:

EUROCONTROL Action Plan for Air-Ground Communications Safety: