Read-back or Hear-back
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|Category:||Air Ground Communication|
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).
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'.
Accidents and Incidents
The following events include the missing of an incorrect read back as a factor:
- 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.)
- 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.)
- 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.)
- B742 / A320, Frankfurt Germany, 2006 (On 12 January 2006, an Air China Boeing 747-200 which had just landed at Frankfurt failed to correctly understand and read back its taxi in clearance and the incorrect readback was not detected by the controller. The 747 then crossed another runway at night and in normal visibility whilst an A320 was landing on it. The A320 responded by increased braking and there was consequently no actual risk of collision. The controller had not noticed the incursion and, in accordance with instructions, all stop bars were unlit and the RIMCAS had been officially disabled due to too many nuisance activations.)
- 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.)
AGC Safety Letters:
EUROCONTROL Action Plan for Air-Ground Communications Safety: