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
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:
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.
On 27 September 2019, an Airbus A320 and an Embraer 145 both inbound to Barcelona and being positioned for the same Transition for runway 25R lost separation and received and followed coordinated TCAS RAs after which the closest point of approach was 0.8nm laterally when 200 feet vertically apart. The Investigation found that the experienced controller involved had initially created the conflict whilst seeking to resolve another potential conflict between one of the aircraft and a third aircraft inbound for the same Transition and having identified it had then implemented a faulty recovery plan and executed it improperly.
On 26 August 2019, an Airbus A320 attempted two autopilot-engaged non-precision approaches at Birmingham in good weather before a third one was successful. Both were commenced late and continued when unstable prior to eventual go-arounds, for one of which the aircraft was mis-configured causing an ‘Alpha Floor’ protection activation. A third non-precision approach was then completed without further event. The Investigation noted an almost identical event involving the same operator four months later, observing that all three discontinued approaches appeared to have originated in confusion arising from a slight difference between the procedures of the aircraft operator and AIP plates.
On 19 December 2008, an Aeroflot Airbus A320 descended significantly below its cleared and acknowledged altitude after the crew lost situational awareness at night whilst attempting to establish on the ILS at Oslo from an extreme intercept track after a late runway change and an unchallenged incorrect readback. The Investigation concluded that the response to the EGPWS warning which resulted had been “late and slow” but that the risk of CFIT was “present but not imminent”. The context for the event was considered to have been poor communications between ATC and the aircraft in respect of changes of landing runway.
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.
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.
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.
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.
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.
On 3 May 2017, an Airbus A330 and an Airbus A319 lost prescribed separation whilst tracking in opposite directions on a radar-controlled ATS route in eastern Myanmar close to the Chinese border. The Investigation found that the response of the A330 crew to a call for another aircraft went undetected and they descended to the same level as the A319 with the lost separation only being mitigated by intervention from the neighbouring Chinese ACC which was able to give the A319 an avoiding action turn. At the time of the conflict, the A330 had disappeared from the controlling ACCs radar.
On 5 July 2012, an Airbus A319 entered its departure runway at Naha without clearance ahead of an A320 already cleared to land on the same runway. The A320 was sent around. The Investigation concluded that the A319 crew - three pilots including one with sole responsibility for radio communications and a commander supervising a trainee Captain occupying the left seat - had misunderstood their clearance and their incorrect readback had not been detected by the TWR controller. It was concluded that the controller's non-use of a headset had contributed to failure to detect the incorrect readback.
On 22 November 1994 a McDonnell Douglas MD 82 flight crew taking off from Lambert- St. Louis at night in excellent visibility suddenly became aware of a stationary Cessna 441 on the runway ahead and was unable to avoid a high speed collision. The collision destroyed the Cessna but allowed the MD82 to be brought to a controlled stop without occupant injury. The Investigation found that the Cessna 441 pilot had mistakenly believed his departure would be from the runway he had recently landed on and had entered that runway without clearance whilst still on GND frequency.
On 30 October 2014, a descending Airbus A320 came close to a Boeing 737-800 at around FL 220 after the A320 crew significantly exceeded a previously-instructed 2,000 fpm maximum rate of descent assuming it no longer applied when not reiterated during re-clearance to a lower altitude. Their response to a TCAS RA requiring descent at not above 1,000 fpm was to further increase it from 3,200 fpm. Lack of notification delayed the start of an independent Investigation but it eventually found that although the A320 TCAS equipment had been serviceable, its crew denied failing to correctly follow their initial RA.
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.
AGC Safety Letters:
EUROCONTROL Action Plan for Air-Ground Communications Safety: