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Read-back or Hear-back
|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 radar 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 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:
- A320, vicinity Oslo Norway, 2008 (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.)
- C525 / B773, vicinity London City UK, 2009 (On 27 July 2009, a Cessna 525 departing from London City failed to comply with the initial 3000 ft QNH SID Stop altitude and at 4000 ft QNH in day VMC came into close proximity on an almost reciprocal heading with a Boeing 777-300ER. Actual minimum separation was approximately 0.5nm laterally and estimated at between 100 ft and 200 ft vertically.)
- 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.)
- B190 / B190, Auckland NZ, 2007 (On 1 August 2007, the crew of a Beech 1900 aircraft holding on an angled taxiway at Auckland International Airport mistakenly accepted the take-off clearance for another Beech 1900 that was waiting on the runway and which had a somewhat similar call sign. The pilots of both aircraft read back the clearance. The aerodrome controller heard, but did not react to, the crossed transmissions. The holding aircraft entered the runway in front of the cleared aircraft, which had commenced its take-off. The pilots of both aircraft took avoiding action and stopped on the runway without any damage or injury.)
AGC Safety Letters:
EUROCONTROL Action Plan for Air-Ground Communications Safety: