Readback & Hearback Error Between Pilots and ATC

Readback Errors

To enable the transfer of clear and precise information, proper communication methods are required. This function is a core social requirement. In aviation communication is an essential pre-requisite to safety. The problem in communication between Air Traffic Controllers (ATCos) and pilots arises when either party fails to recognise and identify an error that occurs during communication during the readback and hearback process and from therein they are both unable to recover from the situation. An incident may then occur and consequently lead to a more serious accident after a series of events that were facilitated by these errors in communication. This Wiki contribution will try to address the psychology surrounding hearback errors1.

Read Back Errors From Pilots2

These types of errors occur when;

  • The air traffic controller delivers information to the pilot swiftly.

The ATCo delivers the information in a manner that is efficient for him, but that is too fast for the pilot to remember or write down. 80% of readback errors that occur when the pilot reads back the instruction incorrectly are associated with high speed/fast delivery by the ATCo. However if the information is given slowly, the pilot has time comprehend it and/or write it down. The error rate associated with low speed delivery of information from the controller to the pilot is only 10-20%3.

  • The instruction from the controller contains a lot of information for the pilot.
  • When the instruction given by the controller varies from what the pilot is used to.

Before the ATCo delivers a message, he/she has to phrase it using the correct phraseology. If for any reason it differs from the ‘usual clearance’ that the pilot is expecting they have to switch their thought process to the topic at hand and carefully listen to the instruction. If there is new information and the pilot’s mind is elsewhere, the pilot may struggle to 'hear’ the entire transmission. It’s in such an instance that the pilots brain ‘fills in the gaps’, so as to construct a complete transmission. This filling in of the gaps by the brain may not always be the correct information and errors begin to occur as the pilot assumes he has the proper instruction.

The Pilot-Controller Communication Loop (image embedded from [] on 30 Sep 2010)

Why ATC Fails To Detect Readback Errors

In general humans aren’t adapted to monitor for the unlikely chance of an error. From a Human Factors perspective the number or readback errors is low, thus the brain often takes shortcuts (for example; assumptions and pattern matching) 2.

Read back is held in the short term memory, also referred to as the ‘working memory’. This working memory only has a capacity of about 7 "items". This means that when the memory is overloaded with a lot of information at once it takes shortcuts to try and make sense of the information. For the ATCo to check that the pilots readback is correct they have to compare the initial information first conveyed to the pilot with the readback received. For the brain to do this it tries to ‘pattern match’ both the initial and the received message. The brain considers; the length of the message, pitch contours and meaning as well as the number of parts in both of the messages2.

This system used by the brain in general does work very well. However, in an environment with vast workload such as that of the air traffic controller when they have to concentrate on numerous tasks at once, the brain takes shortcuts and accepts more of an approximation of the message. It is at this point that errors go unnoticed. For example if a message has the same beginning and ending words in a transmission the difference between 120 and 210 may go unnoticed2.

ICAO requires that Annex 11 is implemented during all safety-related instructions or clearances during readback by pilots to ATC.

Safety Related Clearance That Must Be Readback4

  • Runway in use.
  • ATC Route clearance.
  • ATC Transponder (SSR) Code.
  • Altimeter setting.
  • Clearance & instruction to enter land and take-off on, hold short of, cross or backtrack on a runway.
  • Heading and speed.
  • Transition levels.

Discovering These Errors

There is no sure way to catch all of these errors, but the occurrences can be reduced by ATC in the following ways;

  • Slow the rate speech when reading safety/clearance information.
  • If aware that the message is abnormal, place emphasis on the different information.
  • Try and minimize the amount of information per transmission. Working memory can only handle 7 items at any given time. By doing this the pilot’s brain is given proper processing ability to understand the information.
  • Numerals must be pronounced using the phonetic language to avoid confusion of some numbers sounding the same.

When ATC Listens To Readbacks They Should;

  • Acknowledge that as humans, when busy, our brains perform shortcuts. Therefore listen more to the message conveyed.
  • Pay attention to the information at the middle of the message as often it is the middle of the message that is incorrect2.
  • Also be aware that when we speak in our native language we are less vigilant for miscommunications.

For the communication loop to be complete between the ATCo and the pilot, any readback from the pilot requires a ‘hearback’ by the controller. These communication errors can be avoided by clearly understanding the psychology around hearback errors as well as mutually understanding each other’s operating environment and the whole communication process. Also, being able to determine measures that can reduce these errors will lead to better and safer air traffic communications in general.

Controller Pilot Data Link Communications (CPDLC)

Advances in technology should address the errors associated with read back errors. The most significant development has been with CPDLC, where the voice element of the ATS and Pilot exchange is replaced by digital transmission. Importantly there is involvement of the human interface at either end. The CPDLC includes the standard message elements associated with standard phraseology. Critically the technology retains flexibility that allows both participants in the communications system to communicate as they did with voice so that the system remains safe and efficient.

The main advantages of CPDLC are:

  • improved safety - there is less room for misunderstanding between pilots and controllers, and
  • increased capacity - by reducing controllers workload such as voice communication exchanges, there is more scope for the core tasks of coping with air traffic.
  • Never having to say "say again?"
1. Isaac, A. (2007). Effective Communication in the aviation environment: Work in progress. Retrieved on the 27, September, 2010, from skybrary
2. Fifield, S. (2008). Safety Bulletin: Readback Errors.
3. The Safety Crusader. Fighting against the slips, trips, stumbles and fumbles: Readback-Heardback. Retrieved 30 Sep 2010, from Safetycrusader
4. Airbus (2004). Effective Pilot/ Controller Communications. Lack or Readback or Incomplete Readback. Retrieved 30 Sep 2010, from airbus
5. Eurocontrol. The Benefits of CPDLC. Retrieved 30 Sep 2010 from eurocontrol

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Contributors to this page

Melanie AttanMelanie Attan

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