"Ask ATC" Blog

Hear-back, Read-back

Aviation is an industry that is no stranger to checklists. While some might view the use of checklists as borderline obsessive-compulsive, the truth is that lives depend on air traffic control instructions being understood and complied with. The best check-and-balance system we have in aviation is the read-back.

Implemented as a measure to protect against misunderstanding, pilots of IFR aircraft are required to read back any instruction or clearance issued by the air traffic controller. If the controller hears a discrepancy between the instructions they have issued and what is read back by the pilot, they will re-state the clearance or the instruction. It works almost every time.

A prevailing factor that arises from the investigation of aircraft incidents is what has been termed the "hear-back error" - the pilot or controller effectively hearing an instruction other than what was actually given by the controller. The hear-back error is essentially an error of habit and not naturally one of inexperience. It can and often does strike the most experienced pilots and controllers, all of whom are susceptible to a complacency that sets in after receiving or issuing similar instructions time and time again.

For example, a controller may issue initial descent to 10,000 feet on a regular basis for a particular RNAV arrival. If a pilot is flying this arrival for the first time, he or she will very likely be listening attentively for each instruction as he or she is not saddled with the expectancy that may accompany a pilot who routinely flies in that airspace. Some pilots may fly the same arrival several times a day, leading to an unconscious expectation that the same altitudes will be issued each and every time. Such expectations lie dormant and unnoticed until the controller issues descent to a different altitude than normal due to traffic or temporarily restricted airspace. In such cases, the pilot often tends to hear the altitude they are expecting to hear rather than what was actually issued. As the transgression occurs at the subconscious level of the brain, it may be difficult for the pilot to believe they have done this until they hear their own voice played back to them on the incident tape.

It is not only pilots who are susceptible to this form of complacency - air traffic controllers habitually fall victim to exactly the same type of error. After giving a routine clearance, a controller may be so accustomed to hearing a particular read-back (they may issue descent to "one-zero thousand" 15 or 20 times in an hour) that they begin to unconsciously expect the pilot to read back the altitude correctly and may not catch the erroneous transmission as phones from other sectors ring and other pilots call up on secondary frequencies.

In discussing errors of habit, it is important to understand that this phenomenon is actually our brains trying to make things easier for us. This is what makes this error so insidious and so dangerous. When engaged in periods of focus such as sequencing arrivals or setting an aircraft up for an approach, our brain tends to look for patterns. It wants to free up processing power for other tasks by putting seemingly routine tasks in the category of "habit." If we allow this to happen, our brain may try to filter information such as a crucial altitude from our conscious thoughts. It is a danger that flight crews and controllers must contend with regardless of our level of experience.

A long-standing moniker used in flight training to convey the pilot's priority chain is "aviate, navigate, communicate." As pilot-in-command, flying the airplane must always be the first priority. That being said, when it comes time to communicate, if one's attention cannot be firmly focused on actively receiving and confirming instructions from air traffic control, then errors leading to catastrophic events can and do result.

The air traffic control system is predicated on clear and concise communication between pilot and controller. Precise communication becomes ever critical in congested airspace where traffic volumes dictate that messages are received, understood and acknowledged correctly the first time. For pilots who are native speakers of English, this is a daunting task in itself. For non-native speakers it becomes an even greater obstacle. Aviation English is not the same as conversational English. It requires specialized training beyond that of generic language comprehension as the consequences of misunderstanding air traffic control can be disastrous.

The challenge of bringing non-native speakers of English to international standards is a massive, yet crucial, step toward mitigating communication errors worldwide. Nevertheless, effective communication always comes down to personal discipline and investing the necessary time and attention to ensure that each ATC instruction is clearly understood and implemented correctly.