On a typically busy evening in the dark skies over Seattle, Air Traffic Control is sequencing two Boeing jetliners-a 737 and a 757-for an ap-proach to the Seattle-Tacoma International Air-port. The pilot of the first airplane, a 737-300, has been slowing and is awaiting instructions to descend to 10.000 feet, as shown on the arrival route chart. The controller issues the instruc-tions, and the 737 begins descending from 12.000 feet to 10.000 feet.

The controller continuously watches the radar, and after observing the 737’s Mode-C (indicating it has left 12,000 feet), he transmits to the 757 to “descend and maintain one two thousand and reduce airspeed to two hundred fifty knots”.

Because the 757 pilot is experienced flying this route, he knows ATC will issue a final altitude of “one zero thousand”. The 757 pilot immediately acknowledges the ATC instructions by saying: “Boeing 757 reduce to two hundred fifty knots, descend and maintain one zero thousand”. Sensing a problem, the controller questions the 757 pilot:

Controller: “Boeing 757, where are you going? Your assigned altitude was one two thousand!”

Pilot: “Center, we understood our clearance was to one zero thousand. We read back one zero thousand!”.

Controller: “Negative Boeing 757! Turn right to zero nine zero degrees and climb immediately to one two thousand. You have traffic at twelve o’clock, one one thousand, two and a half miles!”.

This scenario actually happened. The pilot acted upon an Air Traffic Control (ATC) clearance he was expecting to receive -not- the clearance he actually received. The controller did not hear the erroneous readback, because his attention was diverted to resolving another air traffic control problem many miles away.

Overview

This is know as a “hearback” error, a situation that occurs constantly in pilot-controller com-munications. The result of these hearback errors may be:
· Deviations from assigned altitudes.
· Non-adherence to crossing restrictions.
· Turns to incorrect vector headings.
· Flights over the wrong assigned routes.

Each year in ATC environments around the world, these erroneous actions cause traffic con-flicts, narrowly missed mid-air collisions, go arounds and aborted takeoffs.

Readback/hearback is a means of mutual verifi-cation between pilots and air traffic controllers. The flight crew readback of ATC instructions, especially those involving numbers, serves as a double-check between pilots and air traffic con-trollers to catch communication errors. In recent years, this double check has become less effec-tive at preventing ATC communication errors.

The challenge of Effective ATC
Communications

Effective communication is traditionally defined as: message sent = message received. In the air traffic control environment, such effective com-munication between controllers and pilots is es-sential.

An air traffic controller’s communication respon-sibility is to issue correct ATC instructions to the correct airplane and listen to the pilot’s readback to make sure the pilot has heard the message correctly. The controller is obligated to initiate the connection in the communication, and to confirm the communication has been heard and under-stood.

The pilot’s responsibility is to be a partner with the controller. The pilot must clearly indicate he understands the communication - and will com-ply with the controller’s instructions.

In ATC communications, there is an essential built-in redundancy: the controller issues a clear-ance, the pilot responds, and the controller veri-fies the communication. But studies show this redundancy is frequently missing from controller-pilot-controller exchanges of information. When pilots readback ATC clearances, they are, in es-sence, asking a question: “Did we correctly un-derstand your instructions?”.

Frequently, the air traffic controller does not hear the pilot’s readback and does not respond. Sometimes this happens when the controller is not listening due to additional workloads - or is listening passively. Many pilots say they trust the controller when the readback is acknowledged. Therefore, pilots believe that just as they always readbacks, air traffic controllers always listen to them. Pilots frequently interpret silence as ac-knowledgment of the communication and that the communication is correct; the pilots then comply with the instructions - as they perceive them.

Examples of these perceived approvals include:

· “I read back the instructions. The controller didn’t say anything, so I descend”
· “The air traffic controller did not challenge our readback, so we climbed”

Pilot dependency on a controller’s listening role is a surprising departure from the pilot’s normal philosophy of healthy suspicion about placing trust in others, particularly those outside the flight deck. Pilots have shown a consistent behavioral pattern of overriding trustful dependence upon the controllers’ listening function in the exchange of information. Pilots often accept half-heard, doubtful, sometimes guessed-at numbers, as valid instructions if the air traffic controller does not challenge the readback.
An example of this trust is when a pilot mistak-enly accepts a clearance to below known terrain altitude, and descends toward that wrong alti-tude. Later, the pilot may say he misunderstood the controller, but was relying upon the controller to correct any mistakes in the readback.

The Human Factor

Like all of us pilots are human. When it comes to listening, we hear what we expect to hear, we hear what we want to hear, and frequently we don’t hear what we didn’t anticipate hearing. Pi-lots often don’t hear what they don’t anticipate hearing - amendments to just-issued clearances. The result is ATC instructions that are mis-read, tuned out, guessed at  - and at times, even de-termined by an informal vote among crew mem-bers. There is often an apparent reluctance of crew members to re-confirm doubtful numbers with the controller.

These doubtful numbers heard by flight crews and controllers communicating with each other are apparently caused by the similar sound of several number combinations:
· Five and nine (5 and 9)
· Seven and seventeen (7 and 17)
· Five and fifteen (5 and 15)
· One zero thousand and one one thousand (10.000 and 11.000)
· Flight level two zero zero and Flight level two two zero (FL200 and FL220)

Mis-hearing of the numbers occurs most fre-quently when single, one-sentence clearance messages call for two or more separate pilots actions, for example: “Cross Olympia VOR at one one thousand, descend and maintain one zero thousand, reduce speed to two five zero knots, altimeter at two niner niner five”. Pilots acknowledging a clearance of this type often do not understand why the controller does not cor-rect any errors made during the readback. Said one pilot: “It is my opinion that I could read back my Social Security number and most controllers would not question it”.

Controllers often report similar frustrations when describing communication errors. Their verbal mistakes often pass unnoticed in the continuing flow of messages to many airplanes. Only by re-viewing ATC voice tapes after an incident could participants hear what, in fact, had been said - and to whom. One controller summed up his er-ror this way: “Tis human to err. I guess this mis-take proved I was human”.

Key Communication Problems

Some of these key communication problems demonstrate just how human both controllers and pilots can be:
· Frequency Congestion.
· Similar-sounding Call Signs.
· Pilot Frequency Monitoring Failures.
· Controller Frequency Monitoring Failures.
· Pilot Failures to Verify Doubtful Communica-tions.

Frequency Congestion. Radio frequency con-gestion may result in both mis-communications and non-communications. It also encourages communication shortcuts and deviations from standard phraseology. It is probably the worst communication problem confronting the aviation system.

Similar-sounding Call Signs. These call signs are the source of many miscommunication. An example is when a communication intended for “Boeing 775” is heard and acted upon by “Boeing 755” - two different airplanes.

Pilot Frequency Monitoring Failures. Many communication problems arise because pilots are not “guarding” frequencies carefully. They miss clearances directed to them, or intercept clearances intended for other airplanes.

Controller Frequency Monitoring Failures. Controllers often cite distractions as the reasons for not monitoring frequencies with their full at-tention. As a result, they occasionally miss the pilot’s initial establishment of ATC communica-tions, or fail to detect erroneous readbacks. Con-troller frequency monitoring failures can often be related to workload; during busy periods, control-lers may shift their attention to “Boeing 755” as soon as they have given a clearance to “Boeing 775” - without waiting for a readback.

Pilot Failures to Verify Doubtful Communica-tions. Pilots often advise they have doubt about an ATC communication, but choose to clarify it with another crew member, rather than with ATC. If the other pilot is not listening carefully, the crew may end up doing what they thought they were told, or what they expected to be told. This problem seems to be caused by:
· Frequency congestion, which makes it diffi-cult to verify ATC communications.
· Personal pride of flight crews, which makes them reluctant to admit to a monitoring fail-ure.

Interestingly, language differences between pilots and controllers do not seem to create ATC communication problems. In fact, language dif-ferences may actually reduce readback/hearback problems.

Many controllers believe flight deviations due to mis-communications have little impact on the ATC system until a traffic conflict occurs. The ATC conflict alert system activates when com-munication errors occur that place two airplanes in close proximity to each other. Unfortunately, many of the communication errors that caused the airplanes to activate the conflict alert are compounded by the same human factor ele-ments present in the original erroneous commu-nication.

Readback/Hearback Error Categories

The 389 readback/hearback errors examined at Seattle Air Route Traffic Control Center can be divided into the following categories. Other ATC locations throughout the United States - and worldwide - experience a similar percentage of errors:
· Altitude: The pilot’s readback of altitudes are often different from those assigned.
· Radio Frequency: Pilots readback incorrect radio frequency changes different from those assigned.
· Wrong Airplane: Pilots acknowledge clear-ances issued to other airplanes.
· Incorrect Heading: Pilots either turn the air-plane the wrong direction, or readback an in-correct heading.
· Restrictions: Usually, these involve crossing restrictions designed to comply with ATC in-ter-facility requirements. Some of these errors occur with restrictions needed to provide separation from other traffic.
· Airplane Call Sign: Most often, the pilot omits his call sign on the readback; however, sometimes the pilot incorrectly identifies his own call sign.
· Navigational Route: The pilot reads back a route to be flown that is different from the ATC clearance issued.
· Incorrect Altimeter Settings: Altimeter set-tings are occasionally read back incorrectly.

Summary

The most significant finding of the study comes in the form of a precaution to flight crews: just because a controller does not challenge a read-back does not necessarily mean the readback is correct.

Pilots must explicitly question any doubtful or unusual aspects of clearances, rather than de-pending upon controllers to detect readback er-rors. Likewise, controllers must force themselves to actively listen to the acknowledgments and correct any incorrect clearances or call signs.

What steps can each pilot and controller take to safeguard against readback/hearback errors?

Non-clearance Altitudes. Air traffic controllers must increase their awareness that pilots may interpret an altitude mentioned for purposes other than a clearance as an instruction to go to that altitude. An example of these non-clearance alti-tudes is when a controller advises of traffic alti-tude.

Cautionary Messages. Controllers must antici-pate and recognize situations where confusion may occur, and deliver cautionary messages such as warnings for similar call signs. Pilots must listen to their radios closely when advised of these situations.

Amount of Numbers. Controllers must reduce the amount of numbers given in each transmis-sion. For example, controllers shouldn’t issue an altitude, heading and altimeter setting all in the same transmission. This is frequently too many items for the flight crew to respond to when they are busy.

“Magic” Numbers. Pilots and controllers must increase their awareness of the readback when using the “magic” numbers that are frequently misunderstood. For example, be careful when using numbers like one zero thousand and one one thousand, and Flight Level two zero zero and Flight level two two zero.

Listen Carefully. Finally, pilots and air traffic controllers must commit themselves to improv-ing their communications by listening carefully to each other!

Conflict Alert

ATC’s “conflict alert” function is similar to the TCAS present in flight decks. TCAS presents flight crews with traffic warnings, displaying the traffic and providing solutions to resolve the con-flict.
Conflict alert is a computer-generated system that causes a flashing electronic display of air-plane symbology on the controller’s radar screen. This display alerts the controller that traffic sepa-ration is lost - or is about to be lost.

Editor’s note: Every day throughout the world, millions of communications take place between pilots and air traffic controllers. Their relationship is a vital link in the safety and efficiency of the commercial aviation industry. But all too often, errors occur in these pilot-controller communica-tions - errors that have potentially disastrous consequences