by Bob Wright and Marcia Patten

"We were operating about one hour late which put us in XYZ area at the same time as [company flight] 552. Our number was 522. Controller cleared 522 direct, descend and maintain 4,000...I acknowledged and we complied. We had not heard 552 on frequency yet. Nor had we heard him respond to the same clearance. We had blocked each other and not known it until a phone conversation later. Suddenly the Controller said, '552, Where are you going?' 552 [replied], 'You cleared us direct down to 4,000.' ATC was silent for about 10 seconds, seemed longer... A target showed on TCAS at 12:00 o'clock, 2,000 feet below us. If [we had] continued we would have had a near midair. ATC continued to give both 522 and 552 a lot a strange vectors--obviously for traffic. I queried ATC about it and he said, 'You guys keep getting your flight numbers mixed up.' I know he said 522 in the original clearance, but he meant it for 552. Also 552 was expecting that clearance, so he responded. In retrospect, it was strange that we would be cleared from 9,000 to 4,000 in such a high density area. I thought maybe the traffic was light at that time." (#266870)

The Captain of Flight 522 adds:

"No matter how it happened, this is a classic illustration of how dangerous similar callsigns can be, and how a very simple slip by a pilot or controller could result in disaster. My personal feeling is that, given the number of similar callsigns that I hear, my company does not work very hard at 'de-conflicting' them...The current efforts still leave many problems out there looking for the worst possible time to happen." (#266985)

"If I Called the Wrong Number, Why Did You Answer the Phone?"

No one factor "causes" the situations reported here. Rather, as can been seen in the above report, a combination of factors on both sides of the radio leads to incidents. The purpose of this article is to inform readers where aircraft callsigns come from, and how similar callsigns can complicate communication.

Innumerable Numbers

The Aviation Safety Reporting System (ASRS) receives a large number of reports regarding callsign similarities and confusion, as air traffic and radio transmissions increase. Most reports indicate only momentary confusion or minor infractions of a clearance. Others relate incidents as severe as near midair collisions (NMACs) or serious losses of separation.

Numbers are always a potential problem in aviation as they can refer to altitudes, airspeeds, headings, frequencies, transponder codes, flight numbers or various other flight elements. ATC instructions full of numbers are often delivered rapid-fire, received and read back by a pilot in a noisy cockpit, then heard back and accepted by a harried and hurried controller. At some time in their careers, most pilots have been waylaid by all the numbers in a clearance such as, "Aircraft 46261, cleared for takeoff runway 26, wind 250 at 16 knots, turn left heading 210, climb to 2,600 feet, squawk 1216, contact departure on 126.2." (Another article on "the number crunch" can be found in ASRS Directline #2, Fall 1991, "One Zero Ways to Bust an Altitude.")

Letters may pose a problem, too. B, C, D, E, G, P, T, V, and Z are the largest group of letters that can be easily misheard. Other similar-sounding letters include M and N, I and Y, F and S, and A, J, and K. Proper use of the phonetic alphabet can eliminate much of the confusion of similar sounds. Still, similar-sounding words may be misunderstood, especially when transposed, for example, Delta Alpha and Alpha Delta, and Kilo Echo and Echo Kilo.

Take a Number, Please

Where do aircraft numbers come from? FBOs, general aviation aircraft, and most non-scheduled air taxis use the aircraft tail numbers or N-numbers (the numbers or the number-and-letter combination) as their radio callsigns. Most scheduled air taxis and commuter and air carrier airlines use flight numbers rather than N-numbers as their callsigns.

The FAA does not assign the flight numbers used by most air carriers and commuters. If it did take on this task, it would probably have as much difficulty as the air carriers themselves. Assignment of flight numbers is typically a function of an airline's marketing department. Sometimes it appears that Marketing chooses the quickest, easiest method of assigning numbers to newly-created flights. Consequently, some flight numbers have only one number that is different, some have numbers that are transposed, some just happen to sound similar, even though they may contain few, if any, of the same numbers. This problem appears to be increasing, as airline mergers and buyouts have led to operators blending flights under the same carrier name, but with a decreasing pool of available flight numbers.

For example, air carrier A buys commuter B and gains 30 new flights per day. The easiest way to assign flight numbers is to take a block of unused numbers, say, 4101-4130, and assign those numbers in order of departure time. Often odd numbers are assigned to one compass direction, and even numbers assigned to the opposite direction. This can apply to North-South flights and to East-West flights. So, for our fictitious air carrier A, SFO-PDX flights could be 4101, 4103, 4105, etc., and PDX-SFO flights could be 4102, 4104, 4106, etc. If air carrier C, which may also have undergone mergers, also has flights to and from PDX or SEA at approximately the same times using the same or similar block of numbers, there is a potential for major confusion while these aircraft are sharing the same airspace.

The FAA does assign aircraft tail numbers. Usually the numbers are assigned at random. However, owners or operators of corporate, FBO, or air taxi aircraft may request specific or "personalized" N-numbers, like many state vehicle license plates. The result can be a whole family of aircraft with similar or similar-sounding numbers and letters. These aircraft look very impressive sitting all in a row on the ramp, but their numbers can create a nightmare for both pilots and controllers if several of the aircraft depart in the same direction at the same time.

What is Similar?

"Similar" generally means having a resemblance to one another or to something else, or like but not completely identical.

What seems or sounds similar to one person may not sound similar to another. Hence, a pilot may be utterly confused about which aircraft a controller is giving instructions to, while the controller is frustrated and impatient with the pilot's hesitation or failure to comply with an instruction. What sounds similar to a pilot in a noisy cockpit may not appear to be a problem to a controller looking at a radar scope. Likewise, a controller looking at flights 404, 1441, and 4124 on flight progress strips or a radar screen may feel muddled with all the similar numbers, while the pilots of these flights may not have any trouble hearing their respective flight numbers called out to them.

Aircraft with similar callsigns arriving or departing a destination at the same time sets the stage for a mix-up. Even with a thorough understanding of the potential for confusion, this Captain admits to being caught off-guard on occasion:

• "Every morning, air carrier A flight 123 leaves [one airport] while air carrier B flight 123 leaves [a nearby airport], both headed for the same destination.... Both flight crews of both flights have been aware and alert for the obvious probable problems of this situation. In spite of their alertness, at least a dozen clearances to one of the aircraft have been read back or at least questioned by the other. On one occasion, the wrong aircraft left its cruise altitude based on the clearance intended for the other."

From his recommendation, it sounds as if the Captain has given up on company avenues, and is seeking ATC intervention:

• "If the operators cannot prevent this type of situation, each ATC Center should not accept two or more aircraft with the same numbers in the callsign." (#210928)

Similar Numbers + Human Error = Callsign Confusion

Although the callsign problem originates in the management arena, flight crews and controllers add human error to the equation. Any number of human factors can combine to cause miscommunication.

Say What?

Overall radio and communication technique appears to be a major contributor to callsign confusion. Use of an abbreviated callsign, although a common practice and completely legal, can invite a misunderstanding. Related to communication technique is the issue of readback/hearback, frequently cited in reports of callsign confusion. Often this is a case of selective listening--a pilot's expectation of a particular clearance, and a controller's expectation of a correct clearance readback.

• "Operating flight 961, we received and acknowledged a clearance... A few minutes later a flight 691 received and read back the same clearance. I commented on the similar callsigns and asked the other crew members to listen carefully. A short time later, the Controller...asked us to stop our descent...and said something to the effect, 'I believe that clearance was for another aircraft.' Nothing more was said on the radio and there appeared to be no conflict.
  
  "This is a classic example of the confusion that arises when there are similar callsigns in the same airspace. Everyone hears what they expect to hear. If the Controller misstates a callsign, he probably will expect to hear a readback from the aircraft he intended to address and the mistaken readback will not register. If a pilot is expecting a clearance, he can mishear a callsign. I have heard more errors recently involving the transposition of callsigns (961 versus 691), than errors between similar sounding callsigns (1468 versus 468). Everyone must listen carefully to clearances and readbacks and, to the extent possible, the airlines must try to separate similar callsigns." (#268738)

Too Busy to Listen

Workload and fatigue, particularly in bad weather, can take their toll on people's ability to concentrate and perform to the usual standards. For flight crews, fatigue can be the culmination of schedule pressures, long days, and multiple takeoffs and landings at the same airport with quick turnarounds. The addition of frequency congestion can seriously muddle communication.

• "There were three aircraft on frequency with similar sounding callsigns. We all had been 'stepping on' each other's radio transmissions and on Center. The callsign of the other aircraft was very similar to our callsign on our [previous] leg (this was our seventh and last leg of the day)." (#268344)

High traffic volume, the presence of airline hubs, and combined position operation (for example, working Ground and Tower simultaneously) can overload ATC personnel and leave pilots frustrated, as the next report illustrates:

• "I believe the Controller was working 2 frequencies and the fact that there were similar flight numbers on each frequency were the contributing factors. Controllers working separate frequencies make it very difficult for us to maintain as high a situational awareness as we could. On a single frequency we are able to hear if another flight answers the Controller's instructions when we think the instructions are for us. That raises the question in our minds and we are able to ask, 'Was that last call for us?' Similar flight numbers, be it on a single frequency or separate frequencies, are a major drawback." (#246226)

Juggling Act

Modern electronics--GPWS, TCAS, FMS, and ACARS, for example--abound in an aircraft cockpit. All that complex equipment, as well as cockpit checklists, company communications, and PA announcements, may produce additional distractions for pilots trying to listen for critical radio transmissions. A Captain provides an example of distraction and divided attention:

• "The copilot...turned the radios over to me so he could do a passenger announcement. I acknowledged a frequency change. I then checked in to the new frequency, using the callsign of my previous flight. The Controller, who I think was expecting me, gave me a clearance for the correct callsign. I acknowledged, apologized for the mistake, and continued without incident. In retrospect, I feel I was too quick to accept the clearance. I could have easily taken someone else's clearance...You lose your system of double check when one pilot is off the air getting ATIS, doing a passenger announcement, or talking with company." (#292850)

Confusion Conclusions

Phraseology. Communication technique is still pilots' and controllers' primary defense. Use of proper phraseology with full callsigns in every radio transmission can eliminate many of the types of miscommunications cited here.

1. Attention Management. Attentiveness to radio traffic and timeliness of transmissions can ease frequency congestion.
   
   
2. Cockpit Communication. Communication within the cockpit can be critical--all crew members need to be "in the loop" at high workload times or during crucial radio communications to prevent readback/hearback errors. Whenever there is doubt about a clearance or instruction, crews should clarify the matter among themselves and with ATC.
   
   
3. Procedures. Adherence to procedures can also play a major role in alleviating callsign confusion. Careful readback and confirmation of clearances are all-important. Pilots and controllers both need to question transmissions that are out of the ordinary or don't sound "right," and ask for a repeat if a transmission is not clear or may have been stepped on.
   
   
4. Technical Solutions. Technical improvements may help some aspects of the problem. Pre-Departure Clearances (PDCs) have cut down somewhat on congestion of the clearance delivery frequencies, but the FAA has not yet provided a system for confirmation or acknowledgment of the correct clearance being correctly received by the flight crew. (For more information, see ASRS Directline #5, Spring 1994, "PDCs--the Problems with Pre-Departure Clearances.") Data Link may improve problems related to frequency congestion, blocked transmissions, speech rate, foreign and regional accents, and pronunciation (manner of speech) and enunciation (speaking distinctly or with clarity). Human error can still occur, however, in keying the input into the Data Link system, or in trying to read a poorly-lit ACARS screen or a poor quality print-out.
   
   
5. Company Policy. Company planning and marketing departments in all segments of aviation (air carriers, commuters, corporate departments, and FBOs) need to be mindful of the potential for callsign confusion.
   
   
6. For air carriers assigning a new block of flight numbers, a quick check of the Official Airline Guide might help determine if conflicts exist with other aircraft using similar numbers along the same routes, or into and out of airports at the same time. Coordination between a company and its regional affiliate can eliminate a situation such as Company B 454 and Company B Express 1454 arriving at the same destination at the same time.
   
   
7. For non-air carriers using sequential tail numbers or transposed tail numbers (e.g., 58SH/68SH or 404SH/404HS), a request for assignment of less-confusing but still meaningful N-numbers might be an option.

Dealing with Conflict

Currently, pilots can go through channels within their companies to request changes for numbers that are a problem. Pilots might also consider taking this issue to ALPA or ATA, to bring attention to the situation in an industry-wide forum. This might encourage more coordination between airline companies. Controllers have the option of talking to their supervisors about ongoing problems with similar callsigns, and the supervisors can take a case to airline representatives. But there is no promise of action. Pilots and controllers need to continue to bring callsign problems to the attention of management, and as always, all are encouraged to submit reports to ASRS.