Tuesday, September 18, 2012

For real this time, decoded

I've been having a week (well, more like two, actually - but we'll save that for another time), so you guys got some extra study time on this one. But time's up, so pencils down.

Several of you figured parts of this out, and it's also apparent that we have a couple of ringers in the crowd: welcome aboard. The photo below highlights some of the contributing factors:

Decoded, this is a pair of departures that share a flight number (927) and departure time (1820z, which is 10:20 AM PDT). They also feature very similar aircraft types (A319 & A320) and ICAO airline identification code (VOI & VRD).* For the purposes of our tale, the other information on the strips is immaterial.** But that's not where the story ends:

Because this is where the two airplanes are parked on the field: Volaris operates out of Terminal 2, and Virgin America is right next door at Terminal 3. Which means that not only do they have very similar-looking strips, said strips are going to be adjacent on the ground controller's counter, and both aircraft will be on the same ground control (and subsequently tower) frequency. As you can also see from the above photo, both aircraft can be together on the same section of taxiway, and I have personally seen these two flights one right after the other as they taxi to the runway.

Thus, in this scenario we have two issues: The similar-sounding callsigns, as well as the similar-appearing strips that will be close to, if not next to, each other at the ground (and perhaps the tower) controller's position. Each poses its own challenges, the end result in each case that somebody may get confused about who's who and what they're doing (or supposed to be).

Similar Strips

The ground controller on the north side of the airport is generally the first controller to talk with both aircraft, although the strips will originate at the clearance delivery position. Because of the many similarities between the strips, it's very easy for the ground controller to grab the wrong strip when one of the pilots calls for push. If not caught right then, the ground controller may then form his/her taxi plan using the wrong flight plan information, which could then result in an aircraft being sent to the wrong runway, or airplanes reaching the runway in an unworkable sequence. If still not detected, the confusion could spread to the tower controller, who may then put an airplane on the runway that won't be able to depart. Or, worse yet, the airplane will depart and then turn the "wrong way" - and into the "wrong" sector's airspace. Or worse still - right into another aircraft. A bad day ensues for all.

By the time the airplane gets onto the runway, however, at least three controllers should have looked at the strip, the data tag on the ground radar, and (if the weather allows) the airplane itself. Many chances for this mis-match to be detected and straightened out. At some level, it always has been. Even so, we're working with the companies involved to see if one or the other could be assigned a different flight number, which would resolve the issue for good.

Similar Callsigns

As some of you have described in your comments, we have radio phraseology techniques to keep similar-sounding callsigns sorted out (discussed below), but it still requires extra diligence on both ends of the radio - and with one of the players being a foreign carrier (meaning English is not their native language), it's easy to see how things may go awry.

This particular instance arises almost daily at LAX, and so most of us are aware of it and each has his/her own way of dealing with it. One of the more common techniques is to find some other airplanes to put between the two so that they can't both be at the runway at the same moment.

Once they are in the air and the tower controller is done with them, the situation is resolved: Virgin America turns north for San Francisco and calls one radar controller, while Volaris turns south for Mexico City and switches to a different one.

As you can see below, however, these two are not the only flights that do this. I personally received this pair of aircraft back-to-back while working the tower frequency on the south side a couple of weeks ago:

Similar callsigns are a fact of life at someplace like LAX. As you can see, though, this pair doesn't have as many similarities as the first pair, and is in fact more representative of a typical similar callsign scenario. The aircraft types (B752 & MD11) and parking locations (Terminal 5 & South Cargo) have a greater disparity, although both aircraft still came to be on the same ground and tower frequencies concurrently.

The standard technique for dealing with aircraft with similar callsigns is to advise each aircraft about the other:

"Delta 619, caution similar callsigns, EVA 619 also on frequency."

After one acknowledges, then the other:

"EVA 619, caution similar callsigns, Delta 619 also on frequency."

In subsequent transmissions to either aircraft, the carrier name may be appended to the end of the callsign for emphasis:

"Delta 619, Delta, wind 2 5 0 at 1 1, Runway 2 5 Right cleared for take off."

"EVA 619 heavy, EVA, caution wake turbulence, Runway 2 5 Right line up and wait."

It's also considered good practice to warn the next controller if you're about to give him a couple of aircraft with similar callsigns, as well as whether or not the pilots have already been advised.


* - The FAA uses ICAO identifiers for flight plans; in the IATA system, these two are Y4 and VX, respectively. We don't use the IATA identifiers because in some cases they can be confused with aircraft registrations. Example: The Honduran airline
Aerolineas Sosa has been assigned the IATA code "P4". However, aircraft registered in Aruba carry registrations beginning with "P4". These would appear identical on a flight plan strip, leading to inevitable confusion in radio communications.

** - For those with curious minds, the rest of the Volaris strip decodes as follows:
  • VOI927 is the aircraft callsign: Volaris 927
  • LATT shows which printer generated this strip, in this case LA Tower
  • A319/Q is the aircraft type: Airbus 319. The /Q is an equipment suffix that describes the aircraft's navigation equipment capabilities to ATC
  • 801 is the CID, or computer identification number, that our center computer uses to differentiate this particular flightplan from all the others currently in its system
  • 7726 is the assigned aircraft transponder code for this flight; normally no other aircraft will be assigned this code while this flight is active
  • P1820 is the proposed departure time of 1820z (ATC uses Zulu time, or Coordinated Universal Time (UTC), formerly known as GMT - Greenwich Mean Time). Once the aircraft has departed, this becomes a "D" for departure time, or subsequently an "E" for enroute time over a designated fix; and eventually an "A" for arrival time at the filed destination
  • 370 is the requested cruising altitude: 37,000 feet, or Flight Level 3 7 0
  • KLAX is the departure airport; if this were an enroute or arrival strip the fix referred to by the "E" or "A" time would appear here
  • +LAXX6 MZB+ is the departure procedure to be assigned to this flight; the "+" signs indicate that this is not part of the route filed by the airline/pilot, and will have to be issued by the Clearance Delivery controller
  • KLAX LAX OCN MZB J1 TIJ UJ5 GOLFO UQ103 AGU *** MMMX is the route of flight filed by the airline/pilot. In this case, the route is so long that it won't all fit on the strip, and this is indicated by the " *** " - which tells the controller that the portion of the route between AGU and MMMX (Aguascalientes and Mexico City, respectively) has been truncated on the strip. The entire route can be seen by requesting what we call a "Full Route", which generates a strip that lists the whole thing, but in a much harder-to-read format that isn't suitable for regular operations (example below, although a different flight entirely that happened to be convenient when I was putting this together). Volaris' route reads: Departing Los Angeles International Airport direct to the Los Angeles VOR, then direct to the Oceanside VOR, then direct to the Mission Bay VOR, then via Jet Route 1 to Tijuana, then via the Upper Juliet 5 route to Golfo intersection, then via Upper Quebec 103 to Aquascalientes . . . (truncated route) . . . landing Mexico City. When the Clearance Delivery controller issues this clearance, he'll include the assigned departure and then tie it into the filed route thus: "Volaris 927, cleared to the Mexico City airport via the LAXX 6 departure, Mission Bay, then as filed."
  • If there were any remarks in the flight plan, they would appear on the bottom line preceded by a special symbol, as seen on the Virgin America strip: TCAS ACARS EQPD Decoded, that means this aircraft is equipped with Traffic Collision Avoidance System and Aircraft Communications Addressing and Reporting System. All sorts of things can appear in the Remarks section; it's like a free text area for the pilot or dispatcher to include whatever information is thought to be pertinent. Sometimes an emergency phone number, sometimes the callsign decoded if it's a new or unusual one, occasionally "Happy Holidays to ATC"
  • The empty boxes on the end of the strip are generally determined by local procedures; at LAX typical examples are: ATIS Code the pilot reported, which taxiway/runway intersection he's been assigned, or what sort of delay program or time this flight has been allotted

As promised, here's a full-route strip for you to peruse. Compare to the strips in the earlier photos. More information, but this format is not nearly as easy to read:


  1. Thanks so much for that thorough explanation. It certainly give great insight into the types of issues encountered by not only the tower controller but ATC in general.

    Keep up the good work and thanks again.

  2. Color me slightly confused. I see the VRD/Vol similarity, but you address VRD on the radio as 'Redwood,' right? Do the pilots really only listen for the flight number?

    Slightly OT, will you be in the tower tomorrow (Fri) for the big NASA party?

  3. Thank you for the terrific explanation. Are these paper strips being phased out for an electronic system at LAX? The NavCanada system north of the border has switched to an electronic system for some time. They still use 'strips' but they are represented on a computer screen rather than as a physical piece of paper in each control center and tower.

    YYC Dispatcher