Loud and Clear

What's ATC trying to say?

September 1, 2004

Radar service terminated , frequency change approved." How many times has each of us heard this phrase? Simple, isn't it? The meaning is clear. Or is it? What exactly is the air traffic controller telling us when he uses these words?

The FAA definitions for the most common ATC phrases are available in the pilot/controller glossary at the end of the Aeronautical Information Manual, but even this resource is often overlooked. When it comes down to it, most of us learn phraseology by listening to it being used on the radio. There is very little in the way of explanation. It's ATC jargon by osmosis, more or less.

In the case of "radar service terminated," it's worth clarifying the meaning of the word terminate and pointing out some of the potential pitfalls when we don't fully understand its use.

When a controller identifies an aircraft on his radar display, he generally advises the pilot that he is in "radar contact." The pilot can then expect to receive all the services available to an identified target. These include several things. One is traffic advisories. Once the controller sees where you are, he is in a good position to let you know when you get close to other targets. Another is terrain advisories. This kind of advisory can be critical if you are in clouds, it is nighttime, or you are otherwise operating in restricted visibility. (See " Safety Pilot: Going Bump in the Night," August Pilot).

Sequencing to an airport is another radar service. For an IFR flight this might include vectors to the final approach course, a service not available to non-identified aircraft. For VFR flights, this may mean sequencing and spacing. Information on one's position relative to weather radar returns is yet another advantage for the pilot whose plane is identified.

Finally there is what, for lack of a better term, I will call "assistance to pilots." Let us say that you are flying VFR to a small airport within a big terminal area like Dallas. While legal VFR, the prevailing visibility is barely four miles and you are having trouble finding your destination. Obviously, if the controller knows where you are and where the airport is, he is in a good position to suggest a heading.

When a controller uses the words "radar service terminated," all this goes away. In most cases, he can still see you on the radar (if he actually loses your target, he will advise you "radar contact lost "), but he is no longer going to watch over you. He may terminate service when you are approaching the airport and he clears you from his frequency to go to the tower or advisory frequency.

Service may be terminated when you pass from the jurisdiction of a controller who has radar to the jurisdiction of one who does not. There was a time when non-radar approach controls were not uncommon in the United States. These were approach controls in which controllers separated aircraft by use of pilot reports instead of tracking them on radar. Such non-radar approach controls are common in Mexico, the Bahamas, and many other parts of the world. When you fly IFR to Freeport, Bahamas, for example, you will first be worked by Miami Center. Unless you are very low, they will have little trouble tracking you on radar most of the way to Freeport. At some point, however, you will pass into Bahamian airspace and the Miami controller will be required to have you contact Freeport Approach. While Miami can still see you, Freeport, lacking radar, cannot. As Miami switches you over, you will hear "radar service terminated...."

Probably the most common use of this phrase is when a VFR aircraft receiving radar service in Class B or Class C leaves that airspace. Since flight through Class B or C requires being in contact with the appropriate controller, there has to be some way of ending that service and sending the pilot on his merry way. The words "radar service terminated, squawk one-two-zero-zero, frequency change approved" accomplish this. Again, the controller may be able to see you for some time afterward, but service is no longer being given.

Radar service also is terminated when an aircraft on an IFR flight plan cancels its IFR clearance. The controller responds with, "Cancellation received, squawk one-two-zero-zero, frequency change approved." Translation? "We got your cancellation, don't forget to change your transponder to VFR, see you later." The words "radar service terminated" are not actually stated in this case, but that is implied.

Sometimes controllers use the word terminate in a way unfamiliar to most civilian pilots. I recently overheard the following conversation between a center controller and a military fighter. The controller speaks first: "Snake two-six, advise termination." "Roger, center, Snake two-six, terminate." "Snake two-six, copy termination, advise prior to exiting the area."

What's up with all that? Generally speaking, to a military pilot receiving radar service en route to a military operations area (MOA), terminate means that he is established within the MOA, is in visual meteorological conditions (VMC), and wishes to leave the ATC frequency so as to accomplish his training mission without having to monitor ATC chatter. He retains his IFR clearance, switches to a discrete training frequency so that he can communicate with his wing pilots, and switches back to ATC when he is ready to exit the MOA.

For the most part, the use of the word terminate does not cause any grave misunderstandings. Fortunately when a pilot misinterprets the word it is unlikely to cause a serious problem. It can, however, serve to make extra work for the controller, slowing down the system and branding the culprit as a rookie.

Let's say that a noninstrument-rated pilot has flown often from a reliever airport in a busy terminal area. He has routinely received radar service within Class C airspace and is used to hearing the phrase "radar service terminated, squawk one-two-zero-zero, frequency change approved" at some point on his outbound flight. Now this pilot earns his instrument ticket and starts flying IFR cross-country to other airports. After being cleared for a visual approach to a nontowered airport he hears "radar service terminated, frequency change approved." The controller's instructions sound very much like what the pilot has been used to hearing. Without thinking he changes his transponder code to 1200 and leaves the frequency. But wait! The controller never said to squawk VFR. After a couple of minutes, the controller notices that the transponder code associated with this IFR flight is no longer being painted on the scope. There is a VFR code being displayed in the same approximate location, but can he be sure it's the IFR flight? Maybe it's another aircraft operating VFR that the controller had not noticed earlier? "Oh my God, did my guy go down?" the controller asks himself. The pilot does not respond to the controller's calls, but then again the controller did just advise him that frequency change was approved. "Should I call search and rescue, or not?"

The pilot should have left his transponder alone, contacted unicom for advisories, and returned to the ATC frequency when he was ready to cancel his IFR clearance. In all likelihood this error won't cause any great harm; but it is definitely a distraction for the controller. In the ATC world distractions are bad.

Here's another example; this one is more common.

An aircraft departs VFR from a reliever airport underneath Class B airspace. The pilot calls approach control, receives the proper clearance, and climbs to 6,500 feet. This particular pilot's policy is to get flight following all the way to his destination, and at some point he mentions to the approach controller that he would like to have that service.

Shortly afterward, as he exits Class B airspace, the pilot hears the following. "Cessna One-Two-Three, radar service terminated, squawk one-two-zero-zero, contact Denver Center on one-two-eight point seven." The pilot acknowledges, changes his transponder, and calls on the new frequency. "Denver Center, Cessna One-Two-Three, with you at six thousand, five hundred." There is a pause before the controller responds, often the first sign that something may be amiss. Finally the controller answers, "Cessna One-Two-Three, no, sir, you're not with me. Where are you?" The pilot answers, "Center, we just got handed off from approach control. We're 30 miles southwest of XYZ." "Well, you weren't handed off to me. Squawk four-three-zero-one and remain VFR. What is your destination?"

It gets resolved. Center identifies the flight and the pilot gets his flight following. But he is vaguely confused and annoyed at the center controller's attitude. What went wrong?

To begin with, one needs to understand a little about how approach controls relate to centers.

When approach control identifies a VFR aircraft on radar, they may or may not routinely make a handoff to the outlying center. Most approach controls do if they have the time. A handoff means that control and communications are transferred from one controller to another. One controller is telling another, "Here the guy is; are you ready to work him?"

In an automated handoff, information about the flight is entered into the ATC computer. The computer causes a data block to appear on the center's radarscope and the data block begins to flash on and off. When the center controller is ready to accept the flight, he makes a computer entry that "accepts" the handoff. The approach controller receives an indication of this and at that point instructs the pilot to "contact Denver Center on one-two-eight-point-seven." There is no mention of the radar service being terminated because service has been smoothly handed off from one controller to another with no break.

Without automation as the VFR flight approaches the boundary between approach control and center, the approach controller calls on a land line. "Hey, center, you want to work a Cessna to Colorado Springs?" The center controller, who up until now has had no information about this particular flight, replies that he will (if he is busy, he might not). At that point the approach controller briefs him on the details. "Four-two miles south of Denver, code zero-two-two-seven, is a Cessna one-two-three, a Skyhawk to Colorado Springs." The center controller looks on his radar display at the position indicated, sees the code on his radar and responds, "Skyhawk one-two-three, radar contact." The pilot is then instructed to contact the center and his service continues uninterrupted.

All this occurs behind the scenes. The clue to the pilot that a handoff has been made is the fact that he is never advised "radar service terminated." When the pilot checks on the new frequency, he should do so fully confident that the new controller knows who he is.

Now let us assume that the approach controller had no time to enter the data into the computer or make that land line call. Center has no information. As the Cessna leaves approach control airspace, the controller says, "Radar service terminated, squawk one-two-zero-zero, frequency change approved." Most pilots understand that they are on their own. But sometimes, the controller will add one bit of extra information. No handoff has been made to center, but the approach controller figures he'll help out by giving the center frequency so that the pilot won't have to look it up himself. This is what happened to the pilot in our story. "Radar service terminated, squawk one-two-zero-zero, contact center on one-two-eight-point-seven." Since a frequency was included, to the pilot it sounded like a handoff; but it was not.

When the pilot checked on with center using the words "with you" the center controller was momentarily confused. In ATC jargon, with you is only used by pilots who have already been identified and are in the system.

Bottom line? If you are instructed to change frequency with no mention of a change to your radar service, you can assume that the next controller knows who you are. But hearing the words "radar service terminated" is one key that the next controller has no knowledge of you. Being told to squawk VFR is another.

An aircraft squawking one-two-zero-zero is just one of many. The minute the controller takes his eye off that target, he can no longer be sure when he looks back whether he is looking at the same airplane, or someone else who just popped up squawking the same code. In ATC there are exceptions to almost every rule. But if you are VFR and are told to squawk one-two-zero-zero, in almost every case you are no longer being tracked or receiving radar service. Even if the controller gives you another frequency, assume that you are starting all over with a controller who knows nothing about you.

As a last resort, if you are unsure of your status, ask the controller in plain language.


Robert Snow, AOPA 376566, is an air traffic controller and airline transport pilot with 7,500 hours.