Good question. This pilot, like the vast majority of pilots, is unaware of the controller regulations that govern calling traffic. But this is knowledge that could benefit any pilot who uses the air traffic control system. Most of the time the distinctions among the various situations fall into the "nice to know" category. Merely looking in the direction indicated and scanning for the traffic is enough. Sometimes, however, being able to properly interpret what the controller is saying can be critical.
There are actually three separate rules that govern traffic calls. The first is simply called a traffic advisory. The purpose of traffic advisories is to make the IFR pilot aware of VFR traffic in his vicinity. For example, let's say that a pilot is cruising at 7,000 feet. Another aircraft flying VFR is converging with him at 6,500. With 500 feet between them there is no reason why these two aircraft should be in any danger. The problem is that unless the two aircraft are in Class B or Class C airspace (where controllers provide positive separation to both IFR and VFR aircraft) the controller has no control over the VFR aircraft. In fact, it's entirely possible that the VFR pilot is not talking to any air traffic controller and is completely unaware of the presence of the airplane just 500 feet above. If the pilot of the VFR airplane at 6,500 should decide to climb to 8,500 at the precise moment that the aircraft at 7,000 is approaching, a very dangerous situation would occur. The controller's purpose is to make each of the aircraft he is working aware of the other's presence so that each pilot can spot the other aircraft and take action should it become necessary. Again, the controller may be working one or both aircraft involved.
In this example, the pilot at 7,000 feet is at an IFR altitude. He would most likely be in communication with the controller. The aircraft at 6,500 feet might be in communication with the same controller if he were receiving VFR flight following, or he may be on his own, talking to no one. In the first case the controller would provide an advisory to both aircraft, in the second case, only to the one at 7,000 feet. One would assume that if the VFR aircraft were being worked by the controller and had received an advisory, it would delay any change in altitude until it had passed the other aircraft. But pilots should understand that it's the VFR pilot's decision whether or not to change altitude. The IFR aircraft is being controlled; its route and altitude will not normally change without authorization from ATC. Assuming the VFR aircraft is outside Class B or Class C airspace, it is not under ATC control, and that pilot's assessment of the potential risk may not be the same as yours or mine.
When a pilot receives an advisory, he should scan the area indicated and respond to the controller with either "looking" if he does not see the other aircraft or "traffic in sight" if he does. Once the pilot has advised "traffic in sight," the controller will cease calling advisories. Otherwise he will continue advisories until the traffic has passed. He will then transmit the words, "Traffic no longer a factor." It goes without saying that, although this is the ideal procedure, in reality busy controllers sometimes do not find the time to make every call.
It is up to the controller to decide which traffic to call and which not to. The ATC handbook says that a controller should provide advisories on targets that are a factor in the judgment of the controller. As with any endeavor governed by individual judgment, there will be variations. For example, a radar controller who is not a pilot and has never had to try to pick out distant traffic in the haze may call traffic that is passing six miles off your wingtip. If the traffic is a single-engine Cessna, the pilot receiving the advisory would have virtually no chance of seeing the traffic. Another controller might not consider calling traffic unless it is going to pass within two miles. In either case, the pilot receiving the advisory should continue looking until he hears, "Traffic no longer a factor," and should not be unduly concerned if he never spots it.
The second rule controllers use is known as merging target procedures. Merging target procedures are called for when two IFR aircraft target symbols on the radar scope are going to touch and they are separated by the minimum appropriate altitude. Let's go back to Cessna Two-Eight-Charlie at 7,000 feet and imagine that the second aircraft was IFR at 8,000 feet. An advisory was issued. Had the other aircraft been at 10,000 feet (more than the minimum appropriate altitude - IFR aircraft are separated by 1,000-foot intervals below 29,000 feet), no advisory would have been given. The cue to the pilot that this is a merging target procedure and not a mere traffic advisory is the fact that the other aircraft was called at an IFR altitude. This implies that both aircraft are being controlled. This of course leads to the question; "If both aircraft are under control and are assigned safe altitudes, why bother with calling traffic?"
For the answer I turn to a story that has been floating around air traffic control rooms for decades. I have no proof that the story is true, but I suspect that it, or a close variation of it, did happen.
Sometime in the 1960s, two jet airliners operating IFR were converging head-on. One was assigned 21,000 feet; the other was at 22,000. The weather was hazy, with an indistinct horizon. The pilot of the airplane at 21,000 feet looked up suddenly, saw the other aircraft coming at him, and became convinced that the other aircraft was at the same altitude. He pulled back on the yoke to take evasive action. (Since passengers are generally free to walk around the cabin of transport category aircraft, airline pilots can be counted upon to pull back on the yoke when faced with sudden action if they have any choice at all. This way passengers are forced to the floor or their seats instead of floated to the ceiling.) Fortunately there was no collision, but there was a very near miss as the pilot tried to avoid an aircraft that was already legally and safely separated from him. From this incident came merging target procedures. The controller is making the pilots aware of the fact that the other traffic is safely separated so that no one panics and takes unnecessary, and potentially dangerous, evasive action.
Merging target procedures are applied to jet aircraft at all altitudes and propeller driven aircraft at altitudes above 10,000 feet. In the real world, however, most controllers, rather than having to remember when they should and when they shouldn't apply the procedure, go ahead and call traffic for any aircraft that will merge and are separated by the minimum IFR altitude.
The last rule that controllers must follow is the safety alert. Safety alerts are traffic advisories given when there is a very real possibility of a collision. Contrary to popular belief, controllers may not just sit and watch two VFR aircraft run together. Once a controller is aware of a safety issue, assisting the pilots involved becomes his top priority.
Let us say that you are receiving VFR flight following from an air traffic control center. You are at 6,500 feet. Unknown to you, the controller has been watching another VFR aircraft squawking 1200 approaching you head-on. The other aircraft's Mode C transponder has been reflecting a climb. The aircraft was at 5,600 feet when the controller first noticed him, then 5,800, 5,900, 6,100.... It looks as though the aircraft will reach 6,500 feet about the time his target merges with yours. You will hear the following phraseology: "Cessna Two-Eight-Charlie, traffic alert, 12 o'clock, three miles, opposite direction, altitude indicates 6,100 and climbing."
The clue that this is a safety alert and not just routine traffic is the phrase "traffic alert." Normally, when calling traffic the controller simply says "traffic." The addition of the word "alert" completely changes the meaning, however. With that one word the controller is telling you that you are in danger. The two methods of calling traffic are straight out of the ATC handbook. Since they sound so similar, however, many controllers modify the phraseology slightly and say "safety alert" instead of "traffic alert." The idea is to use a word pilots are not used to hearing controllers say ("safety") to alert them to the fact that this is not just a routine traffic advisory. If you ever hear an advisory that contains the words "alert," "safety," or something similar, take it very seriously.
What happens if you get a safety alert but don't see the traffic? The threat aircraft is almost always operating VFR. If both you and the threat aircraft were operating IFR, the controller wouldn't let the situation get this far in the first place. If by some error the situation has escalated to this point, you can bet that the controller already is issuing instructions to resolve it. But if the threat aircraft is VFR, you should understand that although the controller has a responsibility to issue a safety alert, he has no legal responsibility to resolve the situation. Many controllers feel a moral responsibility to take action and will issue a vector away from the other aircraft. Others will simply issue advisories. Remember, other than in Class B or Class C airspace, positive separation is not applied between two VFR aircraft or between IFR and VFR aircraft.
Controllers have a responsibility in safety alert situations to assist the pilot in avoiding the other aircraft if the pilot so requests. In other words, you hear a safety alert and scan for the traffic but don't locate it. You may tell the controller, "Give me a vector." He will. Just don't wait until the last minute. On the scale typically used by a center controller (100 miles or more), once you are down to about two miles, the controller can no longer resolve your position well enough to be sure whether you are better off turning right or left.
Some pilots are incredulous that the FAA doesn't require controllers to take more positive action in safety alert situations. There is a reason for this. Pilots must understand how the situation looks from the controller's point of view. The scale used on the radar scope by the controller will vary with the geographic location and whether the facility is an ap- proach control or center. In a center environment, two aircraft that will miss by a mile and a half may have targets that will touch. Obviously these two aircraft look very close to the controller, and a safety alert will be given. But as a pilot, I am not particularly concerned about an aircraft that is going to pass a mile and a half off my wing. If the controller vectored me aggressively, most of the time it would be for no real safety reason. I prefer the system as it is: The controller alerts me. I scan for the traffic, assess the information, and can ask for a vector. I don't want ATC flying my airplane for me. Give me the information and I will exercise my authority as pilot-in-command to decide what I am going to do about it.
One last tidbit about traffic calls concerns the altitude of the other aircraft. You will sometimes hear "traffic, two o'clock, westbound, at six thousand, five hundred..." Other times you'll hear "traffic, two o'clock, westbound, altitude indicates six thousand, five hundred." What's the difference? In the first case the controller uses the word "at" because he knows the altitude is accurate. How ? There is only one way. The controller is talking to the pilot who has, at some point, confirmed his altitude. This is known as "validating Mode C."
In the second case, the controller is not talking to the pilot and only observes the Mode C altitude on the radar scope. He has no way of confirming that the readout is correct and will use the words "altitude indicates."
Understanding the nuances behind traffic calls and the three basic rules that govern air traffic controllers can help you to stay safe as you fly. The system is there to help you. Use it.
