The Separation Game

From the cockpit, the airway looks deserted. So what's ATC's problem, anyway? Separation standards, that's what. To understand why controllers do what they do, it helps to know the rules that they're required to play by...not to mention the infamous "snitch patch."


It’s sometimes difficult for pilots to understand why air trafficcontrollers do the things they do. They often reject our requestsfor altitude changes, vector us away from our destinations, oroccasionally request what seem like unnecessary speed adjustments.These delays are bad enough when IFR in real weather but they’redownright aggravating when you’re IFR in good weather. If it’sclear-and-a-million miles and there’s not another soul in sight.Trust me fellas, I’m not gonna hit anybody.

A pilot’s impatience is understandable. After all, our aircraftare supposed to be an efficient means of transportation. Anytimewe’re deviated from the planned route or altitude, it costs timeand money. Unfortunately, nobody ever seems to tell the pilotwhat’s going on. Therein lies the most profound disconnect betweenpilot and controller, one that fosters mutual distrust.

It helps to know just what it is a controller is supposed to do.Simply stated, ATC’s job is to "safely expedite the flowof air traffic." To achieve this, controllers must keep theairplanes in their charge pointed more or less in the directionthey’re supposed to be going and sufficiently separated to keepthem from running into each other. Separation is the key wordhere because when the clouds are full of airplanes, pilots can’ttend to separation on their own; they have to rely entirely onATC.

Money-Back Guarantee

Let’s begin our discussion by reviewing some of the ways controllerskeep us from running into each other. First of all, ATC’s primaryguarantee is a basic vertical and horizontal separation standardfor IFR aircraft. Note the emphasis on IFR. In some cases, ATCis obligated to separate VFR traffic as well and there are instanceswhen VFR aircraft are handled more expediently.

But in general, the separation guarantee extends only to IFR aircraft.These days, separation is is usually (though not always) achievedby radar so I’ll assume we’re flying in radar contact. Separationstandards vary according to who owns the airspace you’re flyingin.

Vertically, no IFR aircraft may come within 1,000 feet of anotherIFR aircraft, thus if you’re cruising westbound at 6,000 feet,your protected airspace extends from 5,000 feet to 7,000 feet.The minimum horizontal separation, on the other hand, dependssolely upon your distance from the radar antenna. IFR aircraftwithin 40 nautical miles of the antenna site must be given a minimumhorizontal separation of three nautical miles. Those flying beyond40 miles from the antenna must be separated by five miles. Inreality, this means that you’ll be guaranteed five miles of separationwhen working with an enroute facility (center) and five mileswhen working with a terminal facility (approach, departure, ortower).

These are minimum separation standards. The controller may throwin a mile or two of additional separation as a fudge factor soin practice, you’re likely to get seven or eight miles from acenter and five with approach. There’s good reason for the extraspacing. A controller’s life is made very unpleasant if he orshe allows two airplanes to bust minimum separation. Center radars,that is enroute facilities, are equipped with a sinister devicecalled a snitch patch. Essentially, thesnitch watches over the controller’s shoulder, sounding an alarmif two aircraft bust minimum separation.

Here’s an example that puts the snitch into the pilot’s perspective:Let’s say your aircraft is equipped with an onboard computer thatnotifies your boss when you exceed a speed of 200 knots. Companypolicy is to ground pilots without pay for six months if theyset off the alarm. The computer doesn’t care if you’re slow orarrive late at your destination, it only cares if you’re overthe 200-knot limit. How would you deal with it? I’m sure you’ddo the same thing I’d do; fly 50 knots slow, just in case. Ifyou picture a number of aircraft, each with different performancecharacteristics all converging on the same VOR and all askingfor a lower altitude so they can set up for the approach, youcan see why controllers get maxed out.

Tricking the Snitch

Enroute separation standards, as monitored by the snitch, resultin some truly ridiculous situations. Several years ago, I wasI was IFR northbound on V-225 near Key West. I was being heldlevel at 4,000 feet by Miami center, even though I was anxiousto get more altitude between me and the Gulf. The reason? Oppositedirection IFR traffic. A Cherokee was southbound on V-225 at 5,000feet, 12 miles ahead. I knew the controller couldn’t give me higherbecause my climb would take me right through the Cherokee’s cylinderof protected airspace.

Once the controller called the traffic passing off my left wing,I assumed I’d be cleared to climb. Not so. I had to wait untilthe Cherokee was at least five miles behind me. Climbing any soonerwould have set off the snitch. Even if I’d seen the traffic, whichI hadn’t, it wouldn’t have made any difference. The ATC rulebooksaid that visual separation wasn’t allowed in ARTCC airspace.

Since that episode, the FAA has liberalized the separation rulesfor center controllers somewhat. In the case of opposite-directiontraffic, if one of the pilots reports seeing the other aircraftpassing visually and the controller verifies that their radartargets have passed one another, he no longer is required to maintain1000′ and five miles between them…the FAA powers-that-be havefinally admitted that such aircraft are unlikely to back intoone another. More recently, the rules have changed to permit centercontrollers to utilize visual separation between aircraft flyingbelow FL180.

But these rule changes are relatively new, and their use is strictlyat the controller’s discretion. Some controllers use these toolsregularly to expedite traffic flow, and others stick strictlyto the 1000-and-5 minima.

As general aviation pilots, we can exercise a couple of tricksto get around this situation which aren’t available to our aircarrier or military brethren. We can, in effect, put the IFR separationstandards on hold temporarily and get treated like a VFR kindaguy. Perhaps the two most useful ways of doing this are the VFRclimb or descent and the VFR-on-top clearance.

The VFR climb or descent, if requested by the pilot and approvedby the controller, allows you to operate VFR while you’re changingyour altitude. Once you reach your new altitude, you’re IFR again.Of course, you have to meet VFR cloud clearance and visibilityrequirements during the climb or descent. Had I requested a VFRclimb off Key West, I probably wouldn’t have had a problem. However,because of increased VFR separation standards, this trick mightnot work in a TCA or a TRSA.

A VFR-on-top clearance is similar. It allows you to operate VFR(at the appropriate hemispheric altitude), but also keeps youin the IFR system. Since you’re still an IFR aircraft, controllersare required to provide you with traffic advisories and safetyalerts in ARTCC airspace and the appropriate separation in a terminalarea. You get the best of both worlds. You’re still IFR if youneed to punch through some clag on the approach but your separationstandards are greatly reduced.

But there are a couple of catches. First of all, pilots have toto ask for VFR climbs, VFR descents, and VFR-on-top. Controllersare specifically prohibited from suggesting these, which is asit should be. A really adventurous controller might say somethinglike this: "Say again…understand you’re requesting VFR-on-top?"Take the hint and you’ll get what you want.

Because they aren’t all that common, a controller may occasionallyforget that VFR-on-top clearances are authorized. So if you’venegotiated one with one controller and you’re later instructedto contact a new controller "for further advisories,"he may think you’re VFR. Start asking questions. And if you requestVFR-on-top and are told to squawk 1200, you better speak up becausethe controller didn’t get it.

Terminal Talk

Once an aircraft enters the terminal area, controllers and pilotshave more separation options. Besides smaller horizontal separation(typically three miles), controllers are allowed to authorizewhat’s called diverging course separation. The concept is simpleenough to grasp. If the controller observes on radar that twoaircraft have passed and are on diverging courses, that’s allthe separation that’s needed. He is therefore not required toissue vectors or altitude changes to maintain the three-miles-and-1000’standard generally in force in the terminal area.

Unlike the new opposite-direction rule for center controllers,the terminal controller can use diverging course separation withoutvisual confirmation by a pilot. Of course, that would never workfor center controllers, because an ARTCC scope gets its radardata from a mosaic of radar sites, each of which views the worlda little differently. It’s possible, for example, for two targetsto come from separate radars, even though the airplanes are rightnext to each other. So the targets may not have passed each otheror diverged, even though it looks that way on the radar screen.

In addition to the normal three-mile separation rule, terminalcontrollers are permitted to use something called "conflictresolution" in Class B and C airspace. Conflict resolutionapplies to separating an IFR aircraft from a VFR aircraft, andmeans simply that the controller can see space between the twotargets on his screen — "green between" in controller-speak.If the targets don’t touch, then the aircraft are considered separated.If they do touch, then the aircraft must be separated verticallyby at least 500 feet. It’s easy to see how much more efficientthis is in a high-density terminal area and it’s safe if the weatheris VFR. Keep in mind, though, that IFRs must still be separatedfrom other IFRs by the standard three-and-1000.

Cleared for the Visual

Visual separation may be new stuff to center controllers, butit has always been allowed in terminal areas and is heavily usedby terminal controllers. The requirement couldn’t be simpler.If the pilot reports that he has traffic in sight, he may be instructedto "maintain visual separation with that traffic." Inother words, get as close as you like but don’t hit him.

Additionally, if a tower controller can see both aircraft, hemay also use visual separation. The difference lies in who hasresponsibility for the separation. Obviously, if you report havingyour traffic in sight, you’re responsible. That’s why it’s importantto notify ATC should you lose sight of your traffic. This is notime to play "I’ve got a secret." If the tower is makingthe call, the controller is responsible for separation. He’llpoint out the traffic for you and, if you’re in the pattern, hemay offer to call your turn to base or final.

Visual separation standards go hand in hand with visual and contactapproaches. The important thing to remember about a visual approachis that the airport can’t be reporting weather that’s less thanVFR in order for a controller to authorize it. In a control zone,that’s at least a 1,000 foot ceiling and three statute miles ofvisibility. At an uncontrolled field with no weather reporting,the pilot will be told "no weather available for Podunk muni"but the controller can base his approval on weather reports fromnearby airports or from pilot reports. In order to get vectorsto a visual approach, the reported ceiling must be at least 500feet above the minimum vectoring altitude (MVA) at an airportthat reports weather.

If weather isn’t available, you can still get vectors, providingthe controller has some assurance that VFR conditions exist orif you’ve been told no weather is available. You’ll usually haveno way of knowing what the MVA is in the area you’re in. The firstclue will be the controller’s denial of a request for vectorsto the visual. Here’s an example: You’re approaching an airportfrom the west, where it’s bounded by mountains. Another aircraftis approaching from the flatlands east of the airport. The otherguy might get vectors to the visual but you won’t because you’rein an area where the MVA is higher, due to terrain. There willbe circumstances when you’ll be vectored for an instrument approachin marginal or good-VFR weather. If you don’t want the approachand can see the field, by all means report it in sight. You’llbe immediately cleared for the visual, traffic permitting.

Believe it or not, there are instances when an airport is reportingIMC weather conditions when pilots are calling the field fromten miles out. Can you be cleared for the visual in this situation?No, even though you can see the field, the rules say no visualapproach if the reported weather is less than VFR.

I’ve heard some very interesting exchanges between controllersand pilots in these circumstances. It usually happens when thefield is reporting something like two-and-a-half miles in fogbut really has inflight visibility of ten miles or better. Pilotswill call the field and ask for the visual, only to be denied.Sometimes, though, the controller is pitching but the pilot’snot catching: "Sir, I understand you have visual CONTACTwith the runway, but the field is IFR, and I’m unable to approvea visual approach."

He’s hinting at a contact approach. Unlike its "visual"cousin, the contact approach must be requested by the pilot. Itcan’t be initiated (or even suggested) by controller. To approveit, the controller has to be sure that the reported visibilityat the airport is at least one mile and the pilot has to remainclear of clouds. And, of course, the airport has to have an approvedinstrument approach procedure. Having the airport in sight isnot required. According to the AIM, the controller will provide"approved" separation. That means, among other things,that the controller won’t assign a specific altitude but willclear the pilot for the approach at an altitude at least 1,000feet below any other IFR traffic.

Runway in sight

I always thought that once I was cleared to land, I owned theentire runway. But that’s not necessarily so. Just as there areseparation standards for centers and approach/departure, controlsso too are there standards for the runway. Runway spacing is basedon three categories of aircraft. For simplicity’s sake, we’llsay that category I aircraft are propeller-driven singles, notincluding high performance aircraft such as the T-28. CategoryII aircraft are light, propeller-driven twins weighing 12,500pounds or less. And category III aircraft are all others, up tojet transports. We’ll just call them singles, twins, and largeaircraft.

The basic runway separation standard for arrivals is simple: anarriving aircraft should not cross the threshold until the aircraftahead of it has landed and taxied off the runway. However, during daylight if the visibility is good enough to estimate distanceby using suitable landmarks, a controller has the option of compressingthe spacing. Here are some examples: If a single is landing behindanother single, only 3,000 feet of runway separation is required.So despite what your instructor may have told you about automaticgo-arounds, it’s okay to land with another airplane on the runway.If you don’t want to, fine. Exercise your option to go around.If a twin is landing behind another twin or a single, 4,500 feetis required. And if either aircraft is large, 6,000 feet is required.At night or if distances can’t be accurately determined, fullrunway separation will be used.

For departures, the basic standard calls for no takeoff clearanceuntil the preceding aircraft has crossed the departure end ofthe runway or turned to avert any conflicts. When visibility allows,the same standards that apply for arrivals are used for departures,giving the controller the option of moving more metal on a busyday. However, at an airport with lots of mixed traffic, wake turbulenceseparation may slow things down some. Any aircraft departing onthe same runway behind a heavy jet is supposed to be held fortwo minutes to allow the heavy’s wake to dissipate. The departingpilot can ask for more separation or waive it entirely but ineither case, he should make the request before taxiing onto therunway.

Life With the Snitch

No discussion of separation would be complete without mentionof the Operational Error Detection Program, otherwise known asthe "snitch patch." Although it sounds like someplaceB’rer Rabbit would be sent for tattling on Piggly Wiggly, in actualitya "patch" is any add-on program authorized for FAA computers.This particular patch, a subroutine of the conflict alert programthat preceded it, "snitches" whenever it observes less-than-standardIFR separation. Only enroute facilities (ARTCCs) have it whichis a good thing, because if TRACONs had snitches, you couldn’tcarry enough fuel for all the holds you’d be stuck in.

The program is extremely complex, and takes into considerationwhether or not you’re climbing, descending, or at your cruisealtitude. In reality, it should trigger any time two IFR aircraftbeing worked by center pass within five nautical miles of eachother horizontally and within 1,000 feet vertically. But there’sa bit of slop built into the system so it doesn’t go off unlessyou’re 300 feet off your altitude and within 4.8 nautical milesof another IFR aircraft. So really, minimum separation standardsget squeezed a tad before snitch kicks in.

But when it does kick in, the snitch is a wonder to behold. Justbefore or as the snitch is about to occur, the data block on thecontroller’s screen begins flashing. It’s supposed to give a three-minutewarning before the separation bust actually happens but the warningis not entirely reliable. When a conflict is confirmed, the areamanager’s computer will immediately print a brief report of theincident.

If it’s not obvious why the conflict occurred, the manager willgo directly to the controller responsible and ask for the details.If the controller was at fault, he or she may be in hot waterbut the pilot is off the hook. But if the pilot has screwed up,he or she may be hearing from the FAA, via letter or telephone.It could be simply that your Mode C hiccuped, in which case bothyou and the controller are off the hook. The supervisor writes"Mode C error" on the computer strip that kicked outat his station, and all is right with the world. Case closed.On the other hand, if the controller asks for your altitude andyou reply that you’re more than 300 feet high or low, stand byfor the ram. You may have just admitted careless and recklessoperation, as well as operation in violation of an ATC clearance.

But be clear on one point: the snitch only squawks when an altitudebust results in loss of separation. It doesn’t actually monitoryour altitude. Keep in mind that the snitch makes a lot mistakes,sounding the alarm when no conflict has occurred. So don’t helpit out with hand-wringing confessions. If you’re off the markby 300 feet or more, you might delay your reply until you’re backin the ball park, and, as it were, out of the snitch patch. It’salso not a bad idea to file a NASA ASRS form, just in case.