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."
September 19, 1995
It's sometimes difficult for pilots to understand why air traffic
controllers do the things they do. They often reject our requests
for altitude changes, vector us away from our destinations, or
occasionally request what seem like unnecessary speed adjustments.
These delays are bad enough when IFR in real weather but they're
downright aggravating when you're IFR in good weather. If it's
clear-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 aircraft
are supposed to be an efficient means of transportation. Anytime
we're deviated from the planned route or altitude, it costs time
and money. Unfortunately, nobody ever seems to tell the pilot
what's going on. Therein lies the most profound disconnect between
pilot 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 flow
of air traffic." To achieve this, controllers must keep the
airplanes in their charge pointed more or less in the direction
they're supposed to be going and sufficiently separated to keep
them from running into each other. Separation is the key word
here because when the clouds are full of airplanes, pilots can't
tend to separation on their own; they have to rely entirely on
Let's begin our discussion by reviewing some of the ways controllers
keep us from running into each other. First of all, ATC's primary
guarantee is a basic vertical and horizontal separation standard
for IFR aircraft. Note the emphasis on IFR. In some cases, ATC
is obligated to separate VFR traffic as well and there are instances
when 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) achieved
by radar so I'll assume we're flying in radar contact. Separation
standards vary according to who owns the airspace you're flying
Vertically, no IFR aircraft may come within 1,000 feet of another
IFR 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, depends
solely upon your distance from the radar antenna. IFR aircraft
within 40 nautical miles of the antenna site must be given a minimum
horizontal separation of three nautical miles. Those flying beyond
40 miles from the antenna must be separated by five miles. In
reality, this means that you'll be guaranteed five miles of separation
when working with an enroute facility (center) and five miles
when working with a terminal facility (approach, departure, or
These are minimum separation standards. The controller may throw
in a mile or two of additional separation as a fudge factor so
in practice, you're likely to get seven or eight miles from a
center and five with approach. There's good reason for the extra
spacing. A controller's life is made very unpleasant if he or
she allows two airplanes to bust minimum separation. Center radars,
that is enroute facilities, are equipped with a sinister device
called a snitch patch. Essentially, the
snitch watches over the controller's shoulder, sounding an alarm
if 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 that
notifies your boss when you exceed a speed of 200 knots. Company
policy is to ground pilots without pay for six months if they
set off the alarm. The computer doesn't care if you're slow or
arrive late at your destination, it only cares if you're over
the 200-knot limit. How would you deal with it? I'm sure you'd
do the same thing I'd do; fly 50 knots slow, just in case. If
you picture a number of aircraft, each with different performance
characteristics all converging on the same VOR and all asking
for a lower altitude so they can set up for the approach, you
can see why controllers get maxed out.
Tricking the Snitch
Enroute separation standards, as monitored by the snitch, result
in some truly ridiculous situations. Several years ago, I was
I was IFR northbound on V-225 near Key West. I was being held
level at 4,000 feet by Miami center, even though I was anxious
to get more altitude between me and the Gulf. The reason? Opposite
direction IFR traffic. A Cherokee was southbound on V-225 at 5,000
feet, 12 miles ahead. I knew the controller couldn't give me higher
because my climb would take me right through the Cherokee's cylinder
of 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 until
the Cherokee was at least five miles behind me. Climbing any sooner
would have set off the snitch. Even if I'd seen the traffic, which
I hadn't, it wouldn't have made any difference. The ATC rulebook
said that visual separation wasn't allowed in ARTCC airspace.
Since that episode, the FAA has liberalized the separation rules
for center controllers somewhat. In the case of opposite-direction
traffic, if one of the pilots reports seeing the other aircraft
passing visually and the controller verifies that their radar
targets have passed one another, he no longer is required to maintain
1000' and five miles between them...the FAA powers-that-be have
finally admitted that such aircraft are unlikely to back into
one another. More recently, the rules have changed to permit center
controllers to utilize visual separation between aircraft flying
But these rule changes are relatively new, and their use is strictly
at the controller's discretion. Some controllers use these tools
regularly to expedite traffic flow, and others stick strictly
to the 1000-and-5 minima.
As general aviation pilots, we can exercise a couple of tricks
to get around this situation which aren't available to our air
carrier or military brethren. We can, in effect, put the IFR separation
standards on hold temporarily and get treated like a VFR kinda
guy. Perhaps the two most useful ways of doing this are the VFR
climb or descent and the VFR-on-top clearance.
The VFR climb or descent, if requested by the pilot and approved
by the controller, allows you to operate VFR while you're changing
your altitude. Once you reach your new altitude, you're IFR again.
Of course, you have to meet VFR cloud clearance and visibility
requirements during the climb or descent. Had I requested a VFR
climb off Key West, I probably wouldn't have had a problem. However,
because of increased VFR separation standards, this trick might
not 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 you
in the IFR system. Since you're still an IFR aircraft, controllers
are required to provide you with traffic advisories and safety
alerts in ARTCC airspace and the appropriate separation in a terminal
area. You get the best of both worlds. You're still IFR if you
need to punch through some clag on the approach but your separation
standards are greatly reduced.
But there are a couple of catches. First of all, pilots have to
to ask for VFR climbs, VFR descents, and VFR-on-top. Controllers
are specifically prohibited from suggesting these, which is as
it should be. A really adventurous controller might say something
like 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 occasionally
forget that VFR-on-top clearances are authorized. So if you've
negotiated one with one controller and you're later instructed
to contact a new controller "for further advisories,"
he may think you're VFR. Start asking questions. And if you request
VFR-on-top and are told to squawk 1200, you better speak up because
the controller didn't get it.
Once an aircraft enters the terminal area, controllers and pilots
have more separation options. Besides smaller horizontal separation
(typically three miles), controllers are allowed to authorize
what's called diverging course separation. The concept is simple
enough to grasp. If the controller observes on radar that two
aircraft have passed and are on diverging courses, that's all
the separation that's needed. He is therefore not required to
issue 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 without
visual confirmation by a pilot. Of course, that would never work
for center controllers, because an ARTCC scope gets its radar
data from a mosaic of radar sites, each of which views the world
a little differently. It's possible, for example, for two targets
to come from separate radars, even though the airplanes are right
next to each other. So the targets may not have passed each other
or diverged, even though it looks that way on the radar screen.
In addition to the normal three-mile separation rule, terminal
controllers are permitted to use something called "conflict
resolution" in Class B and C airspace. Conflict resolution
applies to separating an IFR aircraft from a VFR aircraft, and
means simply that the controller can see space between the two
targets 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 vertically
by at least 500 feet. It's easy to see how much more efficient
this is in a high-density terminal area and it's safe if the weather
is VFR. Keep in mind, though, that IFRs must still be separated
from other IFRs by the standard three-and-1000.
Cleared for the Visual
Visual separation may be new stuff to center controllers, but
it has always been allowed in terminal areas and is heavily used
by terminal controllers. The requirement couldn't be simpler.
If the pilot reports that he has traffic in sight, he may be instructed
to "maintain visual separation with that traffic." In
other words, get as close as you like but don't hit him.
Additionally, if a tower controller can see both aircraft, he
may also use visual separation. The difference lies in who has
responsibility for the separation. Obviously, if you report having
your traffic in sight, you're responsible. That's why it's important
to notify ATC should you lose sight of your traffic. This is no
time to play "I've got a secret." If the tower is making
the call, the controller is responsible for separation. He'll
point out the traffic for you and, if you're in the pattern, he
may offer to call your turn to base or final.
Visual separation standards go hand in hand with visual and contact
approaches. The important thing to remember about a visual approach
is that the airport can't be reporting weather that's less than
VFR 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 of
visibility. 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 from
nearby airports or from pilot reports. In order to get vectors
to a visual approach, the reported ceiling must be at least 500
feet above the minimum vectoring altitude (MVA) at an airport
that reports weather.
If weather isn't available, you can still get vectors, providing
the controller has some assurance that VFR conditions exist or
if you've been told no weather is available. You'll usually have
no way of knowing what the MVA is in the area you're in. The first
clue will be the controller's denial of a request for vectors
to the visual. Here's an example: You're approaching an airport
from the west, where it's bounded by mountains. Another aircraft
is approaching from the flatlands east of the airport. The other
guy might get vectors to the visual but you won't because you're
in an area where the MVA is higher, due to terrain. There will
be circumstances when you'll be vectored for an instrument approach
in marginal or good-VFR weather. If you don't want the approach
and can see the field, by all means report it in sight. You'll
be immediately cleared for the visual, traffic permitting.
Believe it or not, there are instances when an airport is reporting
IMC weather conditions when pilots are calling the field from
ten miles out. Can you be cleared for the visual in this situation?
No, even though you can see the field, the rules say no visual
approach if the reported weather is less than VFR.
I've heard some very interesting exchanges between controllers
and pilots in these circumstances. It usually happens when the
field is reporting something like two-and-a-half miles in fog
but really has inflight visibility of ten miles or better. Pilots
will call the field and ask for the visual, only to be denied.
Sometimes, though, the controller is pitching but the pilot's
not catching: "Sir, I understand you have visual CONTACT
with the runway, but the field is IFR, and I'm unable to approve
a visual approach."
He's hinting at a contact approach. Unlike its "visual"
cousin, the contact approach must be requested by the pilot. It
can't be initiated (or even suggested) by controller. To approve
it, the controller has to be sure that the reported visibility
at the airport is at least one mile and the pilot has to remain
clear of clouds. And, of course, the airport has to have an approved
instrument approach procedure. Having the airport in sight is
not 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 will
clear the pilot for the approach at an altitude at least 1,000
feet below any other IFR traffic.
Runway in sight
I always thought that once I was cleared to land, I owned the
entire runway. But that's not necessarily so. Just as there are
separation standards for centers and approach/departure, controls
so too are there standards for the runway. Runway spacing is based
on three categories of aircraft. For simplicity's sake, we'll
say that category I aircraft are propeller-driven singles, not
including high performance aircraft such as the T-28. Category
II aircraft are light, propeller-driven twins weighing 12,500
pounds or less. And category III aircraft are all others, up to
jet transports. We'll just call them singles, twins, and large
The basic runway separation standard for arrivals is simple: an
arriving aircraft should not cross the threshold until the aircraft
ahead of it has landed and taxied off the runway. However, during
daylight if the visibility is good enough to estimate distance
by using suitable landmarks, a controller has the option of compressing
the spacing. Here are some examples: If a single is landing behind
another single, only 3,000 feet of runway separation is required.
So despite what your instructor may have told you about automatic
go-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 feet
is required. And if either aircraft is large, 6,000 feet is required.
At night or if distances can't be accurately determined, full
runway separation will be used.
For departures, the basic standard calls for no takeoff clearance
until the preceding aircraft has crossed the departure end of
the 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 busy
day. However, at an airport with lots of mixed traffic, wake turbulence
separation may slow things down some. Any aircraft departing on
the same runway behind a heavy jet is supposed to be held for
two minutes to allow the heavy's wake to dissipate. The departing
pilot can ask for more separation or waive it entirely but in
either case, he should make the request before taxiing onto the
No discussion of separation would be complete without mention
of the Operational Error Detection Program, otherwise known as
the "snitch patch." Although it sounds like someplace
B'rer Rabbit would be sent for tattling on Piggly Wiggly, in actuality
a "patch" is any add-on program authorized for FAA computers.
This particular patch, a subroutine of the conflict alert program
that preceded it, "snitches" whenever it observes less-than-standard
IFR separation. Only enroute facilities (ARTCCs) have it which
is a good thing, because if TRACONs had snitches, you couldn't
carry enough fuel for all the holds you'd be stuck in.
The program is extremely complex, and takes into consideration
whether or not you're climbing, descending, or at your cruise
altitude. In reality, it should trigger any time two IFR aircraft
being worked by center pass within five nautical miles of each
other horizontally and within 1,000 feet vertically. But there's
a bit of slop built into the system so it doesn't go off unless
you're 300 feet off your altitude and within 4.8 nautical miles
of another IFR aircraft. So really, minimum separation standards
get squeezed a tad before snitch kicks in.
But when it does kick in, the snitch is a wonder to behold. Just
before or as the snitch is about to occur, the data block on the
controller's screen begins flashing. It's supposed to give a three-minute
warning before the separation bust actually happens but the warning
is not entirely reliable. When a conflict is confirmed, the area
manager's computer will immediately print a brief report of the
If it's not obvious why the conflict occurred, the manager will
go directly to the controller responsible and ask for the details.
If the controller was at fault, he or she may be in hot water
but 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 both
you and the controller are off the hook. The supervisor writes
"Mode C error" on the computer strip that kicked out
at his station, and all is right with the world. Case closed.
On the other hand, if the controller asks for your altitude and
you reply that you're more than 300 feet high or low, stand by
for the ram. You may have just admitted careless and reckless
operation, as well as operation in violation of an ATC clearance.
But be clear on one point: the snitch only squawks when an altitude
bust results in loss of separation. It doesn't actually monitor
your altitude. Keep in mind that the snitch makes a lot mistakes,
sounding the alarm when no conflict has occurred. So don't help
it out with hand-wringing confessions. If you're off the mark
by 300 feet or more, you might delay your reply until you're back
in the ball park, and, as it were, out of the snitch patch. It's
also not a bad idea to file a NASA ASRS form, just in case.