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An Intro to En-route Control


This VATUSA Center Study Guide is designed to expand on the knowledge you have gained from the previous guides.
Many of the basic radar skills youR17;ll need in the Center such as vectoring and speed adjustment were covered in the Approach/Departure Guide.
However, the perspective was from a terminal environment serving a single airport. A Center controller is much more involved with the "Big Picture."
Each center is responsible for many thousands of square miles of airspace and for the airports within that airspace.
Centers control IFR aircraft from the time the aircraft departs an airport or leaves the terminal area's airspace or until the aircraft approaches the airspace controlled by a
terminal area or if the airport does not have terminal area control, until the aircraft lands.


Center controllers are responsible for climbing the aircraft to their requested altitude while, at the same time, ensuring that the aircraft is properly separated from all other aircraft in the immediate area.
Additionally, the aircraft must be placed in a flow consistent with the aircraft's route of flight. This effort is complicated by cross traffic, severe weather, special missions that require large airspace allocations, and traffic density.
The vast amount of airspace and multiple airports, some with and some without an approach control or tower, make working in a Center a challenge. There will be times when you find yourself busier than
you ever thought possible as you control aircraft arriving and departing from airports hundreds of miles apart. It may seem a bit daunting at first, but donR17;t let that discourage you. Keep at it and practice all the things youR17;ve learned.
That is the best way to become proficient.


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Altimeter Settings and Lowest Usable Flight Level
(7110.65, 4-5-4)


As the Center controller, you must always issue the altimeter in the following situations:



  • To en route aircraft below FL180 at least once while in your airspace or any time you issue descent below the lowest usable flight level. Use the altimeter for the airport closest to the aircraftR17;s position.

  • To arrivals when 50 NM from their destination if there is no approach control. In this case, use the destination airport altimeter.

Flight levels are based on a standard altimeter setting of 29.92. If the local pressure falls below that there is no longer 1,000 feet separation between normal altitudes and flight levels. Some flight levels become unusable as the pressure drops. Which ones depends on how far the pressure drops. You may not assign these unusable flight levels to aircraft:

  • If the local pressure is between 29.91 and 28.92 FL180 is unusable and may not be assigned. FL190 becomes the Lowest Useable Flight Level.
  • If the local pressure is between 28.91 and 27.92 FL180 and FL 190 are both unusable and may not be assigned. FL200 becomes the Lowest Useable Flight Level.
  • In the unlikely event the pressure drops below 27.92 you are probably in a hurricane. FL210 then becomes the Lowest Usable Flight Level.

Finally, when issuing the altimeter setting at the Center position, always be sure to identify it's source e.g. "KSFO ALTIMETER 3002"

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Altitude Assignments (7110.65, 7-3-1)

Remember from the Basic ATC study guide that aircraft flying below FL290 on a North and Eastbound (360-179) course are assigned ODD altitudes; South and Westbound (180-359) aircraft get EVEN altitudes. And starting at FL290 only odd flight levels are used alternating east and westbound. Well from time to time traffic, weather, or aircraft performance limits prevent following these rules you may assign any altitude so long as the aircraft remains within your airspace. If it will enter another controllerR17;s airspace, the Wrong Altitude For Direction Of Flight (WAFDOF) must be coordinated with and approved by the receiving controller. Regardless of the direction of flight any altitude you assign must be at or above the Minimum Enroute Altitude for the airway being used or the Minimum Instrument Altitude for the area the aircraft is in.

Another altitude you may assign if the pilot requests it is VFR-ON-TOP. As the name implies, the aircraft is on an IFR flight plan but flying under Visual Flight Rules On Top of any clouds or other weather. Do not use VFR-On-Top in Class A airspace. When VFR-On-Top the pilot flies at the appropriate altitude for direction of flight plus 500 feet (North-Eastbound ODD + 500, South-Westbound EVEN + 500). The pilot is responsible for his own separation from other traffic. You only provide traffic advisories and safety alerts as necessary.

When you assign VFR-On-Top as an altitude there are four things you must do:

  1. Tell the pilot the height of the tops (from PIREPS) or that no tops report is available, e.g. "CLIMB TO AND REPORT REACHING VFR-ON-TOP, TOPS REPORTED 12,000" OR "NO TOPS REPORTED."
  2. Ensure separation from all other traffic until the aircraft reaches VFR-On-Top.
  3. Issue alternate instructions in case the aircraft canR17;t reach VFR-On-Top, e.g. "IF NOT ON TOP AT FL180 MAINTAIN FL180 AND ADVISE."
  4. Re-clear the aircraft when it reports reaching VFR-On-Top, e.g. "MAINTAIN VFR-ON-TOP."

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ARTCC Separation (7110.65, 5-5-10)

Centers are responsible for much more airspace and use a different radar system than Approach Controls. Therefore, the separation standards they apply are slightly different as well.

Vertical separation standards at the Center level are as follows:

  • Up to and including FL290 - 1,000 feet.
  • Above FL290 - 2,000 feet.
  • Above FL600 between military aircraft - 5,000 feet.
  • In oceanic airspace, above FL450 between a supersonic and any other aircraft - 4,000 feet.

Radar separation standards at the Center level are as follows:

  • Below FL 600- 5 NM.
  • At or above FL 600- 10 NM
  • When accepting a handoff on a departure - 3 NM increasing to 5 NM if the aircraft are on diverging courses or the first aircraft is and will remain faster.

Note: Centers may also use visual separation except in Class A airspace.

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Descent Procedures

Once the aircraft is at its cruising altitude the next thing to determine is when to start its descent. The actual point can be affected by terrain, traffic, and adjacent airspace. Here is a simple formula to give you a general idea of when to start a typical aircraft down.

  1. Subtract the destination airport elevation (rounded to the nearest 1000 feet) from the aircraftR17;s current altitude (in thousands of feet). Then,
  2. Simplify by dropping the thousands. Then,
  3. Multiply the resulting number by 4. Then,
  4. The product is the number of flying miles from the destination where the aircraft should start its descent.

To use Chicago-OR17;Hare (ORD) as an example:

  1. Airport elevation - 668 (round up to 1,000)
  2. Aircraft altitude - FL310 (31,000 feet)
  3. 31,000 - 1,000 = 30,000
  4. Simplify to 30
  5. 30 x 4 = 120

An aircraft inbound to ORD at FL310 should start descent approximately 120 flying miles from the airport.

Military jet aircraft descend even faster than civilian aircraft (4-6,000 fpm vs. 800-1,500 fpm). The formula for figuring their starting point is even easier. Add 10 to the first two digits of the flight level. FL370 = 37 + 10 = 47 miles.

Another handy tool for determining when to start an aircraftR17;s descent is to use the term "DESCEND AT PILOTR17;S DISCRETION." You must still assign an appropriate IFR altitude to maintain e.g. "DESCEND AT PILOTR17;S DISCRETION, MAINTAIN 8,000" but this shifts the burden of choosing the right point to start descent from you over to the pilot. If necessary to have the aircraft at a certain altitude by a certain point you may include a restriction, e.g. "DESCEND AT PILOTR17;S DISCRETION, CROSS 40 DME AT AND MAINTAIN 10,000." In this case the pilot can start whenever and descend at any rate he wants as long as he crosses 40 DME at 10,000.

"DESCEND AT PILOTR17;S DISCRETION" does not relieve you of you responsibility to assign an altitude which ensures separation from terrain, obstructions, or other traffic. If an aircraft calls outside of your airspace requesting descent, inform the aircraft of the altitude to enter your airspace and advise the pilot to "OUTSIDE CONTROLLED AIRSPACE, DESCEND AT YOUR OWN RISK, CROSS 50 DME AT OR ABOVE 17,000, MAINTAIN 17,000."

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Operations at Uncontrolled Facilities (7110.65, 4-3-4)

When working a Center there will be many times when you are controlling aircraft into airports without an approach control or tower. Some of them donR17;t have them full-time or at all in real-life; others do but they arenR17;t always staffed in VATUSA. If itR17;s the latter situation, you have the option to provide these tower and approach services yourself. However, you are not required to do so and there will often be times you may not be able to due to workload, experience, widely scattered operations, or many other factors.

If you issue an IFR departure clearance to an aircraft at an airport without a tower you have several options:

  • Release the aircraft when you issue its IFR clearance. This way the aircraft may depart any time it is ready.
    • "CLEARED TO KNBR AIRPORT AS FILED MAINTAIN 13,000. RELEASED FOR DEPARTURE."
  • Include the term "HOLD FOR RELEASE." This way the aircraft has its clearance but may not depart until you call it back and issue a departure release.
    • "CLEARED TO KCBS AIRPORT AS FILED, MAINTAIN 14,000, HOLD FOR RELEASE."
      • "RELEASED FOR DEPARTURE."
  • Include a release time and time check with the clearance. This way the aircraft may not depart until the release time.
    • "CLEARED TO KKRQ AIRPORT AS FILED MAINTAIN FL190, RELEASED FOR DEPARTURE AT 1700. TIME NOW 1632."
  • Issue a clearance void time and a time check. This way the aircraft may depart anytime up until the void time but not after it.
    • "CLEARED TO KUAT AIRPORT AS FILED, MAINTAIN 8,000. RELEASED FOR DEPARTURE, CLEARANCE VOID IF NOT OFF BY 1345 TIME NOW 1315."

Arrivals may be vectored to final as in an approach control. However, in a center you will often be controlling other traffic some distance away. This may make it impractical to zoom in enough to give accurate vectors. Most arrivals to uncontrolled airports should be sent to the Initial Approach Fix and cleared for the complete approach.

More and more SB/FSINN pilots have the necessary approach plates and are able to fly the approach from the IAF without assistance from ATC. All they need is routing to the IAF (airway, STAR, or feeder route), an altitude if going direct, and approach clearance, e.g. "CLEARED DIRECT OHBEE, CROSS OHBEE AT OR ABOVE 5,000, CLEARED ILS RWY 27 APPROACH." If a pilot does not have the approach plate the simplest option is to have the aircraft hold over a nearby fix until you are able to provide vectors to the final approach course

Another option is to provide the pilot with the necessary approach information, although this is often more work than holding the aircraft until you can vector. The required information consists of five (5) items:

  1. Initial approach fix altitude.
  2. Direction and distance from the holding/initial approach fix within which procedure turn is to be completed.
  3. Altitude at which the procedure turn is to be made.
  4. Final approach course and altitude.
  5. Missed approach procedures if considered necessary

In real-life you must wait until the first aircraft has landed or canceled IFR before you may clear the second aircraft for approach to an uncontrolled airport. However, because ASRC/VRC always has radar contact, if you choose to you may have more than one aircraft cleared for approach at uncontrolled airports. At uncontrolled airports the pilot decides if he will make a circling approach. Do not include circling instructions in the approach clearance. Arrivals and departures at uncontrolled airports are usually handled as one in-one out. Hold the departure until the arrival has landed or canceled IFR or hold the arrival until the departure is airborne.

If you want to try mixing-and-matching you have two choices.

  1. Because ASRC/VRC always has radar contact you may apply the arrival/departure separation of 2 NM if separation will increase to 3 NM/5 NM within one minute.
  2. You may also use non-radar separation and assign a void time to the departure. If you issue a void time tell the pilot to advise his intentions within 30 minutes (or less) after the void time and issue a time check, e.g. "CLEARANCE VOID IF NOT OFF BY 1710, IF NOT OFF BY 1710, ADVISE NOT LATER THAN 1725 OF INTENTIONS. TIME 1655."

Non-radar separation between an arrival and a departure is:

  • When takeoff direction differs by at least 45 degrees from the reciprocal of the final approach course, the departure must depart before the arrival leaves a fix not less than 4 NM from the airport.
  • When takeoff direction does not differ by at least 45 degrees from the reciprocal of the final approach course, the departure must depart and be established on a course diverging by at least 45 degrees from the reciprocal of the final approach course before the arrival leaves a fix not less than 4 NM from the airport inbound.

If there isnR17;t a fix 4 NM from the airport:

  • When takeoff direction differs by at least 45 degrees from the reciprocal of the final approach course the departure must depart at least 3 minutes before the arrival estimating the airport.
  • When takeoff direction does not differ by at least 45 degrees from the reciprocal of the final approach course the departure must depart and be established on a course diverging by at least 45 degrees from the reciprocal of the final approach course 5 minutes before the arrival is estimating the airport or before it starts procedure turn.

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NAVAID Use Limitations and GPS

When assigning routes based on VORs or NDBs remember these three (3) things:

  1. The typical range of an FS VOR above FL180 is 130 NM. Below FL180 it can be as little as 40 NM depending on terrain and the aircraftR17;s altitude.
  2. The range of most NDBs is approximately 50 NM but this also varies with terrain and altitude.
  3. Unless the aircraft is on an airway you must constantly watch the aircraft on radar and provide course corrections as necessary.

Global Positioning Systems such as the SB/FSINN FMS are making direct routings more common. Using GPS you are no longer limited to only clearing aircraft direct to VORs or NDBs. You may clear the aircraft direct to virtually any navaid, intersection, DME fix, or airport.

Two things you should be careful of when using GPS:

  1. Not all SB/FSINN pilots are proficient or even familiar with operating the FMS. A direct routing in the flight plan does not necessarily mean the pilot really wants it. He just may not know how to plan a route. Unless the aircraft equipment suffix indicates GPS/FMS use direct routings to other than VORs and NDBs with caution.
  2. The pilot may not have entered the fixes you assign into his FMS. It may take a while for him to reprogram or he may refuse the direct routing and require vectors or routing via VORs or NDBs.

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