VATUSA
Virtual Air Traffic Simulation - USA Division
Issuing Altitude and Crossing Instructions
Issuing descent and crossing instructions is probably one of the most important tasks that you will have as a center controller. In this lesson, we will be discussing all things related to altitude. Let's begin by looking at lowest usable flight levels.
Lowest Usable Flight Level
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 Usable 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 Usable 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.
Altimeter Issuance
You are required to issue the altimeter in any of 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 aircraft's position.
- To arrivals when 50 NM from their destination if there is no approach control. In this case, use the destination airport altimeter.
Issuing Crossing Restrictions
"Takeoffs are optional, landings are mandatory."
At some point in time, an aircraft will have to descend in order to reach the terminal airspace surrounding its destination. We are going to cover two different ways to descend an aircraft (other than the obvious "DESCEND AND MAINTAIN" that you learned during your S3 training). First, a more in depth look at the STAR.
Profiled vs Non-Profiled Arrivals
For the purposes of center operations, we are going to categorize all STARs as one of two possibilities: profiled or not profiled. How do you tell the difference? A profiled STAR is one that has multiple mandatory crossing restrictions.
This is a question best explained via a picture. Lets look at a non-profiled STAR, the BRADFORD 5 ARRIVAL into KORD.
You can see blue boxes around the crossing restrictions. Notice how there are no 'mandatory' crossing restrictions and instead the crossing restrictions say 'Expect to cross'. In instances like these the controller must actually instruct the aircraft to descend to cross the fix at an altitude. Does it necessarily have to be the altitude on the chart? No, You can have them cross the fix at a different altitude if needed. Just be sure to coordinate with the affected controller.
Phraseology
The phraseology for issuing this type of descent instruction is simple:
"DESCEND TO CROSS (fix/navaid) AT AND MAINTAIN (altitude)"
"Delta 2602, Descend to cross the Bradford VOR at and maintain FL240"
"Delta 2602, Descend to cross BENKY at and maintain 11,000. O'Hare altimeter 29.96"
Don't forget to issue the altimeter if he will be descending below FL180.
Now let's look at the EAGUL THREE ARRIVAL (RNAV) into KPHX.
Again, you can see boxes around the crossing restrictions to help you easily identify them. You can see from the chart that the pilot (if descending via this STAR) is going to cross TINIZ at or below FL300 and 280 knots, PAYSO at FL240 and 280 knots, etc. So the presence of these on the STAR indicate that it is profiled. It is important to know that although these restrictions exist, profiled arrivals do not activate themselves. You must still issue the following phraseology.
Phraseology
"DESCEND VIA THE (arrival) ARRIVAL"
"Delta 2602, Descend via the Eagul Three Arrival, Phoenix altimeter 29.96"
There is no regulation indicating you are required as a controller to issue the "DESCEND VIA THE" instruction if the pilot is flying a profiled arrival. This is purely the controller's choice. The controller could have the aircraft cross any fix at any altitude (using the non-profiled phraseology). The controller could even have an aircraft descend via the arrival, but change a crossing restriction on the star. The phraseology in that case would be:
DESCEND VIA THE (STAR/FMSP) ARRIVAL EXCEPT CROSS (fix, point, waypoint) (revised altitude information)
"Delta 2602, descend via the Eagul Three Arrival expect to cross HINEY at 5,000"
Issuing Crossing Restrictions for aircraft not on a STAR
For aircraft not on a STAR, you can have them descend to cross a fix in their flight plan at an altitude in the same way that you would have issued the crossing instruction for an aircraft on a non-profiled STAR. "CROSS (fix/navaid) AT AND MAINTAIN (altitude)"
. You can even have an aircraft cross a certain DME of a navaid at a certain altitude. Just be sure to coordinate when necessary.
"Skyhawk N5204N, Cross Knox at and maintain 3,000"
"Skyhawk N5204N, Cross 10 DME south of Knox at and maintain 3,000"
Another handy tool for determining when to start an aircraft's descent is to use the term "DESCEND AT PILOT'S DISCRETION." You must still assign an appropriate IFR altitude to maintain e.g. "Descend at pilot's discretion, maintain 8,000" but this shifts the burden of choosing the right point to start descent from you over to the pilot.
"DESCEND AT PILOT'S DISCRETION" does not relieve you of your 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, (crossing restriction)"
"Delta 2602, Outside controlled airspace, descend at your own risk. Cross 20 DME south of FLM at and maintain 11,000"
When to Issue the Descent
Before you can issue the descent instruction, the next thing to do is 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.
- Subtract the destination airport elevation (rounded to the nearest 1000 feet) from the aircraft's current altitude (in thousands of feet). Then,
- Simplify by dropping the thousands. Then,
- Multiply the resulting number by 4. Then,
- The product is the number of flying miles from the destination where the aircraft should start its descent.
To use Chicago-O'Hare (ORD) as an example:
- Airport elevation - 668 (round up to 1,000)
- Aircraft altitude - FL310 (31,000 feet)
- 31,000 - 1,000 = 30,000
- Simplify to 30
- 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.
