Flight Levels, altitude, and barometric pressure

In a previous post, I noted that a clearance might include an initial altitude of 7,000 feet and an assigned cruise altitude of Flight Level 390, which corresponds to 39,000 feet. Why don't we just say "39,000 feet"? Why do we use two different forms of language for talking about different types of altitude?

There are two reasons. One of which is related to communication, and the other to methods of determining altitude.

We use the phrase "Flight Level" when referring to altitudes including and above 18,000 (above sea level). At these altitudes, for safety's sake, we don't worry about intervals of less than a few hundred feet. For communications clarity, over sometimes crackly radios, saying "one niner thousand feet" (19,000) might be confused with "niner thousand feet" (9,000). It's a matter of clarity. Note that flight levels are always given as three digits, basically leaving off the last two digits of the altitude in thousands, so FL390 is 39,000 feet, and FL 200 is 20,000 feet.

More importantly, though, the transition from one system to another has to do with consistency in the measurement of altitude. When operating at lower levels, near airports, it's vitally important that your instruments reflect the correct altitude relative to the airport. If the altitude of the airport is 50' above sea level, then you certainly want your altimeter telling you that you're at 50' when you're sitting on the tarmac. Airplanes tell altitude in a number of ways (GPS, radar altimetry) but the standard method is using barometric pressure. There's a great article on pressure and altimeters here.

Barometers measure the pressure of the air at a given point; essentially a measure of the weight of the air column sitting on top of the barometer. In North America, this is typically measured in inches of mercury, and the standard sea level pressure is considered to be 29.92" of mercury. You know when the weather report talks about "highs" and "lows"? Those refer to the fact that weather patterns affect the density of the air in a given location, and the pressure is greater or lower. You all know how your ears pop when you're driving up a mountain or climbing in an airplane? That's because of the change of air pressure that occurs with altitude. The higher you go, the less air there is between the top of your head and the top of the atmosphere. Airplane altimeters measure this change, and convert it into altitude.

So, when operating near an airport, it is important to find out what the current barometric pressure is at the airport, and to adjust your altimeter accordingly. This calibrates your altimeter to the local altitude, regardless of what the weather is doing. Aviation weather reports always include the pressure for this purpose. However, at high altitudes, the air becomes much thinner, and aircraft are transiting the air from a wide variety of destinations. At a certain height, all pilots will recalibrate their altimeters to the standard reading of 29.92, which virtually guarantees that everybody is working from the same scale, and two planes indicating that they are at 29,000' above sea level will be at precisely the same height. In North America, the altitude at which this recalibration happens is called the transition altitude, and it happens at 18,000 which is the same as FL180. So Flight Levels refer to the uniform, consistent altitude based on the standard pressure of 29.92" of mercury.

See Michael Oxner's article on altitudes and flight levels.

How do you file flight plans?

On VATSIM, first you have to find a place that will give you a reliable flight plan. There are two resources that I use all the time when planning my own online flights, or when I'm checking the plans that pilots have filed.

The first is Simroutes, which is specifically designed for the VATSIM world. You can enter the ICAO code of any airport in either the departure or the arrival spot and see a list of all the routes in the database. As these are typically entered by the air traffic control managers from around the VATSIM world, they generally represent preferred routes between two airports, which becomes very important in high-traffic areas. You can see the route on a map and even download route information for various flight simulator programs.

The second is FlightAware, which I mentioned in the first post. It shows information about real-world aviation, but only for flights that start, end, or transit near United States airspace. It shows routing and altitude, and the IFR analysis page allows you to see the frequency of use for different routes between two airports. Here's the analysis of traffic between Vancouver (CYVR) and Los Angeles International (KLAX).

Clearance Delivery

First Post. I've been controlling for Vancouver FIR for about 10 hours now, and at this point I qualify to operate the Clearance Delivery position. Like in all of VATCAN's member FIRs, a controller starts out small and works his or her way up through a combination of training, practice, observation with an instructor, and passing grades on examinations.

So I'm authorized for Clearance Delivery. This means that when I'm online as that position, I'm known as CYVR_DEL (CYVR is the ICAO code for Vancouver International Airport) I could also log on as CYYJ_DEL for Clearance Delivery at Victoria International. You get the idea: typically, DEL is a position associated with a single airport. The primary job of DEL is to issue clearances to aircraft before they leave. This is, essentially, a route from one airport to another, or to a point in between, at a given altitude, and following a specific path. In really congested airspace, this is essential for getting aircraft to the right place without causing hazardous conflicts.

The components of the most basic clearance are as follows:

• C - Clearance limit. "Cleared to".
• R - Route. "via".
• A - Altitude (both initial, and final).
• F - Frequency.
  
• T - Transponder code.

For instance, I just looked on the wonderful website FlightAware (real-world aviation) at the flight Air Canada 552, which is an Airbus 319 flying from Vancouver to Los Angeles. If he was on VATSIM, and I were to issue a clearance to that pilot, he would contact me on the ground in Vancouver and say
"Vancouver Delivery, Air Canada Five Five Two request IFR clearance to Los Angeles". In front of me, on my computer, I'd bring up the flight plan that he had filed. I'd note the route that he'd requested (YVR SEA J5 SUMMA 4230N 12050W FRA J7 DERBB SADDE6) and the altitude of 39000 feet. I'd check this over and give him the following response (note the conformance with the CRAFT sequence listed above):
C: "Air Canada five five two, you are cleared to the Los Angeles Airport..."
R: "...via the Vancouver Four departure, flight planned route..."
A: "...maintain seven thousand. Expect flight level three niner zero, five minutes after departure..."
F: "... departure frequency one three two point three..."
T: "...squawk four seven zero zero"

To quickly step through each bit...

• Air Canada 552 is cleared all the way to LAX. I could, perhaps, clear him only as far as a middle point if it's a really long flight. This is most common in transoceanic flights, since it's even more critical to keep planes apart over the ocean...
• Just about all jet aircraft are cleared from Vancouver using the Vancouver 4 departure, which is a Standard Instrument Departure. SIDs are published descriptions for what pilots should do after takeoff. It's a lot harder to say "after takeoff, when you're 500 feet above the ground, turn right until you're heading 141°" than "cleared via the Richmond 9 departure". Richmond 9 is the SID typically given to turboprop aircraft at Vancouver. "Flight Planned Route" means that, once the SID has been executed, the rest of the flight is to proceed along the route defined by the pilot.
• The altitude stuff is actually included in the SID, which, if you read the chart, indicates that the pilot is to climb to 7000' and then expect the filed altitude (in this case FL390) 5 minutes after departure. It doesn't hurt to reiterate this for clarity over the radio...
• Why FL390 and not "39,000 feet"? It's something to do with altitudes, which has everything to do with barometric pressure settings. It'll be the subject of a future post. You use "Flight Levels" when you're talking about altitudes above 18,000 feet.
• the frequency given refers to the departure frequency, which is typically the frequency to switch to immediately after you take off. Since this is a very busy time in the cockpit, the delivery controller prepares the pilot with the frequency.
• "Squawk" is a funny thing. Most planes above a certain size, and all commercial aircraft, carry a transponder. This is a device that, basically, transmits a 4 digit code which is picked up and displayed on the radar scope of air traffic controllers, allowing the plane to have a unique identifier. Clearance Delivery assigns this code.
  

So there's a very basic overview to Clearance Delivery.