Tuesday, October 07, 2008

Paper Trails

This is a post for the pilots out there.

When I was in flight training for my Private Pilot's License (PPL), I dealt very little with air traffic control. I did fly out of two different towered fields and of course worked with the towers in that environment. However, all of my ATC experience was limited to take offs, landings, and taxiing. With the exception of one single dual cross country from Miami to West Palm Beach, I never once dealt with a TRACON or an En Route center while I was in flight training.

After I got my PPL, I took a few cross countries on my own where I did talk to ATC for VFR flight following. One of them was up to Stuart, FL, and along the way I talked with Miami Approach, Fort Lauderdale Tower, and West Palm Beach Approach. It really got me wondering how the information on my flight - my call sign, my aircraft type, my departure point and destination - gets passed from one facility to another. I was fully aware that everything was computerized, but I still wondered what each controller at each facility was looking at.

It was only after I'd gone through Oklahoma City and actually started training in ATC that I developed an idea of how the system worked. I thought it might be interesting to do a comparison between what a pilot's seeing and what a controller's seeing.

If you're a pilot that's wondering, "How does ATC know what I want to do?" then hopefully this post should clarify things. Or make you more confused. Hopefully the former. :)

Flying Hot Potato

The only real ATC concept I'll mention here is what is known as the "hand off". Before an aircraft leaves one facility and enters the airspace of another, it must be handed off to the receiving facility. This is essentially a transfer of responsibility for that aircraft and is conducted either via a function on the radar scope or via voice coordination. Once the receiving facility accepts the hand off (and therefore the responsibility) the first controller tells the aircraft to contact the receiving controller on the receiving controller's frequency.

Think of it as a relay race. When one facility has done all they can for you, they pass you on to the next one. And then you get passed on to the next one. So on and so forth. Each facility bears the same amount of responsibility for your safety and the expeditiousness of your flight.

The Plan

As always, I'll try to create a realistic example. Let's make a simple flight plan. Today, you're flying N12354, a Cessna 172, from New Orleans Lakefront Airport to Pensacola Regional. You want to go as direct as possible and cruise at 5000. Unlike me, you're an instrument-rated pilot, so you'll be flying IFR.

1. Preflight

As part of your pre-flight routine, you call up a Flight Service Station or submit your flight plan online. The flight plan includes your call sign (N12354), departure (NEW), your destination (PNS), and your route of flight (in this case, just NEW -> DIRECT -> PNS). You also give them your aircraft type (C172/G), estimated departure time (1200Z), requested altitude (5000'), VFR or IFR status (IFR), and airspeed (110kt). There's other information that gets taken down for the FSS' purpose but isn't relevant for ATC, such as souls and fuel on board.

This information gets entered into the National Airspace System's database, which transmits it to the appropriate facilities. The NAS is based on "fix pairs" - i.e. it draws a line from one point on the route to the next at the requested altitude. It therefore knows which facilities the route passes through at that altitude. In your flight's case, it will be all of these:

2. Tower Departure Strip

While you preflighted your airplane, the NAS was preflighting too. 30 minutes before your proposal time of 1200z, it generates a squawk code (6231) for your flight and a flight progress strip prints out in NEW airport's control tower.

When you're ready to taxi, the ground controller or clearance delivery controller issues you your clearance and your squawk code, both of which are read from that strip.

All of the information on it should be easy to decipher. The only thing that you haven't seen yet is that number (723) on the lower left. That's your flight plan's Computer I.D. Number - or CID - which makes it unique from every other flight plan in the NAS.

The CID serves to differentiate multiple flight plans filed under the same call sign within an overlying En Route Center's airspace. For example, say you just wanted to stop in Pensacola to get gas and a snack, and then continue on to Gainesville, FL. You would have one flight plan filed at 1200Z from NEW-PNS, and then another filed at 1430Z from PNS-GNV. Even though they're both same call sign - N12354 - each flight plan has its own CID.

A note on En Route Centers: Your flight plan goes from NEW (which is in Houston Center's airspace) and arrives in Pensacola (Jacksonville Center's airspace). Each center's host computer uses a different CID and a different squawk code for your flight. The boundary between Houston and Jacksonville centers is also the boundary between Mobile approach and Pensacola approach, so when you cross that line you will be issued the squawk code that corresponds to the center whose airspace you're entering. While you might be technically talking to Pensacola Approach, you'll be under Jacksonville's jurisdiction. Click here to see a map depicting each center's airspace.

3. Terminal Departure Strip
In the interest of keeping things consistent, I'm going to assume New Orleans TRACON is using procedures similar to those used here at Pensacola and a lot of other places, including flight progress strips. Many of the larger places have recently taken strips out of the equation entirely and replaced it with more simplified and/or automated methods. However, at one point or another in the past, strips were part of most facilities' day-to-day operations. So, for the sake of this example, we'll just say New Orleans's still using strips.

At the same time as the strip prints in NEW's tower, an identical departure strip prints out in New Orleans' TRACON. Since they're NEW's overlying TRACON facility, they'll be working you on your way out and need to know your flight plan. When you first call up the tower to taxi, the tower calls the TRACON and tells them to "activate" you. This lets the radar controller know that you'll be taking off shortly and gives him time to get your strip in his hands.

Then, when you're at the runway at the hold short line and ready for takeoff, the tower calls up the TRACON and requests "release" for you. The radar controller checks to make sure that there's nothing that will conflict with your departure from the airport (i.e. another aircraft flying across your departure path at a conflicting altitude) and then grants the release. From that point, the tower has three minutes to get you off the ground. For the entirety of those three minutes, the departure controller must protect that airspace for you.

In addition to the paper strip, your call sign (N12354) and assigned squawk code (6132) appear in the radar controller's tab list on their scope. This lists all aircraft that are expected to be within the controller's airspace within the next 5 minutes or so, including departures, arrivals, and overflights.

Procedural Notes: All airports and TRACONs have different procedures, depending on their letters of agreement and equipment. As I noted, New Orleans may be doing something different than what I'm showing here.
  1. For instance, at a facility like Miami, the activation is done via bar codes on the strips. The tower controller swipes the strip underneath a bar code scanner, which sends out a signal to both the TRACON and the overlying center that the aircraft is taxiing. This is used for flow management, so the departure and en route facilities can get an idea of how much traffic they're going to be getting and which way they'll be going. Pretty neat stuff.
  2. A tower can coordinate automatic releases with its overlying TRACON via letter of agreement or verbal coordination. For instance, our Pensacola tower is required to request release on all of its departures due to the convoluted nature of our traffic and airspace. However, South Whiting NAS tower has automatic releases since their departure cooridor is self-contained. Therefore, they can launch a helicopter without actually requesting permission from us. The only thing they are required to do is call us over the land line to let us know he's taking off. This is called a "rolling call", telling the controller "Heads up, VV123 is on the go.".
  3. Lastly, like I said earlier, many TRACONs don't use strips at all. For instance, Miami and Potomac TRACON do not since they're working a lot of high volume airliner and jet traffic that will be climbing high and fast out of their airspace. All they really need to know is the first fix outside of their airspace so they can send them out that way as quickly as possible. It's quite possible that New Orleans TRACON has gone that way, so if there are any New Orleans controllers out there reading this, give me a shout out and let me know what you're using.
Alrighty. Enough holding short. Without further ado, the tower controller clears you for takeoff.

When you takeoff, the moment your airplane tags up on the departure controller's scope a Departure Message is sent to facilities down the line basically saying, "Hey, N12354 is airborne." Your flight is therefore activated in the NAS.

4. TRACON En Route Strip

Based upon (I believe) your cruising speed, the NAS computer calculates how long it will take you to get to each point on your route. When it estimates that your airplane is less than 30 minutes from the next facility's boundary, an En Route strip is printed at that facility. In addition, you'll appear in the radar scope tab list at each facility.

Note the differences in comparison with a departure strip. This one features your cruising altitude prominently, as that's the altitude you're expected to be level at when you leave their airspace. The two boxes with "NEW" on the arrival strip indicate the previous fix and the coordination fix. Without going too much into it, these basically tell the receiving controller where you're going to enter their airspace. Lastly, the E0100 in field 8 tells the controller at what time you're expected to cross the coordination fix (in this case, 0100 Zulu).

Note: if you were talking to a En Route Center controller (for instance, if you're a corporate jet pilot flying at 31,000 feet), the strips those controllers use are completely different than the ones used by TRACONs. I won't go into detail, but here's an example below. You can clearly see the differences in the format:

5. Terminal Arrival Strip

Okay, you're 30 minutes away from Pensacola TRACON's airspace, and a Terminal arrival strip prints out in the TRACON.

First thing: note that the CID and the squawk code have changed. Remember how I mentioned that each center has its own host computer? Well, when you crossed from Mobile Approach's airspace into Pensacola's Approach's airspace, you crossed the line from Houston Center to Jacksonville Center. Now you're issued the new squawk code so that you sync up with Jacksonville's computer system.

The second thing you should notice is that the TRACON arrival strip has no altitude on it. Facilities have Letters of Agreement with their neighboring facilities that dictate at what altitude arriving aircraft will be at. I go into that a lot more in this post.

The short version: Mobile Approach must have assigned you an odd altitude (you are eastbound) and you must be level at or within 1000 feet of that altitude when you cross the boundary. Of course, since you were already cruising at 5000, you will likely just be left at 5000.

Once you've been handed off and are talking to Pensacola Approach, they'll tell you what kind of approach you can expect or ask you which one you prefer. That will be typically written on the strip (Visual, GPS, ILS, VOR, or NDB) so the controller knows for which one you need to be vectored.

The controller will also initiate a hand off to Pensacola tower by pressing "T" on the keyboard and click on the airplane on his scope. We have a lot of airplanes flying around in the vicinity of Pensacola and our tower doesn't need to be looking at every single one of them. They only care about airplanes that will actually be entering their airspace, which includes aircraft landing, doing practice approaches, or overflights through their airspace. By handing off to the tower, we ensure that relevant airplanes appear on the tower's radar display so they can see what they have inbound.

Below is a shot from PNS tower. You can see the radar feed display on the left. The only full data blocks are those with "T" tags, such as the FRL6092 (on final at 900 feet) and N530AQ (on high downwind at 8000 feet), which are arrivals to runway 26. If you look just southwest of N530AQ, you'll see an aircraft at 600 feet that's on a "P" tag (meaning he's talking to the TRACON's P sector). That "P" airplane is not landing at PNS and is outside of the tower's airspace, so the tower doesn't need to be looking at them.

6. Tower Arrival Strip

At the same time as Pensacola Approach received an arrival strip, another prints out in Pensacola tower.
After the approach controller has cleared you for an approach, you'll be switched to the tower. When you call up the tower, they grab your strip and write down whatever information their procedures call for. Maybe it's the runway you're landing on, maybe what kind of approach you're doing. As I said, every tower is different.

In the end, you're cleared to land, touch down, taxi to the ramp, and power down. Now it's time to go off and enjoy that $100 cheeseburger.

Post-Flight Debrief

For the pilot, that was a relatively simple flight plan. You popped PNS into your GPS, dialed 5000 on the autopilot, and kicked back and relaxed. The scenery along the gulf coast looked gorgeous as you flew along on a beautiful cloudless day.

But behind the scenes, you had quite a few people looking out for you.
  • 2 controllers at NEW: Ground and Local
  • 1 (or more) controllers at New Orleans TRACON: I'm not familiar with their sectorization, but at least one controller will work your departure.
  • 2 (or more) controllers at Gulf Port TRACON: I'm not familiar with their layout, but they probably have West and East sectors.
  • 2 controllers at Mobile TRACON: West and East sectors
  • 2 controllers at Pensacola TRACON: West and East sectors
  • 2 controllers at PNS: Ground and Local
That's at least eleven different controllers. All of them were responsible for your safety at different points of your journey. Using the strips and methods outlined above, they were able to make your flight as expeditious and predictable as possible. It's all about passing and using information effectively.

For the pilots, I hope that sheds some light about what goes on behind the scenes in the towers and radar rooms around the country.

P.S. The photos I used in this post are ones I took at my former flight school, ADF Airways, at Tamiami Airport, Miami, FL. I got to ride in the back on one flight while my instructor took up another student of his for a round of pattern work. So, if you're looking at those pics and thinking "Hey... that doesn't look like New Orleans, Gulf Port, Mobile, or Pensacola", you're absolutely right!


Brian said...

Great post. Here in the Dallas area, the DFW TRACON has gone ¨stripless.¨ The flight strip prints at 30 minutes prior at the departure airport, and then the info is loaded into the ARTS about five minutes later. Sometimes we have to change which tab list a departure is in based on the LOA.

Since everything is automated, we don´t activate flight plans with the TRACON or even scan strips, though they might at DFW Tower. As long as the pilot sets the correct transponder code, when he tags up on departure, the departure message is sent down the line. Our facility generally has automatic departure releases on all high jets, but has to call for release on all props and low jets. We can deal with up to FOUR different departure controllers!

I believe the CID is unique to each center and their host computer. So CID 713 is N12354 in Houston Center but could be AAL734 in Jax Center. That is why you may have to change your squawk code sometimes when crossing center boundaries.

Mark said...

New Orleans TRACON still uses FPS

Blue Cat said...

CID's are usually unique to each enroute center but the beacon code assigned by the tower "should" remain the same throughout the flight.
There are situations where a beacon code may need to be changed but that isn't the norm.
Flying Penguin...I love your blog, it reminds me what I love about being in aviation. I hope you are successful in your training at P31.

Anonymous said...

We don't have Private Pilot Licenses here in the US. Rather the FAA issues Private Pilot Certificates to those that meet the requirements. Take a look at your certificate and you won't see the word "license".