As far as an ATC n00b like myself is concerned, Flight Progress Strips ('strips' from here on) are the lifeblood of well-planned and well-executed controlling. I gather one eventually learns to develop and maintain a three-dimensional 'picture' in one's head of the traffic situation one is controlling, though I haven't yet achieved that mental zenith. In any case the backup against failure of that, the radar, the R/T or any of the other various critical systems that support our work is the strips.
They are the first 'layer' of filtering through which virtually all vertical plans go before being executed. They will tell the controller at the very basest level whether what they intend to do is safe. They further permit the controller to ensure all the traffic under their control will be in a safe configuration should any of the aforementioned tools fail.
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First of all, what are they? Literally, they are paper strips containing information about flights. There is a strip for each flight, containing information such as its callsign, aircraft type, vortex category, cruise speed, current altitude (just so we're clear, I'm not going to bother splitting hairs with terms like height, altitude or flight level in this writeup), requested cruising altitude and other things this writeup doesn't care about. There are also several boxes the controller writes in as they issue various instructions to the flight, or provide it with certain bits of information.
Through various cool electronic means, when a flight is approaching a sector or an airfield a paper strip for that flight is automatically printed out near the control positions for that sector or control tower. The strips are put into individual plastic holders and held on a sloping board in front of the controller. This board has three columns, each split into two or more vertical segments called 'bays'.
The columns either side of centre contain 'pending' bays, which hold strips for expected traffic. If viewed in plan, the bays roughly correspond to the direction from which the flight is entering the controller's airspace: strips for flights from the West will be in a bay on the controller's left. There may be a pending bay in the centre column as well, but mostly this holds strips for flights the controller is currently working: the 'active' bay.
Although this isn't how it's done operationally, during early training we keep active strips in ascending order of altitude: the aircraft flying the lowest is at the bottom of the stack. This gives a rudimentary picture of the vertical relationship of all aircraft we are working, and is a first point of reference when considering climbs or descents. Take the following example:
| DLH404 410 R410 | Lufthansa 404
| BAW123 270 R270 | Speedbird 123
| AFR670 250 R340 | Air France 670
These are three strips in our active bay. Ignore that lots of the aforementioned information is absent. We've got Speedbird 123 ('Speedbird' is the colloquialism for British Airways, in a throwback to the old BOAC logo; ATC similarly address Pan Am aircraft as 'Clipper'), Air France 670 and Lufthansa 404. The Lufthansa is at its requested cruising level of 41,000ft, as is the Speedbird at 27,000ft. The Air France is at 25,000ft, requesting a cruising level of 34,000ft. The Lufthansa is above the Speedbird, which is above the Air France, as shown by the ordering of the strips.
We are going to get the Air France its requested cruising level, one way or another.
Before we give any climb instruction to the Air France we have a look at the strip stack, to see if anything is potentially 'blocking' that climb. The Speedbird could be, since the Air France will have to climb 'through' it to get to its requested altitude. The Lufthansa is at 41,000ft - well above the level the Air France is requesting - so is not an issue. We now move to the radar, to see if the Speedbird actually would block the Air France's climb. The two might be 80 miles apart, in which case we probably wouldn't hesitate to issue the climb ("nah, they're never gonna hit each other" is a phrase you hear in sims a lot, correct or not). If they are closer we may have to use other methods to safely climb the Air France through the Speedbird, but we won't go into those right now. Once the Air France gets its climb, we move the strip so that it is above the Speedbird's strip, accurately reflecting the vertical relationships of all three aircraft.
| DLH404 410 R410 |
250 340 R340 |
| BAW123 270 R270 |
When we give instructions to an aircraft there is almost always some attendant marking of its flight strip. If we issue a climb we draw an upwards arrow on the strip, with a number next to it indicating the cleared level. As the aircraft vacates its previous level we cross that level out on the strip. In our example above where Air France is climbing, we would draw an upwards arrow with '340' next to it (to indicate Flight Level 340), and once the aircraft has vacated FL250 we cross out the '250'. This indicates the level is nominally available for other aircraft.
We know an aircraft has vacated a level either when the pilot reports it or the aircraft's level readout on the radar indicates it is 400ft or more above that level. In our example here, if the pilot did not report leaving FL250, we would consider that level vacated once the altitude readout on the radar for Air France 670 showed '254'.
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It's all very well using the 'active' strips to separate traffic, but chances are you will be seeing more aircraft in the coming minutes than you are actually controlling right now. Yes, I'm talking about the pending bays. The strips in the active bay must not be considered in isolation.
Pre-printed information on the strip also indicates what routing the aircraft will take, so the controller will be able to see where it is exiting their sector. As I said, the strip board and its bays are roughly arranged so they are a logical representation of that sector. Strips for aircraft that will enter the sector from the South-West will probably be in the lowest bay on the controller's left, beneath a divider marked with the name of the navigation beacon or reporting point an aircraft would fly over as it entered the sector from that direction.
These 'pending' strips are further used to plan levels. Since a controller can see where a particular aircraft will be exiting their sector, they can also see what aircraft will be entering in the opposite direction. Even though the 'active' strips may say a climb is safe, the 'pending' strips may disagree. Observe:
| WEST | | DLH404 E 410 R410 |
| PAA657 E 310 R310 | | BAW123 W 270 R270 |
| AFR670 W 250 R340 |
The situation in the active bay is the same as before, though I have added a letter to each strip to indicate the direction the flight is going (this is a gross oversimplification). Again, Air France wants to climb to 34,000ft and will have to climb through the Speedbird, which is maintaining 27,000ft. We may look at the radar and see those two aircraft will not present a problem to each other, but this is not the last stop before giving Air France its climb.
We know Air France is going out of our sector to the West, and look to the pending strips from that direction. There we see that Pan Am 657 is entering from the West, maintaining 31,000ft. This could present a problem for the Air France since it will have to climb through the Clipper's level, and we may have to take some further measures to ensure we maintain safe separation between the two.
The problem is the Clipper may not even be on the radar screen yet. The two aircraft are too far apart for us to judge whether there will be a problem. Air France may have sufficient time and distance to make the 7,000ft 'jump' above the Clipper, but at this stage there is not enough certainty of that. This judgement is made easier in real life since strips contain estimated times the aircraft will pass over certain points, but I am ignoring that here for the sake of simplicity.
Regardless of any possible situation with the Clipper, we do know that we can climb the Air France 'through' the Speedbird, so we give it the minimum climb we know is safe: up to 1,000ft below the Clipper.
"Air France 670, climb flight level three hundred."
"Climb flight level three hundred, leaving flight level 250, Air France 670."
We mark an upwards arrow on Air France's strip with '300' next to it, and since the pilot has reported vacating FL250 we cross out the '250' straight away. Then we move the strip to its appropriate position in the stack: above Speedbird but below Lufthansa. We let things trot along for a little while, check everything else is running smoothly and eventually come back to find that Air France has reached FL300. When an aircraft levels out after a climb or descent, the stalk of the climb/descent arrow is struck out to indicate this:
This descent arrow indicates the aircraft was initially at FL320, was subsequently cleared to FL270, then to FL200, then to FL120. The struck-out numbers indicate the aircraft has passed, or is 'out of' those levels (the aforementioned 400ft loose change), and the struck-out arrow indicates the aircraft has levelled out at FL120. In other words the aircraft was in continuous descent from FL320 to FL120. If any subsequent descent is issued a new arrow is drawn with the appropriate number; the same for climbs.
Back to our wee situation. Air France has levelled out at FL300, and the inbound Clipper is 1,000ft above it at FL310. We now refer to the radar to see how far apart the two aircraft actually are. In giving an intermediate climb like this we have made the judgement much easier to make, since the Air France now only has to 'jump' 2,000ft to get safely above the Clipper. After assessing the distance of the two from each other we decide whether to make the climb, or whether we have to start issuing turn instructions. If the two are head-on, beyond about 50nm apart then there is plenty of time to give the climb, unless either are supersonic or some other such bananas thing.
The basic rule separating aircraft is to do it using the strips first; if you need to do anything beyond what the strips 'permit' (e.g. climbing an aircraft through the level of another), further measures need to be used. Radar can be used to make a judgement, or aircraft can be locked onto appropriate headings before the level swap occurs, but this is beyond the scope of this writeup. Using such a staged approach to separating aircraft guarantees that the controller's actions will remain safe in the event that, say, the R/T fails and the controller is unable to speak to any aircraft. I hope to expand on the last sentence elsewhere.
Air Traffic Control: as easy as listening to Eluvium.