I am so going to regret choosing this topic.
As I imagine is the case with several professions, there are aspects to the ATC world that even controllers and the regulators scratch their heads and argue over the actual facts of. It is sometimes inconclusive that everyone isn't just (hah) winging it concerning some details. I hear instructors arguing over minutiae all the time. They would be the people responsible for teaching others how not to bang aeroplanes together.
On-route status; what is this, and why should you care? Actually the chances are very, very great you needn't care at all; as far as I know none of us are air traffic controllers, only a fraction of one percent of us are pilots, and even pilots don't need to know what this is. But we are all geeks here, yes?
On-route status is related to expedition: aeroplanes getting where they want to go, more quickly. Public transport flights must be planned, and that plan filed centrally, a minimum period of time before the flight is intended to take place. These plans allow ATC units to get details of a flight before it arrives in their airspace, and plan what to do with it when it does. All sorts of interwebs connect ATC units together to permit relatively transparent exchanges of information on what flights are going where, and when.
Part of the information contained in a flight plan is the route the aircraft will be flying; it is rarely a straight line, since an aircraft often passes through the airways of one or more countries on its way to its destination. 'Airways' are fixed, published "corridors" of controlled airspace inside which the movement of aircraft is strictly controlled. Since airlines prefer the protection and efficiency these corridors afford, flights tend to stay within them even though they rarely align perfectly with where an aircraft is going.
One of the most annoying requests a pilot may make of a controller is for a "direct routing." This is no more complicated than it sounds: the pilot simply wishes to skip the convolutions of their pre-planned route and make a beeline for their destination, to get there as quickly as possible (for some reason I think of Ryanair, which awards pilots bonuses for fuel economy). A request for a direct routing is often fairly trivial, at least nominally, but if the controller hasn't given it already there is usually a reason - remembering every controller's goal is to dump all of their aeroplanes onto someone else as quickly as possible - so pestering them about it will probably just piss them off.
As seems often to be the case with my ATC ramblings, a TORRENTIAL ONSLAUGHT small amount of background information should aid understanding the subject itself.
Airspace Divisions
As I have said in previous writeups, airspace is divided into areas of different classifications, each of which place varying limitations on what aircraft can and cannot do without permission. Now, the sky being three-dimensional, it will probably not surprise the reader to hear that such demarcations are vertical as well as horizontal. This is fair enough, but most examples of both are neither consistent nor regular from one area to the next, which can prove quite a head-scratcher initially.
After six years, you probably all know by now that I love my examples. And my ASCII art. Sorry.
Let's use a contrived bit of airspace, viewed first in plan:
N
+-------------------------+-------------------------+
| | |
| | |
W | BLOCK | STACKER | E
| | |
| | |
+-------------------------+-------------------------+
<-------- 100nm --------> <-------- 100nm -------->
S
Oh, if only airspace structures were actually this simple. The south-north boundary here marks the division between two sectors - BLOCK and STACKER. Nominally fairly simple, until I add that we're just looking at one layer of the airspace strata at this point. Take the same bit of airspace in elevation:
| |
| OMG TEH EDGE OF SPACE LOL |
| |
+---------------------------------------------------+ FL660
| |
| |
| MRSA |
| |
| |
+-------------------------+-------------------------+ FL460
| | |
| | |
| FLURRY | LOUD |
| | |
| | |
+-------------------------+----+--------------------+ FL245
| |<------ 80nm ------>|
| | |
| BLOCK +----+ FL150 |
| | STACKER |
|<-------- 100nm -------->|<------- 100nm --------->|
+-------------------------+-------------------------+ 0
/////////////////////////////////////////////////////
WEST //////////////////// SURFACE //////////////////// EAST
/////////////////////////////////////////////////////
I apologise for the crudity of this model; I didn't have time to draw it to scale or to paint it.
So anyway, to summarise: *gulp*
We were looking at the bottom layer of this pile of airspace. Plenty of blocks of airspace like this exist in reality; in this example, BLOCK, STACKER, FLURRY, the LOUD sector and MRSA are all sections of airspace controlled, most likely, by individual controllers.
Why all these divisions? Mostly, the lateral divisions group together traffic flowing in particular directions, or traffic operating near a specific airfield or clutch of airfields. This is by way of simplifying life for the poor saps controlling everything, including those responsible for planning such heady things as Target Sector Flows and Sector Capacities.
And the 'step' of airspace between BLOCK and STACKER sectors? Such contrivances are mostly instituted following practical experience, and usually relate to aircraft departing from airfields and climbing up to their cruising altitudes. Here, it is likely that BLOCK sector has an airfield from which a high volume of eastbound traffic departs, including aircraft which frequently struggled to crest STACKER sector and ended up clipping the corner of their airspace, like so:
| FLURRY | LOUD >>>>> |
| | >>>> |
| | >>>>> |
+-------------------------+-- >>>> -----------------+ FL245
| >>>>> |
| BLOCK >>>> | |
| >>>>> | STACKER |
| >>>> | |
| >>>>> | |
+-------------------------+-------------------------+ 0
/////////////////////////////////////////////////////
WEST //////////////////// SURFACE //////////////////// EAST
/////////////////////////////////////////////////////
Those slow-climbing aircraft would have to go through three sectors on their way up to their cruising level: first BLOCK, then STACKER, then the LOUD sector. At some point, some STACKER sector controllers got together and said to their superiors: "we're fed up of taking aircraft from BLOCK sector that only stay in our airspace for two minutes. Just delegate a bit of our airspace to them and they can go straight to the LOUD sector instead."
So, I imagine after much bureaucratic hoop-jumping, a change in the airspace was effected to give BLOCK sector control of a small portion of STACKER sector's upper airspace, so slow-climbing aircraft could go straight from BLOCK sector to the LOUD sector.
| FLURRY | LOUD >>>>> |
| | >>>> |
| | >>>>> |
+-------------------------+-- >>>> -----------------+ FL245
| >>>>> | |
| BLOCK >>>> | |
| >>>>> +----+ FL150 |
| >>>> | STACKER |
| >>>>> | |
+-------------------------+-------------------------+ 0
/////////////////////////////////////////////////////
WEST //////////////////// SURFACE //////////////////// EAST
/////////////////////////////////////////////////////
Our simple plan map of that slice of airspace would then look something like this:
+---------------+
| FL150 - FL245 |
/| Delegated to |
N / | BLOCK sector |
/ +---------------+
+-------------------------+--/-+--------------------+
| |////| |
| |////| |
W | BLOCK |////| STACKER | E
| |////| |
| |////| |
+-------------------------+----+--------------------+
S
A change not unlike this was once made to a portion of the airspace west of the London Terminal Manoeuvring Area, mainly because of the Airbus A340. These aircraft are such slow climbers that when departing Heathrow they frequently could not climb fast enough to remain inside controlled airspace (the base of controlled airspace usually gets higher further away from airfields), so a horizontal 'stub' of controlled airspace was extended to accommodate them.
Moving on to vertical divisions, these are usually also static. They exist mostly because aircraft tend to accelerate as they climb. Interestingly enough, the thinning air rapidly reduces the range of viable cruising speeds available to an aircraft as it climbs. In fact, when an airliner is flying at its cruising altitude (~35,000ft+), there are only a few dozen knots between the speed at which its wings will fall off because it's flying too fast, and the speed at which it will drop out of the sky because it's flying too slowly. Anyway, I digress: since it is easier for controllers to work with aircraft flying at generally similar speeds, airspace is divided up vertically as well as horizontally.
FL245... well, FL195 really
There are some vertical airspace divisions in the UK which generally extend across the whole country. The first significant level is flight level (FL) 245 - about 24,500ft. This marks the top of what is called the Flight Information Region (FIR), which contains things like Aerodrome Traffic Zones, Control Zones, Control Areas and Airways. Airways are almost all Class A: the most strictly-controlled airspace class. The maximum vertical extent of airways is FL245. Sectors which contain airways frequently have their 'top' at FL245, coincident with the tops of their airways.
It's worth noting, though, that airways are usually only about ten miles wide and never extend right down to the surface. They are surrounded by other, lower classes of airspace so aircraft not permitted to fly through them (aircraft operating visually, for example) can still fly around or under them.
Above FL245, over the whole of the UK, is the Upper Information Region (UIR). It has no vertical limit, but contains various airspace divisions which do, such as FLURRY sector and the LOUD sector in the example above. The UIR used to be Class B airspace up to FL460 until March 2007, when all airspace above FL195 was made Class C to bring the UK in line with similar divisions in the rest of Europe (only three other states in the EU use Class B airspace, and none in such volume as the UK did).
The slightly confusing part is that vertical boundary between the FIR and UIR remained unchanged at FL245. When I was training and we were informed of this the question was soon asked: so what happens to airways between FL195 and FL245? Under the previous system, airways - which you hopefully recall are virtually all Class A - extended up to FL245. But under the revised arrangement, all UK airspace from FL195 to FL660 is Class C.
Why does this matter? Because only instrument flight rules (IFR) flights may enter Class A airspace. IFR flights are navigated using instrument - rather than visual - references. Aircraft can happily fly through cloud or at night and still know where they are going. Visual flight rules (VFR) flights do not enjoy this luxury, and thus the amount of control ATC may exercise over them is limited. ATC cannot, for example, instruct a VFR flight to fly in a particular direction because it may result in the aircraft flying into cloud. This means that IFR flights have to give way to VFR, if it comes to that.
Class C airspace allows both IFR and VFR flights, while still requiring ATC to separate most aircraft from each other. Although the change to Class C was a slight improvement over Class B (at least, from an ATC standpoint: in Class B airspace, not only are both VFR and IFR flights allowed but ATC are required to separate them all), opening up airways to VFR flights above FL195 could raise traffic levels and controller workloads unpredictably, so a caveat was added that VFR flights would generally not be permitted in this airspace:
VFR flight by civil aircraft above FL 195 shall not be permitted unless it has been accorded specific arrangements by the appropriate ATS authority.
...
It is anticipated the demand for VFR access [to Class C airspace above FL195] will be minimal. Such access will be accommodated within the context of safety, capacity and effect on the ATS network as a whole; consequently VFR access to the ATS route structure is only likely to be permitted in exceptional circumstances.
-UK AIP ENR 1-1-4-3 (14th February 2008)
The effect of this is that while airways between FL195 and FL245 are Class C - and thus technically not airways - they retain virtually the same status as if they were Class A. Class C airways, then.
So, we now have Class C airspace across the whole of the UK up to FL660. This Class C airspace contains the Upper Airspace Control Area, which comprises the Upper Air Traffic Service Route (UAR) structure and the Military Mandatory Radar Service Area (MRSA), which you may remember from our earlier airspace sandwich. To generalise, the MRSA comprises all of the upper airspace that isn't part of the UAR structure. Aircraft in this airspace are required to operate under radar control, and when outside of the UAR structure this service is usually provided by the military.
FL460
This is the next significant level in the airspace strata - about 46,000ft. It marks the maximum vertical extent of the Upper Air Traffic Service Routes (UARs) and also the altitude at which civil air traffic control terminates. I am now drifting in proximity to the writeup topic.
If you care to view a couple of the charts of the UK's route structure and compare the low-level routes (below FL245) with the high-level routes (FL245 and above)...
http://www.nats-uk.ead-it.com/aip/current/enr/EG_ENR_6_3_1_1_en.pdf (~8MB PDF)
http://www.nats-uk.ead-it.com/aip/current/enr/EG_ENR_6_3_2_1_en.pdf (~2.2MB PDF)
...you should be able to see some broad similarities in the positioning of certain routes. Air Traffic Service Routes, while they may not have a consistent airspace classification, frequently extend right up into the UIR to a maximum of FL460. The Class A part ends at FL195, but they may continue above that as Class C.
UARs, unlike the airways below them, do not have a declared width. For the purposes of ATC provision though, they are deemed to stretch five nautical miles either side of their centreline. Outside that is the MRSA.
FL660
Not much to be said about this, really. All UK airspace above FL660 - about 66,000ft - is Class G, or uncontrolled airspace. If you can get permission and can get this high, you can do anything you like without having to speak to anyone else. In theory you could even fly VFR at this altitude.
Why does controlled airspace go up this high? Two reasons: the RAF and Concorde. Concorde usually cruised at around 60,000ft and the RAF has plenty of aircraft that can operate at this altitude, so some provision was made for them in airspace structure.
On-route
Now we're here we can finally usefully define "on-route", as it pertains to these Upper ATS Routes. An aircraft is deemed to be on-route if it is within five nautical miles of, and flying parallel to, the centreline of a UAR.
The important aspect of this is how civil aircraft interact with military aircraft in the UIR. Since the Upper ATS Routes are surrounded by military airspace, it is quite possible for RAF jets to be swooping around nearby doing all sorts of crazy nonsense. It is next to impossible to take this into account when planning civil flights; therefore it is the responsibility of military controllers to keep their aircraft separated from civil aircraft with "on-route" status. If a civil aircraft does not have on-route status it is said to be - wait for it - "off-route."
If a civil aircraft is off-route, the civil controller is responsible for separating that aircraft from military aircraft. Since this would involve trying to anticipate the actions of aircraft you neither know exist nor are in contact with, you can probably imagine that it is not a desirable assignment. To say the least, it is more than the job is worth.
Now we finally get to annoying pilots pestering for direct routings. If an aircraft is in the upper airspace when the pilot makes this request, a direct routing will almost certainly take the aircraft off the UAR it is following: if you - the civil controller - grant the pilot's request, the flight will then be "off-route." You then become responsible for separating it from any military aircraft zooming around outside the civil route structure. If a Typhoon comes barrelling along at mach 2 and grazes your aircraft by 100ft, it is your fault. If you value your ATC licence you should not touch this situation with a ten-foot bargepole.
The oddity here is that nothing actually stops you from doing this. Nothing legally prevents a civil controller diverting traffic from the upper civil route structure. Although the Military Mandatory Radar Service Area is all Class C airspace - which aircraft may not enter without a clearance - it and the upper route structure are all the same block of airspace; if an aircraft is cleared to enter the upper route structure, that clearance is "good" for all of the upper airspace. It's a pit waiting to be fallen into.
Fortunately there is a way to sidestep this problem: you can call the appropriate RAF controller, tell them where the aircraft is and what route it wants to take, then sweetly ask the controller for "on-route status" for that flight. If the controller grants it, the effect is that a temporary Upper ATS Route follows the aircraft along its chosen path; it retains its on-route status despite having diverted from the established route structure, and thus the military remains responsible for separating military aircraft from it. The pilot gets what they want, you don't have to listen to their griping, and you don't have to put your licence on the line.
If the RAF don't grant you on-route status for the flight, you can still legally go ahead and point the aircraft onto a direct routing, but don't come crying to me if it gets sliced in two by a blind Tonka driver. At least it's one fewer pilot that's going to bother you.