UK ATC Delays – a case of “Computer says No”

Many folks will have read about the ATC delays being experienced in the UK today.

Sadly, a lot of the explanations have been rather technical, not surprising since they are coming from industry experts or from those working for ATC or airlines who have a good understanding of the industry and the jargon that goes with it (e.g. “sectors”, “flow control”, “slot restrictions”, etc.).

Air Traffic Control Principles: To maintain a good margin of safety, an air traffic controller can only manage (“work“) a certain number of aircraft at the same time, taking into account the amount of communication they need to make on the radio with the planes they are working and the phase of flight those planes are in (e.g. takeoff, landing, climb, descent, cruise).

To manage the amount of traffic the UK airspace needs to handle on a daily basis, the skies are carved up into areas known as “sectors“, slotting together like a big 3D jigsaw.

Sectors: A sector handling traffic which is “in the cruise” – has taken off, finished climbing and is in level flight in the general direction of it’s destination – can generally handle more aircraft than a sector which is working traffic that is descending toward it’s destination.

For example, it takes more work to direct an aircraft to line up with the runway in order to land (several radio transmissions over 10-20 minutes), especially in the cloudy weather we get in the UK, than to tell it to fly in a straight line for 200 miles (one radio transmission in half an hour).

As an aircraft goes on it’s journey, it’s handed off from controller to controller.

The pieces of this jigsaw are set up for the “worst case scenario” – the busiest time of the day, when the most aircraft are flying in the UK and the most Air Traffic Controllers are needed on duty.

Control Positions: The consoles you see the controllers working at, which include screens with radar displays showing where the planes are, and display output which helps the controllers organise the traffic they are responsible for, make a record of their decisions, and communicate with neighbouring controllers. This includes the “telephones” and the VHF radio system for speaking to the aircraft, which are integrated into the console at the control position.

Merging sectors (Band-boxing): At night, when less planes are in the UK’s skies, the traffic is less dense, and fewer controllers are needed. These sectors are combined together, known in industry jargon as band-boxing.

The radars, radios and telephone lines for all the different sectors being combined are re-routed to the control position working the band-boxed sector. This is done using control software, and I suspect it’s this control software which has suffered a failure and become stuck in “night mode”.

It can’t “un-bandbox” the sectors, and re-route the radio and telephone lines to the right place in the control room.

The Net Effect: Because of the failure, it’s not possible to un-bandbox the merged sectors, and delegate control of the airspace to a greater number of controllers. Going back to my first point, a controller can only work a certain amount of planes while maintaining an acceptable margin of safety.

If it’s not possible to have more controllers working the traffic, they simply have to make sure there are fewer planes in the same bit of sky at the same time. To do this, ATC uses a process known as Flow Control, which consists of setting very specific (to within a few minutes) take-off times for aircraft. Because we know a plane is going to fly at a certain speed, at a requested height, along a known route, ATC can work out where a plane should be at a given time, based on it’s take-off time.

If there is too much traffic expected in the same place at the same time, ATC will work out the soonest it could safely work the traffic and then work that backward into take-off times for each plane – the slot time you hear pilots refer to.

Therefore when you hear a pilot talk about missing our slot, they don’t necessarily mean missing their time to use the runway at the departure airport, but their allotted time through some point in the air, maybe a couple of hours into the future.

Obviously, when something like this happens, the flow control and slot restrictions become more severe, also you can’t just go on delaying flights indefinitely. Airlines must also try and do their bit to reduce the strain on the available ATC resources, and they will therefore start making tactical cancellations – for instance reducing the number of flights on a given route, especially if it’s a high-frequency route with a plane every hour or two. Maybe they will cancel 50% of the flights and transfer passengers to those planes which will still operate. This frees up a slot in the airspace to be used for a flight which runs less regularly and makes sure that route is still served.

Where airlines have a fleet of planes of different sizes, so these are the larger carriers such as BA, they may try and combine two flights together on one single larger aircraft (e.g. two Airbus A319s onto a single Boeing 767). Again, this makes more space in the skies.

If you’re travelling today, I hope you get where you’re going eventually. You’ll need a lot of patience though, and resign yourself to being delayed. Sadly, being delayed is one risk you take whenever you travel, and by whatever method you choose.

If you don’t really need to make your journey today, contact your airline and offer to travel another day, they may appreciate having one less person to carry and you will release your seat for someone else that might really need it.