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On August 29, NATS had an issue, which escalated into a fairly major event for UK aviation. It was handled really well, but still leads to a lot of learning for us all so worth a look at what happened, why, and the impact on operations.

The Problem

NATS handles a big chunk of very busy airspace and in order for aircraft to utilise that airspace (whether that is overflying, or flying in and out of airports within it), they have to have a flight plan.

The system that handles flight plans stopped working, or rather the automation of it did. NATS reckon it was due to one dodgy bit of information that entered the system and caused the secondary automatic processing system to suspend itself as it could not guarantee the integrity of safety-related info so ‘failed safely’ to isolate the problem.

With the automatic system suspended, inputting and checking had to be done manually. Flight plans tend to be standardised (for airlines with recurring scheduled flights) and can only be submitted up to 120 hours prior to a flight meaning a whole lot to input manually in a relatively tight time frame.

NATS typically handle around 2600 departures and 2600 landings in a day, with approximately 500 aircraft airborne at anyone time, and a total of nearly 6000 flights in total in a day (that’s about 2 million flights per year).

The Explanation

Surely the pilots know where to go any way?” I heard one irate passenger cry. “Why can’t they fly without a flight plan!”

The whole event did raise the fair question of why we do need flight plans so badly anyway. After all, pilots do roughly know where the airport they are heading to is…

A flight plan is actually a pretty complex thing. It doesn’t just tell ATC where an aircraft plans to fly to and from, it also contains a whole bunch of information regarding how they will actually do that, what an aircraft is capable of, the equipment onboard, how ATC can control and communicate with the aircraft, the rules they will operate under…

This is important because airspace is not just a big space that aircraft zoom about in willy nilly with ATC vaguely ensuring they don’t get in each other’s way. It is complex and crowded, and info in the flight plan allows ATC to manage all those aircraft within the airspace system in space and time!

So, a lot of very important information is contained in a flight plan, which means a lot of very important information to be checked, input, and most importantly integrated into the actual ATC flow management planning and handling.

If you want to understand the full shebang of a flight plan, have a read of this chunk of ICAO Doc 4444 here.

One page with a lot of info (Source: ICAO Doc 4444)

The Impact

Well, first up a big delay with manual inputting (and probably a lot of sore fingers as NATS people sat typing as quickly as they could), but this delay had large and escalating knock on effects:

  • Aircraft cannot depart: Flight plans have to be accepted in order for aircraft to depart airports in the UK and also to operate through the airspace.
  • Airport fill up: With aircraft not departing, gates remain occupied and airport aprons and taxiways become congested.
  • Airborne aircraft have nowhere to go: With no space in airports, they have to divert to alternate ones. This impacted all of the UK airports which meant a lot of potential diversions.
  • Airlines and operators have to cancel. This helps ease the issue for the airports, but leads to a whole lot of logistical issues for the operators – from passenger support, to getting aircraft and crew where they need to be.
  • Airspace flow is severely limited: Traffic flow restrictions are put into place across the whole airspace because ATC do not have the information required to maintain the usual separation and controlling standards.
  • Inbound aircraft have to be prioritised: Aircraft have to land so those unable to be diverted or re-routed take precedence. This is more of an issue for long haul, international flights.
  • Other airspace and airports are impacted: Routes between mainland Europe and the UK cannot depart meaning those airports become congested, further impacting operations into them as well and leading to a domino effect across networks.
  • Overflights are re-routed: NATS controls airspace across the London and Scottish FIRs which link to extremely critical and busy airspace across the North Atlantic (NAT HLA). This traffic has to be re-routed via other airspace such as France and Portugal leading to further filing delays as well as congestion in those airspaces leading to other operational delays.
  • Fuel issues: For airborne aircraft this is a significant threat. For aircraft on the ground, it may require additional fuel but with many aircraft requiring it, further delays occur.
  • FTL issues: Crew who have reported for duty begin to exceed FTLs leading to further logistical issues for operations departments.
  • Cancellations for passengers: Operators have to find ways to accommodate and look after passengers stranded, and place them on other flights. It can take many days to clear backlogs because they cannot simply add more flights once the situation is resolved.
London airports fill up while airspace empties (Source: FlightRadar24)

Crisis Management

The level of knock on effect is significant, and extends right down from the folk in NATS attempting to resolve and communicate about the issue itself, to a barista in Costa Coffee trying to serve the hundredth passenger stranded in an airport…

NATS provided a number of updates and the issue was resolved efficiently (kudos to NATS!). You can read their updates here and find a good explanation of the situation and how it evolved.

EUROCONTROL provided real time information via their Network Operations Portal. You can view the history of the events and updates here. They also provide daily network status information covering all of Europe.

This isn’t the first time there have been issues with ATC services:

  • 2014: NATS services failed due to a technical issue leading to the closure of high level airspace across England and Wales, and several impacting airports around London, and stopping all departures from European airports requiring transit across the UK. A full report is available here.
  • 2023: An accidental deletion of data led to a failure in the live and backup NOTAM database in the USA. NOTAMs provide critical safety information to operators and pilots and the loss of the database led to the first nationwide stop of all traffic in 20 years. Some info is available here.
  • 2023: The Philippines experienced several power outages due to outdated systems. A major outage occurred in January 2023, leading to the closure of RPLL/Ninoy Aquino and the Manila FIR.
  • ATC Zero: ATC zero refers to the breakdown or loss of an ATC controlling a single region or FIR. Concern for these occurring during Covid led to updated contingency plans, particularly in areas like the NAT HLA.

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