NATS’ preliminary investigation has confirmed the root cause that led to last Monday’s air traffic control incident, and the company has reiterated its apology for the associated disruption to passengers, airlines and airports.
The report, submitted earlier this week to the Civil Aviation Authority, identified an issue with a flight plan processing sub-system called Flight Plan Reception Suite Automated – Replacement (FPRSA-R).
The NATS incident
On 28th August 2023, significant disruption was experienced across UK airspace following an incident affecting part of the technical infrastructure that supports NATS’ safe controlling of aircraft.
The reduced levels of flights that resulted from the measures needed to maintain safety due to the technical incident caused significant disruption to the UK aviation system.
While it is not yet clear exactly how many flights were cancelled by airlines, it is likely that the number exceeds 1,500 for Monday 28th August, with more cancelled on Tuesday 29th August as the airlines strived to recover their schedules.
This number is in addition to the delays to flights on 28th August; of the 5,500 flights that did operate in UK airspace around 575 were delayed as a result of the incident.
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Identifying the Culprit: FPRSA-R
The heart of NATS’ air traffic control technical infrastructure lies in its various subsystems. The investigation pinpointed a specific issue with a subsystem known as Flight Plan Reception Suite Automated – Replacement (FPRSA-R).
This subsystem plays a vital role in processing flight plans and ensuring the smooth flow of air traffic.
The incident was not caused by a single flaw but rather by a series of extremely rare circumstances. It was discovered that a flight plan containing two identically named, yet distinct waypoint markers outside of UK airspace triggered a ‘critical exception.’
Both the primary system and its backup entered a fail-safe mode, creating a challenging situation.
In the face of this ‘critical exception,’ the system faced a dilemma. It couldn’t reject the flight plan without a clear understanding of its potential impact, nor could it allow it through and risk providing air traffic controllers with incorrect safety-critical information.
This decision-making process was further complicated by the urgency of the situation.
Steps Toward Prevention
What makes this incident particularly noteworthy is its unprecedented nature. The FPRSA-R subsystem had processed more than 15 million flight plans over the five years it had been in service without encountering such a scenario. This highlights the rarity and complexity of the circumstances.
To ensure that such an incident does not recur, NATS has taken significant steps. The time taken to recover from the incident was largely due to the need to identify the problem, isolate the specific data causing the issue in a controlled manner, and rigorously test it to ensure it could safely return to operation.
CEO Martin Rolfe Comments
Martin Rolfe, the CEO of NATS, expressed his commitment to safety, stating, “Keeping the sky safe is what guides every action we take, and that was our priority during last week’s incident.
I would like to reiterate my apology for the effects it had on so many people, including our airline and airport customers. Incidents like this are extremely rare, and we have put measures in place to ensure it does not happen again.”
A full copy of the NATS Report can be accessed here.
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