The Manchester Airport Group Plc owns and operates three airports: Manchester, London Stansted and East Midlands. Manchester Airport is the third busiest airport in Britain. Their website says Manchester Airport is one of only two airports with two runways. Now actually, lots of small airports have two runways (one asphalt, one grass) and the old British military airfields generally had three at 60° angles to each other, although admittedly, most of these are no longer in use or have been turned into racetracks (Silverstone, Castle Combe, Goodwood and Thruxton). But, even if I presume they mean active runways at modern commercial airports, London Heathrow, Edinburgh and Belfast all have two runways and are all still in the UK, last time I checked.

Anyway, what’s important at the moment is that Manchester has two runways, both of which can be used in the daytime. The two runways are parallel and 390 metres (1,280 feet) apart. At the time, Runway 1 was 06L and Runway 2 was 06R. Manchester uses segregated operations, which means that one runway is used for arrivals and the other for departures. The prevailing wind at Manchester is from the west, so generally, inbound aircraft approach from the east and outbound aircraft depart to the west.

Runway 2 started operations in 2001 with the agreement that the airport would only put both runways into service during peak demand. In 2004, when this incident took place, dual operations were limited to peak times during the day and at weekends.

On the morning of the 29th of February 2004, at 06:45 local time, the airport put both runways in service, using Runway 1 for departures and Runway 2 for arrivals. Each runway had a controller assigned to it and each controller used a dedicated VHF frequency. The two controllers sat next to each other at the top of the Tower, facing south with a view of both runways. Air-1 sat on the left and controlled Runway 1 (departing aircraft), while Air-2 sat on the right and controlled Runway 2 (arriving aircraft).

The two controllers interacted with each other across the desk. These interactions were not recorded.

A third controller, Ground Movement Control, sat at a desk behind Air-1 and Air-2. Ground Movement Control also had a dedicated VHF frequency and was responsible for aircraft travelling between the parking stands and Runway 1.

If you were an aircraft departing from Manchester, you would first speak to Ground Movement Control to taxi from the stands to the runway area. As you approached Runway 1, Ground Movement Control would ask you to switch frequency to Air-1, who would guide you onto Runway 1 and clear you for take-off.

If you were an aircraft arriving at Manchester, you would speak to Air-2 and land on Runway 2. You would stay with Air-2 until you vacated Runway 2. Now, you need to cross Runway 1 in order to get to parking, so you would change frequency to Air-1 who would clear you to cross the runway. Once you were clear of Runway 1, you would speak to Ground Movement Control and taxi to the gate.

It’s a little more complicated from the controller’s point of view. Note that this is the system as it stood twenty years ago, in 2004, which doesn’t quite reflect Manchester procedures now.

The National Air Traffic Services (NATS) uses a system of Flight Progress Strips (FPS). Every aircraft in and around the airport has a strip, which includes aircraft details such as callsign and current clearance. At the time, this was completely paper-based.

For an aircraft leaving Manchester Airport, Manchester ATC personnel generate a strip. The strip is placed in a blue holder and given to Ground Movement Control, who is responsible for the appropriate clearance and taxi instructions from the parking stands to the taxi-ways leading to Runway 1. Ground Movement Control then transfers control of the aircraft to Air-1, passing the strip to Air-1 via a chute. Air-1 is now responsible for the aircraft until it takes off and has departed the airport.

Once the aircraft has taken off, Air-1 transfers control to the relevant airborne frequency and throws away the strip.

For an aircraft arriving at Manchester Airport, Manchester ATC personnel generates a strip which is placed in an orange holder for Air-2. Air-2 is responsible for the landing aircraft on Runway 2. Once the aircraft had landed and is clear of the runway, Air-2 transfers control of the aircraft to Air-1, handing over the flight strip, because Air-1 is responsible for the aircraft crossing Runway 1. Once the aircraft has crossed the runway, Air-1 passes control (and the strip) to Ground Movement Control.

This means that Air-1 ends up with both blue strips (departing aircraft) and orange strips (inbound aircraft). On Air-1’s desk was a recess to hold the flight progress strips, along with a runway designator strip which holds the active runway information. The strips for aircraft which had not been cleared to enter the runway, whether departing or crossing, are placed above the runway designator. When Air-1 clears an aircraft to enter the runway, he or she moves the aircraft’s strip to the bay below the runway designator strip. This bay can hold more than one strip, so if one aircraft was cleared for take-off and a second aircraft was cleared to line up on the runway and wait, both strips would be in the lower bay. The strip with the lowest position has the highest priority.

I know that a key question here is why are they still pushing paper strips around instead of a computerised system?. When I visited the tower in Blackpool in 2007, the same paper-based system was in use and I asked that very question. The controller explained to me that the strips system had been refined over forty years specifically to reduce the chances of human error. Computerised systems don’t automatically protect us from making mistakes and instead lead to a whole new wave of unpredictable errors. This system helped controllers to physically follow the progress of the aircraft and clearances in a straight-forward visual (and physical) manner.

Of course, no system is fullproof, and this day in Manchester stands out specifically because the system failed.

The National Air Traffic Control Service in the UK (NATS) has been shifting to a paperless system called EXCDS (Extended Computer Display System) since 2015, but at the time, it was still being developed. Manchester now uses EXCDS but I suspect it would not have helped in this particular case.

The controller representing Air-1 had eight years of experience and was validated for all positions in Tower and Approach. At the time of the incident, he was a current “On the Job Training Instructor”. He started work that day at 06:55 as Air-2. Traffic was light that morning. He took a half-hour break at 08:00 and then took over Ground Movement Planning for an hour. After a further 30-minute break, he took the position of Air-1 at 10:00 for a ninety-minute shift. He considered the morning traffic to be “light to medium”.

A Boeing 737-237, registered in Ireland as EI-CJI, was on final approach for Runway 2. There were 122 passengers and 5 crew on board. During this approach, the flight crew was advised to keep their speed up and that they were ahead of another 737.

A Boeing 747 landed and changed frequency to Air-1. Then, the Turkish flight, also a 737, landed, vacated the runway and was also transferred to Air-1. Finally, the Irish Boeing 737 landed. As they did, they saw the Turkish flight taxiing on Victor. They were cleared to vacate at Tango, the next exit from the runway.

Air-2 Controller: Keep your speed up you’ll overtake the Turkish on your left. Taxi Delta Zulu One hold short of Zero Six Left.

This instruction means that the Irish Boeing 737 should keep their speed up as they leave the runway and taxi directly to the location marked as DZ1. Air-2 expects them to get to the holding point before the Turkish aircraft, so they will cross Runway 1 second even though they landed third. Once at DZ1, they are to wait (hold short) until they are given clearance to cross Runway 1 (Zero Six Left).

After giving this instruction, Air-2 transferred control of the Irish Boeing 737 to Air-1.

At the same time, an Airbus A321 carrying 220 passengers and eight crew was taxiing towards Runway 1, when it was transferred from Ground Movement Control to Air-1. Air-1 cleared the Airbus to enter Runway 1, asking them to line up and wait.

Air-1 then cleared the 747, the first of the three aircraft to have landed, to cross Runway 1 and then hold on the other side. The 747 crossed the runway and reported when they were clear. Next, they will need to speak to Ground Movement Control for the taxi to the apron.

Once they reported that they had finished crossing the runway, Air-1 cleared the Airbus A321 for take-off. The captain of the Airbus remembered seeing the Irish Boeing 737 turning off of Runway 2 as he received the take-off clearance. He acknowledged that they were clear for take-off and the flight crew started their final checks.

The Irish Boeing 737 changed frequencies and checked in with Air-1, confirming that they were continuing to point DZ1. They had just joined the frequency, so the flight crew had not heard the clearance given to the Airbus A321.

Air-1 initially responded to this call using a different call sign; he seemed slightly confused. Then, he cleared the Ryanair Boeing 737 to cross Runway 1 (Zero Six Left) and wait behind the 747 that crossed earlier.

Air-1 Controller: Thank you and, er, you may as well, er, cross Runway Zero Six Left now. Hold behind the European Seven Four.

The Airbus crew were still running through their checks, speaking in the cockpit, so they did not hear the Boeing 737 cleared to cross the runway.

The flight crew of the Boeing 737 attempted to acknowledge the clearance but at the same time, a fourth aircraft called to check in with Air-1. The two calls at the same time made them close to unintelligible.

Air-1 repeated the crossing clearance for the Boeing 737 and then asked, “Who else was calling?”

The Boeing 737 captain remembered looking left along the runway to confirm that it was clear. Again, they’d never heard any of the calls to the Airbus A321; there was no reason to believe anyone would be on the runway.

The flight crew of the Airbus A321 had missed the initial call to the Boeing but heard it when Air-1 repeated the clearance to cross. The captain said he then saw the Boeing accelerate straight past the DZ1 holding point and realised what was happening.

The Boeing entered the active runway. They were travelling at an average groundspeed of 33 knots, slightly higher than the maximum ground speed for taxying at Manchester, as they had been asked to keep their speed up. After entering the runway, both pilots looked left again and saw the Airbus A321 barrelling towards them.

The Surface Movement Radar showed that when the Irish Boeing 737 entered the runway, the Airbus A321 was 873 metres away (2,700 feet). Manchester Airport’s Runway Incursion Monitoring function monitors Runway 1 to ensure it is clear. It lights an alert in several scenarios, including if a departing aircraft’s speed exceeds 40 knots and the runway ahead is not clear. The labels for the vehicles in conflict change colour to denote the alarm; there is no audio alert.

The Airbus A321 was travelling at over 100 knots (115 mph, 185 km/h) as the Boeing 737 crossed its path.

The labels for the Airbus A321 and the Boeing 737 turned red. The controller did not notice; ATC personnel at Manchester later said that the system was susceptible to spurious alerts.

The Airbus A321 captain called “STOPPING”, presumably while braking as hard as the aircraft was capable of.

The controller realised that the Airbus was taking off from the runway while the Boeing was crossing. His first thought was that an aircraft was taking off without clearance. In his mind, the Irish Boeing had priority for crossing the runway, and that’s the aircraft that he had been watching. He called for the Boeing 737 to hold position. Everything needed to stop.

The controller had made a basic mistake: clearing the Airbus to take-off and then clearing the Boeing to cross before the Airbus had departed. Controllers are human, just like pilots, and mistakes will happen. This is the point of the flight progress slips, whether paper or electronic. They offer a visual indication of the status of each aircraft so that we aren’t relying on memory alone. The controller should have placed the slip for the Airbus A321 in the priority position the moment he cleared it to take off.

When the Irish Boeing 737 joined the Air-1 frequency, the captain waited for a transmission to finish before checking in. That transmission must have been the tail end of the Airbus acknowledging its take-off clearance.

The Boeing 737 ended up ahead of Turkish, as expected. The 747, which had crossed the runway earlier, was still holding at the far end. Air-1 and other controllers at Manchester knew that the Irish operator was “expeditious on the ground” and would want to get to the gate as soon as possible. The controller had this on his mind as he told the Boeing to cross the runway and “hold behind the European Seven Four”.

When Air-1 used the wrong call sign when calling the Boeing 737, it was probably a subtle sign of overload. He had completely forgotten that he’d given take-off clearance to the Airbus and later, his recollection was that the Irish Boeing 737 strip was in the priority position in the bay. It was not possible to determine the actual position of the strips at the moment of the conflict, but it seems clear that either the controller hadn’t moved the Airbus strip down when he gave the flight crew clearance, or that a few minutes later, he moved the Boeing’s flight progress strip below the Airbus, showing the Boeing as the highest priority.

Regardless, two aircraft were cleared to use the same runway at the same time and the strip system meant to catch such a mistake had failed. The last safety net was that the flight crews would notice. If either had heard the clearance given to the other aircraft, they would have reacted. But the Boeing 737 joined the frequency too late to hear the clearance, and the Airbus A321 crew were in the middle of their checklists and thus did not hear the clearance given to the Boeing.

Another aircraft then called to check in, without waiting for the call-and-response to be completed. Air-1 had cleared the Irish Boeing flight to cross the active runway but the Boeing crew had not yet responded. This made for additional activity on the channel and a further distraction.

This time, the distraction actually helped. Air-1 repeated the clearance as he had not had an acknowledgement. The Airbus crew heard the repeated call and realised they were in danger, immediately reacting to the aircraft they now saw entering the runway.

It was enough. The Airbus A321 came to a halt before the taxiway. The two aircraft were 600 metres apart, less than 2,000 feet.

A miss is as good as a mile. The disaster was averted.

The AAIB summary says:

The incident occurred following a human error by a controller. While procedures are continually evaluated to minimise the opportunities for error, human errors will occur. The defence against this incident becoming an accident was the use of effective situation awareness by a flight crew. Whilst the risk of a collision was averted, the incident shows the importance of all crews and controllers maintaining maximum situational awareness at all times.

As Manchester Airport was already in the process of evaluating their procedures and NATS were working towards improving and modernising those procedures, the AAIB declined to make a formal recommendation.

The operator of the Irish Boeing 737 reported that they were now using the circumstances of the incident as a training aid for their crews.