“Confirmation Bias” Takes Over a Cockpit
In a report released July 26, 2006, the Accident Investigation Board of Norway (AIBN) described how a Gardermoen ATC controller’s shouted intervention halted an underway taxiway takeoff, albeit late in the takeoff roll, by a Pegasus Airlines 737-800 on Oct. 23, 2005.
The Turkish aircraft captain and the ATCO were both female. That observation may or may not be relevant to what happened. What’s clear is that the incident derived from “confirmation bias,” loosely defined as wishful thinking predicated on visual evidence. It’s not an uncommon mistake; universal lessons can be learned from it.
TC-APH was operating Flight PGT872 from Oslo to Antalya. The departure runway (RWY) in use was 01L. A “Notice to airmen” (NOTAM) had been issued for RWY 01L: “A1 and A2 clsd, RWY 01L TORA (takeoff run available) 3200M via A3 and backtrack. RWY 01L not available for landing.” The crew was aware of this and the commander was strongly focused on using the available runway from intersection A3.
While taxiing in a southerly direction on TWY N, the crew got take-off clearance on RWY 01L via intersection A3. The first officer acknowledged this with the flight’s call sign, and the controller followed up with clearance to backtrack taxi in a southerly direction along RWY 01L, if necessary. The crew calculated that the runway available from A3 was sufficient for take-off. The stated TORA in the AIP (airfield dimensions manual) for RWY 01L from A3 is 2,696 metres.
In her report, the commander said that when they were given clearance for take-off and turned to the right towards A3, she increased the engine rpm slightly and lit the landing lights. She then saw the yellow dashed line across TWY M, south of A3 which marks the intermediate holding point. “When I saw the yellow dash lines on the left on the taxiway, because of the notam I misinterpreted the lines to be as if I was lining up on the runway centreline,” she recounted.
The commander had a strong focus on the crew making an intersection take-off and misgivings that the available RWY was shorter than normal. She knew that the part of the runway to the south of the aircraft was closed, and interpreted the yellow dashed line to be a delineation marking the closed part of the RWY. At the time, this seemed logical to the commander because it “coincided” with the NOTAM. The pilot had flown to many airports in many countries, and claimed to be accustomed to ground conditions, including markings that were not always in accordance with ICAO standards. An airport operator marking a closed part of the RWY in this non-standard manner was considered by the commander as within the realm of the possible.
Impulse and Impetus
From this point in time, the commander was mentally “on the runway” and, as clearance for take-off had already been given, the take-off procedure was promptly initiated. Because the available runway was shorter than normal, the commander took the aircraft all the way down to the yellow dashed line on TWY M (which she now believed to be the RWY displaced threshold) before turning north (up the taxiway) to ensure that there was no available runway behind the aircraft. In the control tower, this turn was understood as being the crew turning south on TWY M and the air traffic controller initially thought that the crew was continuing to taxi southwards there. The air traffic controller therefore called the crew: “Confirm you are entering runway now, seems like you are turning onto Mike.” Continue right turn, right turn and then left again to enter the runway.” The first officer responded: “Turning right.” He’d also misinterpreted the controller’s directive.
When the right turn had been completed, and with the nose of the aircraft pointing towards the north on TWY M, the commander pressed the TO/GA button and the aircraft accelerated towards take-off speed, quickly reaching 80kts. The air traffic controller immediately understood what was about to happen when she heard the increase in engine rpm. She called the crew immediately, stating clearly and authoritatively: “Hold position, you are on taxiway Mike.” The commander says in her report: “I immediately disconnected the autothrottles, closed the thrust levers and braked to reduce the speed.”
Insignificant Signs
Because the commander had focused on straightening for take-off and the perspective in front of the aircraft when the right turn was completed, she had not noticed the illuminated sign “A3, 01L – 19R” which was positioned to the left of the aircraft at the stop line between TWY M and the RWY. She had already rationalized her subsequent actions and made a commitment to go.
After the takeoff abort, the pilots were evidently shaken to the extent that they failed to carry out the controller’s subsequent directions, turning left towards the runway to enter at A4, instead of right to track back down TWY N to A3 as directed. The controller stopped another taxiing aircraft and cleared PGT872 to enter, backtrack and takeoff.
There’s no doubt that the controller’s attentiveness and assertiveness averted a major accident. TWY M runs from A2 to A7 and is significantly shorter than the western runway. The crew started acceleration for take-off from A3. From this point the distance to the northern end of TWY M is only 1,600 m. A Boeing 737-800 at max takeoff mass requires a RWY distance of 2,100 m for take-off at sea level. There was no intervention by either of the other two pilots in the 737 cockpit.
RWY lighting at Gardermoen is always set at low intensity when visibility is good. These lights are very directional in directions aligned to the RWY. At low intensity, the RWY edge lights are barely visible when you look at the RWY perpendicular to its length. This would’ve contributed to the pilot’s error. Oslo had also chosen to use the opportunity offered in ICAO Annex 14 not to mark taxiway edges with blue lights. It’s possible that blue edge lights would’ve contributed to reducing the probability of misinterpretation. The commander commenced take-off on a paved path having clear green center-line lights, which a runway never has.
The AIBN considered that the most important cause for the incident was to be found in human factors. The commander was experienced, with almost 7,000 flying hours, but came to easily misinterpret the standard ICAO markings so that they “coincided” with her perceptions — an overweening impression that she was on the RWY.
The AIBN has not been successful in finding a clear reason for this, other than the vague phenomenon of confirmation bias (a simplistic definition of which might be self- delusion, where “seeing” is believing). Another conclusion might be that the markings, lighting and intensities used for taxiways and runways aren’t particularly intuitive, particularly as implemented at Gardermoen. For pilots, man-made nocturnal views that aren’t intuitive have the innate capacity to confuse. The main lesson learned should be that precipitate action and assumption can easily kill. When in any doubt, always hasten slowly and seek assistance, even a “Follow Me” vehicle. Stale takeoff clearances aren’t worth the paper they’re not written on. That conclusion agrees with the AIBN’s safety recommendations.
At airports where taxiways run parallel to the runway, there is a risk that air crews may become confused and try to use a taxiway for take-off. The AIBN recommended that Avinor (ATC) consider implementing a procedure where take-off clearance is not issued before the air traffic controller has verified that the aircraft has passed a point where the only remaining possibility for departure is on the intended runway.
At Oslo Airport Gardermoen, Norway, the intensity of runway guard lights and taxiway center-line lights are adjusted simultaneously. This results in runway guard lights being undesirably subdued to a low intensity during periods of good weather conditions during darkness, causing their conspicuousness to be partly lost. The AIBN recommended that the airport operator install a separate light intensity control for runway guard lights.
The AIBN also noted that the Runway Awareness & Advisory System (RAAS) provides an aural alarm “On Taxiway!” if the aircraft’s speed exceeds 40 kts when the aircraft is not on the runway. It was a quiet endorsement of the value of active intervention.