An aviation hazard, known as “Sea of Blue Effect” combined with the fatigue of the cockpit crew could be the reason of the taxiway excursion by an Air India Express flight during the wee hours of 5th September at Kochi international airport.
An analysis of the incident done with the help of aviation experts having many years of experience with Airports Authority of India and Air India excludes all possibilities, but this phenomena, which though specifically mentioned in the Aerodrome Design Manual (Part IV) released by International Civil Aviation Organization (ICAO), is rarely documented and reported in India so far.
On September 5, the Air India Express Abu Dhabi-Kochi flight 452 landed at Kochi International Airport at 2.20AM on runway 27. After landing, the aircraft exited the runway through link C2 and then turned right to go to the apron of the new terminal T3. To reach the designated apron, the aircraft was instructed to exit the taxiway using the link-taxiway F in the left, after B and E. Everything went well till the aircraft reached near the exit E. As per the reports of the incident, the aircraft took the left turn 90m before the actual exit F. So, instead of entering the apron, the aircraft crossed into the space between link taxiways E and F and its rear wheels got caught in the 1m wide open drain that runs parallel to the taxiway. While the wheels descended into the drain, the bottom side of the two engines of the Boeing737-800 aircraft hit the paved surface beneath. The nose-wheel assembly too damaged as the most of the weight of the aircraft got suddenly transferred to it.
The Sea of Blue
To see how an experienced pilot could cross into an open space mistaking it for the link-taxiway, we may visualize what the pilots of the aircraft were seeing from the cockpit of the aircraft during its movement towards exit F.
When the aircraft moved forward at a speed of 18knots, the pilots must’ve been seeing clearly the taxiway stretching ahead. The thick yellow line marking the centre-line of the taxiway too must’ve been visible, thanks to the edge lighting. A little farther ahead on the left, there were four links-taxiways perpendicular to the taxiway that connect the taxiway with aprons. These links -denoted by Roman alphabets B, E, F and G- too had blue edge lights.
When the aircraft initiating the 90 degree turn to enter the link-taxiway, the glow of the blue lights fitted along the edges of the four parallel exit paths can together appear as a huge, rectangular illuminated surface. This optical illusion is the Sea of Blue Effect. It occurs because blue light that travels as shorter, smaller waves gets scattered more than other colors. The light thus scattered from the edges of the five link-taxiways spaced just 125m apart can easily overlap, hiding the open land between them. So the chances are abundant for the pilot either to totally miss the actual exit that got submerged in the ‘blue sea’ or to confuse between the exit (link-taxiway) and the area between the link-taxiways.
And it seems Flight IX452 did commit the second mistake- it turned 90 degrees to the left through the open area between E & F links-taxiways, instead of entering the link F.
But why only this pilot?
Many cockpit crew before him too would have got confused, no doubt. Only that they all could overcome the illusion just in time. And the reason why this pilot succumbed to the playing-of-tricks by blue lights could be the very timing of the flight. IX452 that landed at 2.20 am at Kochi airport was the same aircraft that flown from Kochi to Abu Dubai as IX419 the previous evening. IX419 that took off from Kochi at 5.20pm had landed at Abu Dhabi by 7.50 pm. Within one hour, the aircraft departed to Kochi as IX452, operated by the same crew. That means the pilot who mistook the open land as a link taxiway was continuously flying the aircraft from 5.20pm to 2.20 am, but for a one hour gap from 7.50 to 8.50pm.
And it may also be noticed that the taxiway excursion was happened 20 minutes after the beginning of the Window of Circadian Low. (WOCL, the interval of time from 2.00am to 6.00am, is a period during which people working through night can experience maximum fatigue). The end of the flight coinciding with the WOCL is more dangerous. Seeing that the duty time is almost ended, a fatigued person’s all urges will be to do away with it as soon as possible. It was only natural for the pilot to become more impatient seeing he is almost there- a turn and then the apron and the end of the journey. And this fatigue-triggered impatience could have made him an easy prey of the Sea of Blue illusion.
The other possibilities
- Reduced visibility
It is understood that the visibility at the airport during the time was more than 800m. For an aircraft moving at 20kmph, this visibility is more than enough to see the turns & obstructions ahead.
- Objects on the runway
Obstacles happen to be on the runway can cause the nose wheel lose control, while going over it. But nothing of this sort was reported.
- Faulty nose wheel moving erratically, resulting in unintentional right turn.
This indeed is a chance, especially for this particular aircraft. On March 11th 2014, while operating as IX-193, this VT-AYB aircraft had its nose-wheel damaged during taking off from Lucknow for Dubai. Seeing parts of nose wheel gear on the runway, the ATC called the aircraft back. But the damage was repaired soon after, and the aircraft was flying all these three years after the incident.
- Hydroplaning & skidding
when there is lots of rain water on runway layers of water can accumulate between the wheels of the aircraft and the runway, leading to a loss of traction and loss of control. But the rain was not that strong Tuesday night, for this to happen. Moreover, the plane’s speed was only around 20kmph.