A massive backlog of flights due to adverse weather at the airport here is not news. That it happened due to a summer storm in the month of May, and left airlines unprepared to handle a rush of delayed flights, is news.
A little over 70 flights were diverted on Sunday. According to the metereological department, this was the highest number of diversions on a single day in a decade, including fog-related ones during winters.
What caused it? Meteorologists say a rare phenomenon -- formation of cumulonimbus cloud, coupled with changing patterns of strong winds. Cumulonimbus cloud is a dense and towering vertical formation from water vapor carried by powerful upward air currents. "Such a formation of clouds is the trickiest thing to navigate for pilots. This can stretch to a vertical height of almost 69,000 ft, with a dome-like top often resembling that of mountains," said a meteorologist at the airport.
To get an idea of how high 69,000 ft is -- most aircraft have a highest altitude level of 36,000 ft during flight. So, pilots who had neared Delhi for landing saw the cloud formation from far away, immediately cancelled their descent and climbed up.
"A number of flights cancelled their landings and immediately the entire Delhi airspace was crowded," the official explains.
A pilot who was in the air explained it is not advisable to climb above such cloud formation, as severe icing can be expected, especially at the higher levels. "If any part of a cumulus cloud is at your altitude or higher, no surprise -- stay away," the pilot says.
Add a phenomenon called wind shear - strong wind currents, with changing direction and speed over a short distance. Delhi airport has two runways - number 27, towards the Dwarka (southwest) side and 29/11, towards the south Delhi side. "Landing was taking place from the Dwarka side when the wind direction changed suddenly. So, Air Traffic Control had to suddenly ask all airlines to land from the other side, throwing navigation into a tizzy," a pilot explains.
"Five to six aircraft which attempted to land had to abort their landing or go around because of the wind shear at 2,000-4,000 ft. Pilots also experienced cross-winds while landing. This peak weather activity, between 4.30 pm and around 8 pm, impacted both take-offs and landings severely. We have never reported such bad weather in any summer storm in the past eight to 10 years," an official said.
Such a long phase of inclement weather led to pilots' duty hours lapsing. For instance, IndiGo flight 6E-2977 was supposed to take off at 10:40 pm; it was delayed by seven hours. Angry passengers flooded Twitter with posts, accusing the airline of shoddy service. Some photographs showed sleepless, harrowed passengers seated or reclining on the tarmac in the late hours of the night.