New Delhi, July 2: A Calcutta-educated, Indian-American scientist has solved a quarter-century-old riddle of how the world’s largest known flying bird that lived 6 million years ago could take off, fly, and land.

While paleontologists have known since the early-1980s that the fossils of Argentavis magnificens — discovered earlier in Argentina — belonged to a giant flying bird as large as a Cessna-152 light aircraft, its flight capabilities remained a mystery.

Now, Sankar Chatterjee, curator of paleontology at the Museum of Texas Tech University in the US, has determined just how a bird that weighed around 70 kg and stood 3.5 metres from beak to tail with a wing span of 7 metres could actually fly.

Using software originally developed for the study of helicopter aerodynamics, Chatterjee and his co-workers have shown that Argentavis was incapable of flight powered entirely by wing flapping, but could glide efficiently with the help of air currents. Their findings appear today in the US journal Proceedings of the National Academy of Sciences.

“The flight of Argentavis has been a subject of much speculation,” Chatterjee told The Telegraph over telephone. “It had anatomical features that suggested that it could fly. But it was so large and so heavy. Argentavis probably represents an upper limit for ground take-off,” he said.

The researchers have found that Argentavis — like vultures or the South American condors of today — was a glider that “extracted energy from the atmosphere from flight” — using rising air currents in the rocky Andes mountains or on the grassy pampas to provide lift for itself.

The calculations suggest that Argentavis could not make a successful running take-off from level ground. But even a gentle slope with a light 5-metre per second headwind would have allowed it to lift itself from the ground employing the same technique used by human hang glider pilots.

Chatterjee, who had completed his doctorate in geology at Calcutta University and taught at the Indian Statistical Institute during the early-1970s before leaving for the US, is a leading paleontologist whose work in recent years has focused on the biomechanics of flight in prehistoric birds.

Combining information from fossilised bones of Argentavis and the flight simulation software, the researchers have calculated that as long as the upward speed of rising air was about one metre per second, Argentavis could exploit slope soaring for several hours. Their simulations also show how Argentavis used wings and tail as breaks to initiate landings.

But given its dependence on rising air currents, the bird could not fly at will any time of the day or night. Strong air currents occur by mid-day and disappear in the evening, and “soaring for Argentavis would have been possible only between those times,” the researchers said.

“It’s almost like a take from the Arabian Nights — it was a very aggressive bird that flew over the pampas of Argentina to sweep down from the sky and seize large prey with a formidable beak,” Chatterjee said.