The buildup of ice on the wings and control surfaces of an aircraft flying through freezing rain has long been a major safety concern because it can lead to a crash. Now the phenomenon is getting a new look with a Chinese study published on Tuesday that helps explain why icing occurs.

The well-known culprit in ice formation-supercooled large water drops, or SLDs, with a diameter greater than 1,000 microns－can cause trouble with aircraft control or result in ice-clogged engines.

Safety officials are so concerned about SLDs that the US Federal Aviation Administration enacted new rules in November 2014 requiring that aircraft be able to handle such conditions safely.

Yet, despite the known dangers, significant knowledge gaps have remained.

Now, a research team from Shanghai Jiao Tong University has published the results of an investigation that pointed to a different icing mechanism at work than had been identified previously.

Led by Professor Liu Hong, the team worked in a closed-circuit wind tunnel with an SLD generator and a high-speed visualization system to study abnormal ice buildup on aircraft.

"The thermodynamic effect during the supercooled large droplet impact process has not received sufficient attention," Liu said. "We set out to fill certain knowledge gaps."

The paper was published in the journal Physics of Fluids in the US.

"The most critical significance of our model is that it reflects the heat transfer quantity generated from the thermodynamics of impact," Liu said. "Nowadays, understanding the mechanism of SLD icing has become a significant goal for researchers concerned with air travel safety. So that is our goal in building and testing the most robust model to date."

Test conditions for Liu's Shanghai Icing Wind Tunnel experiments reproduced the meteorological conditions that might be encountered in flight, as well as realistic variable droplet velocities and temperatures.

By reproducing the impingement phenomenon in the lab, researchers observed rapid freezing characteristics in droplets that had diameters of 400, 800 and 1,300 microns.

The results may be used to more closely characterize meteorological conditions to help pilots manage flights safely when they encounter freezing rain and SLD-forming conditions, they said.

"Our results indicate that the droplet size is a critical factor influencing the supercooled heat exchange and effective heat transfer duration," Liu said.

By Xinhua in Washington