Monday, March 05, 2012

Jet engine icing research at Cleveland's NASA Glenn will tackle dangerous flight problem

On a stormy July evening in 2004, more than five miles above the South China Sea, the engines powering a large passenger jet en route to Taiwan suddenly failed.

A simultaneous engine shutdown on a large, modern aircraft is almost unheard of. After a harrowing 75 seconds, the pilots managed to restart them, and the jet landed at the Taipei airport without further problems. But the incident set off alarms in the aviation community.

Minutes before its engines quit, the jet had been skirting thunderstorms spawned by a distant typhoon. Investigators first thought the storms' powerful updrafts had pulled rain high into the atmosphere, temporarily smothering the engines when they sucked in gouts of water instead of air. The jet's pilots had seen and heard droplets hitting the windshield, bolstering the rain theory.

But the jet's radar sweeps were clear, with no echoes from rain. And the temperature at the altitude where the trouble began was a frigid minus-44 degrees, far too cold for liquid water. Researchers eventually concluded the engines must have been choked by tiny ice crystals as small as flour grains – a dangerous, unexpected phenomenon that aviation officials urgently want to learn more about so they can lessen its risk.

Much of that work will take place at Cleveland's NASA Glenn Research Center, where engineers are readying a unique test chamber capable of mimicking the odd weather conditions that threatened the Taipei-bound jet, and have caused more than 150 other in-flight incidents. 

"These things are happening pretty frequently, like one incident every month or so," said Glenn project manager Ron Colantonio. "NASA is working with the aviation community to understand what's causing the problem and how to mitigate it." 

Glenn officials recently unveiled the silvery, boxcar-sized engine icing tunnel during a visit by NASA administrator Charles Bolden. Glenn engineers previously had used the tunnel for other types of jet engine tests. With $15 million in Recovery Act and NASA money, they've retrofitted it with water sprayers that will produce the minute ice crystals believed to be causing the engine problems. Sensors will track the performance of jet engines mounted on a frame in the icy air stream. 

Aviation safety experts have long recognized the danger from ice buildup on wings and other external aircraft surfaces. The hazard is caused by super-cooled liquid water freezing on contact, disrupting smooth airflow and hampering lift. For decades engineers in Glenn's Icing Branch have led international efforts to develop better ice forecasting methods, icing sensors, anti-icing aircraft designs, and improved pilot training. 

But the idea that ice could cause a problem deep inside a jet engine, and at altitudes much higher than where liquid water can exist, didn't seem to make sense. Above 22,000 feet, ice crystals in the atmosphere bounce harmlessly off cold aircraft surfaces and exterior icing ceases to be a problem.

Aviation officials struggled to understand why, beginning in the 1990s, commuter and large transport jet pilots were reporting engine flameouts or partial loss of power. There have been no crashes to date attributed to engine icing. Pilots usually are able to restart or regain full engine power after dropping to lower, warmer altitudes, although the pilot of a small business jet had to make an emergency "dead-stick" landing at the Jacksonville, Fla., airport in November 2005 when neither stalled engine would re-light.

The clues from these incidents were sparse:

The engine failures occurred at high altitudes and cold temperatures, averaging 26,800 feet and minus-17 degrees. They mostly happened as jets were descending, although the problems also cropped up during ascent and level cruising. The most common site was the Asian Pacific region, though incidents were reported throughout the world. Flight paths were near or above convective clouds, with lots of rain down below but nothing on weather radar at flight level.

Researchers know that storm updrafts can shoot lots of moisture to high altitudes, where the drops fast-freeze into ice crystals. The Asian Pacific is a cauldron for such fierce convective storms because of its warm ocean surface temperatures.

Large ice particles show up on aircraft radar and remain in the core of the storm cloud, which pilots already know to avoid. But tiny ice crystals – invisible except on sophisticated satellite scans – accumulate at the cloud's periphery. They're only evident when they strike and melt on a jet's heated windshield, masquerading as raindrops.

How those near-microscopic ice crystals manage to foul a jet engine is mostly speculative, at least until engine testing in Glenn's ice crystal tunnel begins early next year. Here's what scientists think happens:

A jet's forward motion and its engines' mighty air intake draw ice crystals deep inside. At some point, probably in the compressor section, the temperature is above freezing and some of the crystals melt. A thin film of water forms on engine surfaces, trapping more incoming ice crystals and melting them, too. 

As some of the water vaporizes, the engine metal loses heat in a process called evaporative cooling, like sweat cooling the skin. Eventually the metal's temperature drops to the freezing point and ice accumulates. When ice shards slough off, they can choke airflow into the compressor, causing the engine to surge or stall. They also may quench the engine's combustor, causing a flameout, or complete failure. Vibration and mechanical damage may occur if breakaway ice fragments slam into the engine's rapidly spinning blades. 

"Test facilities capable of simulating this [ice crystal] weather condition for turbine engines are not readily available to the industry," a trio of propulsion experts wrote in a 2006 study that outlined the ice particle threat. Previous test rig attempts have had trouble getting the right air temperature and the range of ice crystal sizes and shapes that duplicate what happens in flight. The Glenn chamber should fill that void. 

"This is a one-of-a-kind facility," Colantonio said. "There's no other facility in the world that can generate ice crystals at altitude. We can take [a test engine] from sea level to 40,000 feet. Engine icing can come about in minutes." 

To make certain its icing tunnel is accurately simulating the weather conditions jets experience at high altitude, the Glenn center has outfitted a Gulfstream jet with more than 20 meteorological sensors. Next January, the jet will begin data-gathering flights from Darwin, Australia, where seasonal monsoons should provide plenty of ice crystal-generating storms. 

Glenn engineers will use the results to fine-tune their simulations in the icing tunnel. The tests should help jet engine manufacturers find ways of preventing ice buildup, and help the Federal Aviation Administration develop safety certification standards for future engines. 

For now, aviation officials advise pilots to reduce the risk of engine icing by steering a path at least 20 nautical miles around a storm cell, and avoiding flying over them, where the icy updrafts occur.

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