Jet engines are well understood, predictable and more reliable than ever, but even after seven decades of development there can still be surprises. As if to warn against complacency, a mysterious phenomenon that triggers potentially hazardous engine thrust-loss events has propulsion researchers scratching their heads.

The spate of incidents, ranging from complete multiengine flameouts and surges to unexpected power “roll-backs,” is forcing U.S. and European airworthiness agencies into an urgent rethink on icing certification. With more than 150 such events recorded so far, the phenomenon continues to pose a risk. Given the inexorable growth of longer-range twin-engine operations over remote regions, and no ready explanation yet found, the phenomenon has attracted international concern.

The focus is on a previously unsuspected form of engine icing that affects the hot core, and appears to be caused by the accretion of ice crystals in areas deep within the engine once thought immune to such a condition. NASA, which is leading U.S. research into the issue through its Aviation Safety Program, says “it is a problem whose frequency is alarmingly high.”

The issue has been observed for decades. “Ice Crystals, The New Engine Hazard,” was a report title from Canada's National Research Council (NRC) filed in 1958. But by 1979, interest waned because there did not appear to be a problem, says James D. MacLeod, group leader of Icing and Environmental Certification for the NRC's Gas Turbine Laboratory in Ottawa.

As old as the ice accretion issue is, it is getting some up-to-date testing by the three leading engine makers at new facilities in Canada's Manitoba province. General Electric has opened the Engine Testing, Research and Development Center in Winnipeg, where average winter temperatures are a chilly -8F. Farther north, Pratt & Whitney and Rolls-Royce are using the Global Aerospace Center for Icing and Environmental Research (Glacier) in Thompson, which has been designed to accommodate any size engine from any of the industry's manufacturers. The city averages 260 days a year of below-zero temperatures.

The core icing phenomenon appears to have been on the rise since the 1990s and hit everything from Beechjet 400As to Airbus A330s and Boeing 747s. Although some researchers say there is no consensus yet on whether higher bypass ratio engines are more susceptible, engine makers are not so sure. One senior engineer with a major manufacturer says the push for better performance “increases stall margin and flameout potential. So the newer high bypass ratio engines are more susceptible to all environmental conditions—rain, icing or hail. That's why this is coming up, and why we are seeing problems.”

An Ice Protection Harmonization Working Group at the NTSB has reviewed data for more than 30 years to see how many incidents fall outside normal certification requirements, says MacLeod. The findings indicated 60% of 200 events fell outside normal standards and were characterized by a “loss of thrust, a rollback, loss of thrust, surge, flameout and engine relight. While pilots are becoming aware of the threat, ice crystals do not show up on their radar even though rain does,” MacLeod says. Depending on winds, the ice crystals can trail storm clouds like an invisible tail that pilots do not see on their radar.

At its small altitude chamber in Ottawa, the NRC has found it difficult to create ice at the size suspected of being most dangerous. The laboratory experiments have ground ice into particles of 100-300 or 400 microns, but their equipment is not able to create crystals below 100 microns. Spray nozzles, like those used on ski slopes, are able to deliver crystals as small as 40 microns. “The question is, how big are the crystals?” says MacLeod.

Although some engine events have been seen in mid-latitudes, the bulk of the incidents occur in Southeast Asia. As a result, a team of atmospheric scientists is heading first to Florida, and then to Darwin, Australia, where they hope to find data to unlock the mystery. The research flights will coincide with the introduction of new certification requirements for engine and airframe icing that are due to come into force by the end of 2012. Data from the tests will be used to help devise mitigation strategies and provide manufacturers with a means of showing compliance with the new rules.

Traveling to the tropics for an icing research study seems as counterintuitive as the theory of ice formation in the high-pressure core of a gas turbine; yet this is where the phenomenon most frequently crops up and where suitable atmopsheric conditions occur for testing on a predictable, repeatable basis. “It's a great tropical atmospheric laboratory,” says aerospace engineer Tom Ratvasky of the Icing Branch at NASA Glenn Research Center, Ohio.

After preliminary instrumentation tests in Florida later this year, a modified NASA Gulfstream II will be based in Darwin in early 2013 for use by an international flight-test group. Located at the tip of Australia's Northern Territory, Darwin's temperatures usually hover around 100F, while humidity averages more than 70% during the wettest months. Together with high sea-surface temperatures, these conditions promote mega-storms and towering convective cells that pump massive amounts of moisture-laden air high into the atmosphere.

Unlike traditional engine icing, in which supercooled liquid droplets freeze on impact with exposed parts of the engine as the aircraft flies through clouds, engine-core ice accretion involves a complex process in which particles stick to a warm metal surface and act as a heat sink until the metal surface temperature drops below freezing, thereby forming a location for ice and water (mixed phase) accretion. The accumulated ice can either flow as a block into the core or shed into the downstream compressor stages and combustor, causing a malfunction.

Until recently it was assumed that ice particles would bounce off structures and pass harmlessly through bypass ducts, or melt inside the engine. “Now there is evidence [of] an environment that is conducive to ice accreting where we never thought it would,” says Mike Oliver, NASA Glenn icing branch engineer. “Usually ice breaks off because it is cold, and water will stick. There is a notion now that somewhere is a sweet spot, which we are not testing or certifying for, where there is a combination of water, ice and airlfow that is susceptible to accreting ice.”

Since many of the recorded events were found to have occurred near convective clouds, the early investigators assumed supercooled liquid water, hail or rain is responsible because they are lifted to high altitudes by updrafts. Most events were recorded above 22,000 ft., which is considered the upper limit for clouds containing supercooled liquid water. However, pilots reported that even though they were in clouds at the time, there was no evidence of the usual indications of trouble, including a significant icing on the airframe, or any other remarkable aspect to the weather. According to researchers from Boeing, Honeywell and Environment Canada, some pilots were surprised that investigators were even looking at icing.

“We need a good understanding of the concentration and practical size of [supercooled droplets] in clouds above 22,000 ft.,” says MacLeod. Better instrumentation to improve on previous studies also is important.

By studying case histories in which events took place on aircraft diverting around convective storms, researchers have surmised that the lift associated with these weather events releases extraordinary amounts of condensed water that become ice crystals. They can reach levels approaching 9 grams per cubic meter. “That compares to levels of around 1 gram per cubic meter for air with supercooled droplets normally,” says Ratvasky. “With such a massive amount of ice, that's a tremendous amount of cooling capacity versus liquid water.”

The NASA GII will bristle with wing-mounted sensor probes and nose- and upper-fuselage-mounted radars as its crews try to find out what is really going on inside these monster convective clouds. Twelve instruments will be hung on hard points beneath the wings to measure particle spectrum, total water content, temperature and water vapor. Two radars will collect data to characterize the glaciated and mixed-phase icing environment, while comparing measured reflectivity against observed conditions.

The nose-mounted unit is a Honeywell RDR-4000 X-band weather radar modified to provide raw data, taking measurements below the unit's standard 20-db detection threshold. “If we can develop algorithms to detect these conditions [remotely], we can give it to the manufacturers,” says Ratvasky.

While the X-band work may provide a shorter-term, tactical solution to help pilots steer clear of hazardous core icing conditions, the fuselage-mounted Ka-band radar will collect data that may contribute to more strategic threat-surveillance methods. This three-antenna radar was developed by Environment Canada and previously flown on its Convair 580 research aircraft.

Working with the National Center for Atmospheric Research in Colorado and other weather specialists, NASA is developing versions of existing icing-potential algorithms for the Ka-band radar to “see where supercooled liquid icing exists. The intent is to take data from atmospheric models, weather satellites and radar, and develop products for predicting or now-casting [a form of very short-range weather forecasting] where the threat zones might be,” Ratvasky says.

Airbus proposes a multinational, government-industry project in Europe, called High-Altitude Ice Crystals (HAIC), to dovetail with the U.S.-Canada research. It is expected to start in mid-2012, with studies of the flight envelope of a Falcon 20 that will carry a Latmos 95-ghz multi-beam Doppler radar and other sensors. Those flights also will cover a research envelope applicable to air data probes, a focus of air safety work in the aftermath of the 2009 crash of Air France Flight 447. The HAIC project is expected to be part of the GII flights out of Darwin from January to March 2013.

The Europeans want to develop new detection and awareness systems from 2013-15, leading to flight tests on an A340 in 2016. They expect this will take the technology to the point where it will be ready for full-scale development.