Flaming Out on Ice

And just when the aircraft icing community thought it had seen every­thing—clear ice, rime ice, glazed ice, SLDs, tail plane icing, and freezing rain encountered within the coldest atmospheric conditions possible— a new icing concern was recently discovered in the least likely of places: the interior of jet engines, where parts are often several hundred degrees above freezing. Almost nothing is known about the mechanism behind engine core ice accretion, except that the problem does cause loss of power, even complete flameouts. According to data compiled by Boeing and cited in a number of news media stories and Government reports, there have been more than 100 dramatic power drops or midair engine stoppages since the mid 1990s, including 14 instances since 2002 of dual-engine flameouts in which engine core ice accretion turned a twin – engine jetliner into a glider. "It’s not happening in one particular type of engine and it’s not happening on one particular type of airframe,” said Tom Ratvasky, an icing flight research engineer at GRC. "The problem can be found on aircraft as big as large commercial airliners, all the way down to business-sized jet aircraft.”[1255]

The problem came to light in 2004, when the first documented dual­engine flameout occurred with a U. S. business jet due to core ice accre­tion. The incident was noted by the NTSB, and during the next 2 years Jim Hookey, an NTSB propulsion expert, watched as two more Beechjets lost engine power despite no evidence of mechanical problems or pilot error. One of those incidents took place over Florida in 2005, when both engines failed within 10 seconds of each other at 38,000 feet. Despite three failed attempts to restart the engines the pilots were able to safely glide in to a Jacksonville airport, dodging thunderstorms and threat­ening clouds all the way down. Hookey took the unusual step of inter­viewing the pilots and became convinced the cause of the power failures was due to an environmental condition. It was shortly after that realiza­tion that both the NTSB and the FAA began pursuing icing as a cause.[1256]

Hookey employed some commonsense investigative techniques to find commonality among the incidents he was aware of and others that were suspect. He contacted the engine manufacturers to request they take another look at the detailed technical reports of engines that had failed
and then also look at the archived weather data to see if any patterns emerged. By May 2006, the FAA began to argue that the engine prob­lems were being caused by ice crystals being ingested into the engine. The NTSB concurred and suggested how ice crystals can build up inside engines even if the interior temperatures are way above freezing. The theory is that ice particles from nearby storms melt in the hot engine air, and as more ice is ingested, some of the crystals stick to the wet surfaces, cooling them down. Eventually enough ice accretes to cause a problem, usually without warning. In August 2006, the NTSB sent a letter to the FAA detailing the problem as it was then understood and advising the FAA to take action.[1257]

Part of the action the FAA is taking to continue to learn more about the phenomenon, its cause, and potential mitigation strategies is to part­ner with NASA and others in conducting an in-flight research program. "If we can find ways of detecting this condition and keeping aircraft out of it, that’s something we’re interested in doing,” said Ratvasky, who will help lead the NASA portion of the research program. Considering the number and type of sensors required, the weight and volume of the asso­ciated research equipment, the potentially higher loads that may stress the aircraft as it flies in and around fairly large warm-weather thunder­storms, the required range, and the number of people who would like to be on site for the research, NASA won’t be able to use its workhorse Twin Otter icing research aircraft. A twin-turbofan Lockheed S-3B Viking aircraft provided to NASA by the U. S. Navy originally was proposed for this icing research program, but the program requirements outgrew the jet’s capabilities. As of early 2010, the Agency still was considering its options for a host aircraft, although it was possible that the NASA DC-8 airborne science laboratory based at the Dryden Flight Research Center (DFRC) might be pressed into service. In any case, it’s going to take some time to put together the plan, prepare the aircraft, and test the equipment. It may be 2012 before the flight research begins. "It’s a fairly significant process to make sure we are going to be doing this program in a safe way, while at the same time we meet all the research requirements. What we’re doing right now is getting the instrumenta­tion integrated onto the aircraft and then doing the appropriate testing to qualify the instrumentation before we go fly all the way across the

world and make the measurements we want to make,” Ratvasky said. In addition to NASA, organizations providing support for this research include the FAA, NCAR, Boeing, Environment Canada, the Australian Bureau of Meteorology, and the National Research Council of Canada.[1258]

In the meantime, ground-based research has been underway and safety advisories involving jet engines built by General Electric and Rolls – Royce has resulted in those companies making changes in their design and operations to prevent the chance of any interior ice buildup that could lead to engine failure. Efforts to unlock the science behind inter­nal engine icing also is taking place at Drexel University in Pennsylvania, where researchers are building computer models for use in better under­standing the mechanics of how ice crystals can accrete within turbofan engines at high altitude.[1259]

While few technical papers have been published on this subject— none yet appear in NASA’s archive of technical reports—expect the topic of engine ingestion of ice crystals and its detrimental effect on safe operations to get a lot of attention during the next decade as more is learned, rules are rewritten, and potential design changes in jet engines are ordered, built, and deployed into the air fleet.