Industry and Regulators Confront the Science of Engine Icing
Researchers are gradually coming to understand the physics of in-flight engine icing due to ice crystals. In response to this enhanced knowledge of the subject, civil aviation authorities, such as the European Aviation Safety Agency (EASA) and the U.S. Federal Aviation Administration (FAA), are considering more stringent certification requirements. Meanwhile, manufacturers are factoring in icing protection in their designs but are wary of what they would view as excessively strict rules.
Ice crystals (50 to 200 microns in size) can cause icing in the low-pressure compressor, intermediate casing and even the first stages of the high-pressure compressor. It is not the only icing phenomenon that can affect an engine but is the one that has the deepest impact on the turbomachinery of an engine. It can cause various damages and malfunctions, up to flameout. More than 100 ice crystal and mixed-phase engine events were recorded between 1988 and 2003.
During a conference the EASA organized last October, Rolls-Royce (Booth N23) and Snecma (Booth G16) shared data on in-service icing events in the past decade. It appeared ice crystal encounters happen usually at high altitude and very low ambient temperature. Ice crystals can be found near a convective storm, particularly a mesoscale convective system, which is a storm complex measuring up to hundreds of miles horizontally.
According to records kept by engine makers Rolls-Royce and Snecma, events seem to concentrate in the Pacific Rim region. Morgan Balland and Geoff Jones, icing specialists at Snecma and Rolls-Royce, respectively, suggested the issue could become more serious in the near future. “Traffic is increasing in Asia and tropical climate is a good place for ice crystal formation,” they warned in a joint presentation.
The process in which ice crystals can create in-engine icing in flight are now understood. Ice crystals bounce off freezing surfaces near the front of the engine and enter the core, explained Richard Lewis, Airbus certification manager for icing, speaking on behalf of the international coordinating council of aerospace industries associations (ICCAIA).
Then, in the compressor, ice crystals melt on vane surfaces, creating a film of water. Particles keep impinging on the wet vane. This cools down the vane, which becomes cold enough for ice to form, and this is the start of an ice accretion process. Eventually, the ice breaks off and can damage airfoils and quench combustion. But the phenomenon is not completely understood yet, warned EASA initial airworthiness rulemaking officer Xavier Vergez.
For better physics understanding, an international research project is to gather EASA and FAA teams in Darwin, Australia, for two months early next year. They will study high ice-water content clouds in oceanic and continental convection. A specially equipped Dassault Falcon 20 aircraft will sample three different altitudes, corresponding to three different temperatures: -10-deg C, -30-deg C and -50-deg C, said Fabien Dezitter, coordinator of Airbus’ icing research and technology activities.
“This will help us validate or update our new appendix P of CS-25 aircraft certification,” Vergez said. The appendix P requirements address ice crystals and the mixed phase, where supercooled liquid droplets and ice particles coexist in a cloud. It requires the aircraft to be able to safely exit or fly in such conditions without restriction. EASA’s appendix P is now at the notice of proposed amendment stage, mirrored by an FAA notice of proposed rulemaking process. Other proposed changes may toughen icing requirements under EASA’s CS-E engine certification requirements.
However, these proposed new rules are worrying some industry executives. For example, ICCAIA’s Richard Lewis argued that they should not be allowed to stifle innovation. There is also concern that rules may become too complex and counteract wider efforts to reduce fuel burn.
As Balland and Jones explained, engine design already takes into account ice crystals, and the geometry of a powerplant is a big factor. For instance, optimizing the gas path’s curvature can minimize ice accumulation. Crystals, along with droplets, can be directed away from critical accretion sites.
The engine’s design should also promote ice shedding. In turn, components should have impact resistance wherever ice will hit them so that the ice sheds from the engine as soon as possible. Ice and water should be directed away from the engine, for example through variable and bleed valve doors.
Certification is based on analysis of any given system or operational issue compared to known in-service events, as well as research into the physics of the phenomenon in question. But, according to manufacturers, there is no representative ice crystal environment that can be used for testing and so computer models need to be devised.
Work to resolve this need is already under way. For instance, at the international level, the high-altitude ice crystals project of the Engine Harmonization Working Group is creating a numerical model for ice particle trajectory, impingement and accretion. Other collaborative research programs are contributing to develop the much-needed means of compliance.