Engine core icing phenomenon and a new thecnical solution

Thunderstorm activity (strong convective weather) can lift high concentrations of moisture to high altitudes where it can freeze into very small ice crystals, perhaps as small as 40 microns (the size of flour grains). These crystals that can affect an engine, several engine power-loss and damage events have occurred in convective weather above the altitudes typically associated with icing conditions. The crystals can partially melt and stick to relatively warm engine surfaces. Weather radar can detect large particles such as hail, rain, and large ice crystal masses (snowflakes). Small particles, such as ice crystals in high concentrations near thunderstorms, are invisible to weather radar, even though they may comprise the majority of the total mass of a cloud.

Ice building up on the inlet, fan, or spinner would likely shed outward into the fan bypass duct without causing a power loss. Therefore, in these power-loss events, it is reasonable to conclude that ice must have been building up in the engine core.

The ice crystal icing can occur deep in the engine where surfaces are warmer than freezing. Both older generation jet engines and the new generation of jet engines (high bypass ratio engines with electronic engine controls) can be affected by ice crystal icing.

Engine

Engine

The actual mechanism for ice crystal-related engine power loss takes many forms, depending on the design characteristics of each particular engine type (see table below).

 

table

About 60 percent of recorded ice crystal power-loss events have occurred in Asia. As per the researchers this may be due to the fact that the highest sea surface temperatures are also found in this region. Higher temperature air can hold more water.

Engine power-loss events have occurred in three phases of flight: climb, cruise, and descent. However, most events occur during the descent phase, most likely because of a combination of two factors. First, for icing to occur, the ambient temperature must be below the freezing level, and therefore icing tends to occur at the higher altitude associated with the descent phase. Second, the engine is least tolerant to ice shedding at idle power, which occurs in the descent phase. Second, the engine is least tolerant to ice shedding at idle power, which occurs in the descent phase. Icing at high power and high altitude is possible due to the existence of high concentrations of ice crystals for long distances, such as in the anvil of a large convective storm, and the fact that ice can build up on warm engine surfaces.

 

Researchers have identified several conditions that are connected to engine ice crystal icing events:

  • – High altitudes and cold temperatures. Commercial airplane power-loss events associated with ice crystals have occurred at altitudes of 3 to 12 km, with a median of 8 km, and at ambient temperatures of -5 to -55 degrees C with a median of -27 degrees C. The engine power-loss events generally occur on days when the ambient temperature is warmer than the standard atmosphere. 
  • – The presence of convective clouds. Convective weather of all sizes, from isolated cumulo­nimbus or thunderstorms to squall lines and tropical storms, can contain ice crystals. Convective clouds can contain deep updraft cores that can lift high concentrations of water thousands of feet into the atmosphere, during which water vapor is continually condensed and frozen as the temperature drops. In doing so, these updraft cores may produce localized regions of high ice water content which spread downwind. Researchers believe these clouds can contain up to 8 grams per cubic meter of ice water content; by contrast, the design standard for supercooled liquid water for engines is 2 grams per cubic meter. 
  • – Areas of visible moisture above the altitudes typically associated with icing conditions. This is indicated by an absence of significant airframe icing and the ice detector (when installed) not detecting ice, due to its ability to detect only supercooled liquid, not ice crystals.

 

RECOMMENDATIONS FOR FLIGHT NEAR CONVECTION:

  • – Avoiding flying in visible moisture over storm cells. Visible moisture at high altitude must be considered a threat since intense storm cells may produce high concentrations of ice crystals at cruise altitude.
  • – Flying upwind of storms when possible.
  • – Using the radar antenna tilt function to scan the reflectivity of storms ahead. Assess the height of the storms. Recognize that heavy rain below the freezing level typically indicates high concentrations of ice crystals above.
  • – Avoiding storm reflectivity by 20 nautical miles has been commonly used as a recommended distance from convection. This may not be sufficient for avoidance of high concentrations of ice crystals, as they are not visible on airborne radar.

 

LED – Engine Core Ice Sensor Under Test
While researchers have made good progress in recent years to understand the phenomenon of engine core icing at high altitudes, the scientists are developing a way to detect this danger. As already mendioned,  the conditions that can lead to engine core icing cannot be detected by current weather radars. In addition, clues that sometimes alert crews to the presence of ice crystals, such as loss of power thrust and flight-deck noise frequency changes, only occur when it is too late to avoid areas of potential danger. Podium Aerospace, a division of Canadian-based Podium Energy Corp., has unveiled details of a reflective material sensor (RMS) system that detects ice buildup using light reflectivity. Podium’s system is based on a pair of light-emitting diodes (LED) that project light into the gas path through a small window built into the engine casing. The system, which incorporates an erosion guard and temperature sensor on the upstream side of the window, detects ice crystal growth when ice particles accumulate on the transparent surface and a photo diode co-located with the LEDs senses subsequent changes in reflected light levels.

 

LED

 

The RMS sensor is rigged to communicate directly with the engine’s full-authority digital control system as well as, potentially, the engine-indicating and crew-alerting system. The sensor triggers the activation of additional heating flows to guide vanes as well as the opening of variable-bleed valves to counter icing in real time. A prototype system has been built, and already tested. It has detected changes in reflectivity during simulated icing conditions. Now are planned more extensive testing,  looking at the altitude variable, and special wind-tunnel test work will be carried out. Performance of the unit in initial ground tests at the company’s Montreal facilities has been “stellar,” says Adler (Podium Energy founder and CEO), who adds that engine and airframe manufacturers are in talks to assist in the further development and application of the system.

 

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