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Today, knowledge of the nature of convective weather and the exact mechanism of ice crystal buildup and shedding in the engine is limited. A research program is being developed by an industry icing group to address these needs. It involves flights into convective clouds to measure their prop­erties, as well as ground-based engine testing.

Most of what is currently understood about the environment associated with engine events is based on pilot reports and flight data. Additional pilot reports of high-altitude ice crystal encounters (with or without engine events) will help researchers understand the conditions associated with engine events, ensure that the flight program is directed into the appropriate flight conditions, and help develop cues for these flight conditions.

Pilots encountering conditions such as those described in this article are encouraged to provide as many details about the conditions as possible to their airlines for subsequent use by researchers.


Ice crystal icing conditions have been recognized as a hazard to turbofan engines. Ice can build up deep in the engine core.

Pilots are advised to familiarize themselves with the conditions under which ice crystal icing typically occurs and follow the recommendations in related technical bulletins.

Airline awareness of the potential for ice crystal icing on all engine models/airplane types may provide additional information that will help Boeing and the industry better understand this phenomenon.

For more information, please contact Jeanne Mason at jeanne.g.mason@boeing.com.

Material for this article has been drawn from AIAA 2006-0206 "Ice Particle Threat to Engines in Flight," Mason, Strapp and Chow.


In this infrared satellite image from about the time of an engine event, bright white indicates colder cloud, and therefore at high altitude. The airplane penetrated the upper altitudes of a fully developed typhoon, yet the pilot did not see any flight level radar returns.

The asterisks represent the aircraft path from left to right on descent into Taipei, with the event noted in purple.

  • A commercial airplane on descent, flying in convection conditions, experienced a TAT anomaly. (The anomaly is due to ice crystals building up in the area in which the sensing element resides, where they are partly melted by the heater, causing a 0 degrees C reading. In some cases, TAT has stabilized at 0 degrees C during a descent, and may be noticeable to pilots. In other cases, the error is more subtle, and not a reliable-enough indicator to provide early warning to pilots of high concentrations of ice crystals.)
  • At 38,000 feet (-42 degrees C), the pilot encountered moderate turbulence and noted some lightning in the vicinity.
  • A brief power-loss event occurred at 30,000 feet — the engines restarted quickly.
  • There were no radar echoes at the altitude and location of the airplane.
  • An absence of a response from the ice detector indicated that no supercooled liquid was present.
  • The pilot reported heavy rain at -25 degrees C.
  • Initial report of rain on the windscreen was later determined to be ice crystals, and confirmed by the pilot to have a unique sound.

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