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Ice Crystal Accretion in an Engine
• Frozen ice crystals bounce off cold surfaces; thus airframe icing is
not noticed during aircraft operation in high-altitude ice crystals
• The physics of ice crystal accretion in the engine is not completely
understood, but the accretion mechanism is thought to be:
– When ice particles enter the engine and bombard a warm surface,
a mixture of liquid and ice particles can exist on the surface
– The liquid slows down the incoming ice particles long enough for
heat transfer to take place
– Heat is removed from the metal until the freezing point is reached,
and ice begins to form
– This phenomenon means ice accretion can occur well behind the
fan in the engine core
– Ice shed from compressor surfaces can cause engine instability
such as surge, flameout, or engine damage
Copyright © 2006 the Boeing Company Mason_Forum.14
The Ice Crystal Weather Threat to Engines
TAT Anomaly
• Total air temperature (TAT) anomaly has occurred in many
cases near the time of the engine power loss events
• The airplane total air temperature probe (TAT) erroneously
reporting zero degrees C is known to be evidence of ice
crystals in the atmosphere
• This anomaly is due to ice crystals building up in the area
where the thermocouple resides, where they are partly melted
by the heater, causing the zero degrees C reading
• In some cases, TAT has “flat-lined” at zero 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
• Although TAT is an engine control-system parameter, the TAT
anomaly has not been determined not to be a contributor to
the power loss events. Under these conditions, the engine
control compensates for loss of TAT
Copyright © 2006 the Boeing Company Mason_Forum.15
The Ice Crystal Weather Threat to Engines
Industry Challenges – Designing a Robust Engine
• Zones of high ice particle concentrations are not easily
identifiable by pilots
• High concentrations of ice particles are being penetrated,
even those storms being tracked by meteorological agencies
• The most effective solution is to make the engine capable of
flight in these conditions
• Experimental data characterizing the ice crystal environment
is limited – flight measurements are needed
• Test methods for creating representative ice particles
do not exist
• Facilities for testing engines in these conditions do not exist
• The ice particle accretion and shedding mechanism is not
fully understood
Copyright © 2006 the Boeing Company Mason_Forum.16
The Ice Crystal Weather Threat to Engines
Industry Committee Activities
• Development of Regulatory Requirements:
– Created Appendix D to Engine Certification FAR Part 33 –
ice crystal envelope – similar to Appendix C, built with
theory and experimental data
– Wrote draft rules for engine compliance to ice crystals
• Technology Plan developed to address unknowns:
– Improved instrumentation to measure atmosphere
– Flight trials to characterize atmosphere
(understand particle size, concentration and extent)
– Fundamental physics of ice accretion and shedding
– Test methods and facilities
• Government/Industry partnerships needed to fund
this work
Copyright © 2006 the Boeing Company Mason_Forum.17
The Ice Crystal Weather Threat to Engines
Communication
• Most of what we currently understand about the environment
associated with engine events is based on pilot reports and flight
data
• More pilot reports of high-altitude ice particle encounters
(with or without engine events) would be valuable to:
– Make sure our hypotheses about of the conditions associated
with engine events continues to be supported by flight data
– Make sure the flight program is directed into the appropriate
flight conditions
– Help develop cues for these flight conditions
• Boeing is considering publishing a tech bulletin summarizing ice
crystal weather conditions that are associated with engine power
loss – would appreciate airline feedback
Copyright © 2006 the Boeing Company Mason_Forum.18
The Ice Crystal Weather Threat to Engines
Summary / Conclusions
• Ice crystal icing conditions have been recognized
as a hazard to turbofan engines
• Ice can accrete deep in the engine core where
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