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• One commuter aircraft type suffered engine rollback events
at altitudes between 28,000 – 31,000 feet
• Extensive investigation including flight-testing led to the
understanding that ice particles were accreting on warm
surfaces in the engine core
• Weather instrumentation conclusively established that the
condition that led to engine rollback was a high concentration
of ice crystals
• Industry icing committee in 2003 compared the commuter
aircraft events to the large transport events
• The industry now recognizes that high-altitude large transport
engine events are most likely due to ice-particle icing
Copyright © 2006 the Boeing Company Mason_Forum.7
The Ice Crystal Weather Threat to Engines
Similarities between Commuter and
Large Transport Aircraft Events
1. High-altitude, cold temperature
2. Aircraft in the vicinity of convective clouds/thunderstorms
3. Significantly warmer than standard atmosphere
4. Visible moisture
5. Light-to-moderate turbulence
6. Precipitation on heated windscreen, often reported as rain
7. Aircraft total air temperature (TAT) probe anomaly
8. Lack of observations of significant airframe icing
9. No pilot reports of weather radar returns at the event
location (only large transport aircraft pilots queried)
Copyright © 2006 the Boeing Company Mason_Forum.8
The Ice Crystal Weather Threat to Engines
Weather Connected to Engine Ice Crystal Icing Events
• Convective weather containing ice crystals – isolated
cumulonimbus or thunderstorms to squall lines and
tropical storms
• Deep convective clouds lift high concentrations of water
thousands of feet into the atmosphere
• In a warm atmosphere at the earth’s surface, the clouds
can contain more water which can be lifted to high altitude
– Measurements up to 8 g/m3. The certification standard
for supercooled liquid water for engines is 2 g/m3
• Ice crystals exist from temperatures just below freezing
to well below -40C
• Away from the core of the storm, in the anvil, ice particles
have been measured to be very tiny – the particle size of
baking flour
Copyright © 2006 the Boeing Company Mason_Forum.9
Satellite/Radar Can Detect Ice Crystals
NASA TRMM Satellite/Radar Combined Space-borne
Sensors Show The Nature Of A Convective Storm
Freezing level
White Vertical Cross Section through Hurricane
indicates
regions of
particles with
low reflectivity
such as ice
crystals
Region of high
reflectivity (heavy
precipitation, hail)
Copyright © 2006 the Boeing Company Mason_Forum.10
The Ice Crystal Weather Threat to Engines
Satellite/Radar Can Detect Ice Crystals
(Representation)
Aircraft weather radar
• Flight in ice crystals will look like visible moisture, but will not
produce significant aircraft radar returns
• Using tilt feature on radar should identify heavy rain below – a good
indicator that dense ice crystals may exist above
Copyright © 2006 the Boeing Company Mason_Forum.11
The Ice Crystal Weather Threat to Engines
Recommendations for Flight Near Convection
• It is not practical to avoid all ice crystal conditions; crystals may not be
detected by aviation radar
• Normal thunderstorm avoidance procedures may help avoid regions of
high ice crystal content
• These include:
– Plan a flight path that avoids storm cells by at least 20 nautical miles
– Fly upwind of the storm
– Avoid flying over a storm cell. A fully developed thunderstorm can
reach altitudes of more than 50,000 feet
– Even when there are no radar returns, there may be significant
moisture in the form of ice crystals at high-altitudes
– Utilize the radar antenna tilt function to scan the reflectivity of storms
ahead. Recognize that heavy rain below likely indicates high
concentrations of ice crystals above
Copyright © 2006 the Boeing Company Mason_Forum.12
The Ice Crystal Weather Threat to Engines
Global View of Engine Powerloss Events
• There is large concentration of events in the Asia Pacific
region – this may be due to the fact that the highest sea
surface temperatures are also found in this region
Longitude
Latitude
-150 -100 -50 0 50 100 150
-80
-60
-40
-20
0
20
40
60
80
Concentration of events
Copyright © 2006 the Boeing Company Mason_Forum.13
The Ice Crystal Weather Threat to Engines
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