737 MAX Flight Crew Operations Manual
Supplementary Procedures -
Adverse Weather
MN-FLT-OH-201
SP.16.29
Ice Crystal Icing (ICI)
At temperatures below freezing near convective weather, the airplane can
encounter visible moisture made up of high concentrations of small ice
crystals. Ice crystals can accumulate aft of the engine fan in the engine
core. Ice shedding can cause engine vibration, engine power loss and
engine damage. CFM56-7 engines have experienced several power loss
events resulting from ice accumulation in the engine.
Ice crystals can also accumulate in the fan hub. This can cause vibration
indications above 4 units. Fan ice removal procedures have no effect on fan
hub icing. When clear of clouds, fan hub ice sublimates and engine
vibration decreases over time. Fan hub ice can remain into descent.
Ice crystal icing is difficult to detect because ice crystals do not cause
significant weather radar returns. They are often found in high
concentrations above and near regions of heavy precipitation. Ice crystals
do not stick to cold airplane surfaces.
Avoid ICI conditions. Flight in clouds containing high concentrations of
ice crystals has been associated with engine vibration, engine power loss
and engine damage.
Because these conditions can be difficult to recognize, careful preflight
planning is a key component of in–flight situational awareness. When ICI
is encountered or suspected, do the QRH Ice Crystal Icing NNC to mitigate
the effect on the flight.
Recognizing Ice Crystal Icing
Ice crystals are most frequently found in areas of visible moisture and
above altitudes normally associated with icing conditions. Their presence
can be indicated by one or more of the following:
• appearance of rain on the windshield at temperatures too cold for
liquid water to exist. This is due to ice crystals melting on the heated
windows (sounds different than rain)
• Areas of light to moderate turbulence
• In IMC with:
• No significant airframe icing and
• no significant radar returns at airplane altitude and
• heavy precipitation below the airplane, identified by amber and red
radar returns on the weather radar.
• cloud tops above typical cruise levels (above the tropopause).
• Smell of ozone or sulfur
March 1, 2021