Convective Weather Containing Ice Crystals Associated with Engine Power Loss and Damage

BACKGROUND INFORMATION

Icing conditions are defined as temperatures below 10 degrees C with visible moisture. This definition describes conditions where supercooled liquid drops adhere to airframe surfaces, typically at 22,000 feet and below.

Boeing Recently, several engine power loss and damage events have occurred in convective weather above the altitudes typically associated with icing conditions. Research has shown that convective weather can contain very small crystals of frozen water, perhaps as small as 40 microns (the size of flour grains). The industry is using the phrase “ice crystal icing” to describe this condition.

Ice crystals do not adhere to airframe surfaces, only to engine surfaces, because the ice crystals bounce off cold surfaces, but partially melt and stick to relatively warm engine surfaces. “Glaciated conditions” refers to atmospheric conditions containing only ice crystals and no supercooled liquid. “Mixed phase conditions” refers to atmospheric conditions containing both ice crystals and supercooled liquid. Both glaciated and mixed phase conditions occur in convective clouds and have been present during engine power loss and damage events.

The nature of deep convection and its importance in engine power loss and damage events

Deep convective weather is characterised by significant lifting (thousands of feet) and condensation of water vapour in an unstable atmosphere. Some or all of the following can be found in areas of convection: strong windshear; turbulence; lightning; and high condensed water content, mainly in the form of ice crystals but occasionally in the form of heavy precipitation or hail. Convective weather can range in size from small to large: isolated thunderstorms or cumulonimbus clouds, to convective complexes or squall lines, and finally to tropical storms or hurricanes. Convective weather can extend hundreds of miles laterally and above 50,000 feet vertically.

Engine power loss and damage events have occurred in the periphery of convective weather of all sizes at altitudes between 11,500 feet and 36,000 feet. Typically, the event airplanes were diverting around isolated thunderstorms or crossing the cloud anvils of convective storms, convective complexes or tropical storms. The following properties of clouds away from the convective core appear to be important in the formation of ice on the engine surfaces:

• Small crystal sizes
  Research suggests that most of the mass is concentrated in very small crystals. These small crystals may play a key role in ice formation in the engine because they can melt quickly and provide a liquid film on surfaces that can capture more ice crystals.
• High ice crystal concentrations
  Deep convection is thought to contain very high ice crystal concentrations, analytical estimates are up to 9 grams per meter3 (g/m3). In comparison, the current standard for the maximum concentration of supercooled liquid for engine icing certification is 2 g/m3. If a 9 g/m3 concentration was associated with a conventional icing encounter, it would be extremely severe.
• Ice crystals above the freezing level; usually little or no supercooled liquid present
  The strong circulation associated with the periphery of deep convective clouds can efficiently mix water rising from below the freezing level with ice crystals above the freezing level, causing the mass above the freezing level to quickly become glaciated (converted to ice crystals). This means that, at an altitude above the freezing level, in a mature convective cloud, the majority of the water is likely to be ice crystals.

We know that near the convective core, water and large hail can rise above the freezing level. This mass is detectable by airborne radar. However, at the locations of the engine power loss and damage events, pilots did not report radar returns and reported only light to moderate turbulence. Thus, the conclusion is that the airplanes were not crossing the convective precipitation core at the time of the engine events and were likely encountering ice crystals.

Use of on-board weather radar to detect ice crystals

On-board 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 on-board weather radar, even though they may comprise the majority of the total mass of a cloud.

Sophisticated satellite radar technology has been used to detect crystals smaller than the lower limit of on-board weather radar. Above the freezing level, where icing can occur in a deep convective cloud, satellite radar has confirmed that large particles, which can be detected by on-board weather radar, are only found near the convective precipitation core. Away from the convective precipitation core, satellite radar has confirmed that small ice crystals, which are invisible to on-board weather radar, exist.

For this reason, flight in visible moisture near deep convective weather, even without radar returns, and at temperatures below freezing is very likely to be in ice crystal conditions.

Recognising high ice crystal conditions

High ice crystal concentrations can be found under the following conditions:

• Areas of visible moisture above the altitudes typically associated with icing conditions, indicated by:
  • No significant airframe icing
  • The ice detector (when installed) not detecting ice, due to the ice detectors ability to only detect supercooled liquid, not ice crystals
• Areas above heavy rain
• Areas of light to moderate turbulence
• When rain appears on the windshield above the freezing level, due to ice crystals melting on the heated flight deck windows
• When airplane Total Air Temperature (TAT) is significantly different than expected, often near zero degrees C.

OPERATING INFORMATION

Since ice crystals may not be detected by existing on-board weather radar systems, it is not possible to avoid all ice crystal conditions. However, normal thunderstorm avoidance procedures may help in avoiding high ice crystal content conditions. These include:

• Plan a flight path that avoids reflective regions of storm cells by at least 20 nautical miles.
• Use the radar antenna tilt function to scan the reflectivity of storms ahead. Assess the height of the storms. Recognise that heavy rain below, typically, indicates high concentrations of ice crystals above.
• Fly upwind of storms when possible.
• The most effective procedure is to avoid flying 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. Storm cells that do not produce visible moisture at cruise altitude may be overflown safely.

Boeing Flight Operations Technical Bulletin 75, 1st August 2006