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Refer to figure.
An aircraft is planned to fly from Nairobi ('N', approximately 1°S, 37°E) to Manila ('M', approximately 15°N,121°E). Assuming the track follows the straight line depicted on the chart, and the cruising FL increases from initially FL370 to FL490 halfway through the flight, where along the route is it most likely to encounter a high concentration of hazardous ice crystals?

  • A
    At cruising altitude for the first 600 NM after initial climb.
  • B
    At cruising altitude for the last 600 NM before descent.
  • C
    It is NOT expected to encounter a high concentration at cruising altitude on this route.
  • D
    Clear for the first 1500 NM or so and then likely for the rest of the cruise.

Refer to figure.
Learning Objective 050.09.01.04.03: Identify weather situations and their relevant areas where high concentrations of ice crystals are likely to occur.


Ice crystal icing (ICI) condition refers to aircraft experiencing icing inflight in high altitude due to high concentration of small ice crystals. At very low temperatures, the water vapour turns directly into solid ice crystals by deposition (often referred to as "sublimation" in meteorology). Engine surfaces and pitot tubes are affected. Several engine power-loss and damage events have occurred in convective weather far above the altitudes typically associated with icing conditions. Research has shown that strong convective weather (thunderstorm activity) can lift high concentrations of moisture to high altitudes where it can freeze into very small ice crystals, mostly invisible to the human eye, and to weather radars.

  • Ice crystals do not adhere to cold airframe surfaces because the ice crystals bounce off. However, the crystals can partially melt and stick to relatively warm engine surfaces. It has also been noted that ice crystals impacting heated windscreens can result in pilots observing “rain” as the crystals rapidly melt on contact with the heated windscreen.
  • The main risk of encountering high crystal concentrations appears to be downwind from the tops of large areas of convective cloud - the area where the visible anvil shape is seen when viewed from a distance.
In the case of this question, the first 1000-1500 NM are completely clear of large convective active such as CBs, but after this, the aircraft is cruising at FL490 completely within a region of ISOL EMBD CBs with tops of FL470, and goes through regions of OCNL EMBD CBs with tops of FL500. This area would be a perfect place for ICI to occur, in and above the tops of huge convective clouds with plenty of moisture carried up to the high altitudes.

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