In the last couple of decades, a new hazard to the aviation industry has been identified – that of high altitude ice crystals (HAIC), also referred to as high ice water content (HIWC). This hazard is distinct from the better known and more typical icing conditions that cause airframe icing.
What are high altitude ice crystals?
As you might expect, this hazard is more complex than simply having ice crystals at high altitudes. The hazard is specifically related to very small ice crystals that can exist away from main convective cores (where most ice is created). A key characteristic of this hazard is that the ice crystals are small enough so as not to produce an obvious weather radar return from typical aircraft radars, and so pilots will not be able to see if they are flying towards an area with a high concentration of these ice crystals.
Airframe icing is typically caused by super-cooled liquid droplets freezing on impact with the cold exposed aircraft surfaces. Being frozen, the high altitude ice crystals on the other hand ‘bounce off’ these surfaces. The HAIC hazard is commonly found at altitudes above 22,000 feet, above the theoretical limit for the existence of super-cooled liquid droplets and so away from the risk of airframe icing.
So how do they pose a hazard to aircraft?
If these ice crystals do not accumulate on the aircraft body, one might ask why they are worth bothering about. The reason these crystals are of such interest and importance to the aviation industry is because they have been connected to a number of jet engine power-loss and damage events. This is thought to be due to the ice crystals being ingested into the engine where they melt, creating a thin liquid film over surfaces. This film then captures further ice crystals, which can refreeze on engine surfaces (such as on the stationary stator blades). This can lead to engine events either from further ice accretion (for example ice build-up blocking sensors), or from the ice shedding when the aircraft enters warmer air (e.g. during descent), causing possible power-loss such as engine surge events.
As this is a relatively newly identified hazard, the scientific community still has some way to go in our understanding of the actual atmospheric conditions and processes under which HAIC occur.
In 2012, a HAIC consortium co-ordinated by AIRBUS was set-up involving over 30 partners. Their focus has been on the development of numerical tools and testing appropriate awareness technologies to better detect these hazardous weather conditions. As part of this project, research flight field campaigns have been conducted, looking at the microphysical properties of HAIC regions among other aspects surrounding both the characterisation and detection of this hazard.
There is also ongoing development work at the Research Applications Laboratory (at the United States National Center for Atmospheric Research) using satellite, numerical weather prediction models and radar data to try to identify possible regions of HAIC. Here at the Met Office, similar work is being done in assessing the applicability of satellite and model data in the identification and prediction of these hazardous areas. There is a recognised need for more observational data to be able to better verify these data, whether collected from further research flight campaigns or from standard aircraft which have experienced an engine event thought to be related to the HAIC hazard.
The combination of the knowledge that HAIC is not picked up by current aircraft weather radar, and of the possible impacts it can have if ingested into the engine, highlights the importance of continued research efforts. Engineering solutions are also being explored but in the shorter term at least, there is a requirement for better detection and prediction of this hazard.
Claire Bartholomew- Aviation Scientist