ice crystals - airplane

Ice crystals: A danger to aviation

Tim Takeoff
11.02.2020
2 pictures
5 minutes

Something that initially seems completely normal was, for a long time, considered not especially dangerous in aviation too: ice crystals at high altitude. In fact, they are a danger to both man and machine, particularly in this age of modern turbofan engines.

Ice-crystal icing

At high altitude, flying is relatively carefree, as a rule. Nevertheless, turbulence and thunderstorms, and occasionally volcanic ash, can act as external sources of danger and cause a jet problems during cruising flight. Only in the last ten years has the topic of ice-crystal icing come under scrutiny. But what does this mean to pilots?

Temperatures in aviation

Basically, one assumes that below a static air temperature of – 40°C (above 22,000 feet) no icing is to be expected. The air here is simply too cold to take up water droplets at all. In cruising flight, between 30,000 and 40,000 feet, near the tropopause, you often find temperatures down to – 56°C. Therefore, initially it appears that there is nothing to worry about.

However, if you consider the temperature on the exterior skin of the aircraft, the situation appears very different. Friction between the air particles and the aircraft creates heat. Enough energy to raise the temperature quite a bit. This temperature, which the pilot sees displayed in the cockpit, is called the “Total Air Temperature” (TAT). The temperature in still air (“Static Air Temperature”) is merely calculated in the background by means of a formula, because it cannot be measured directly.

“Super”-cooled water droplets

If you now fly near convective clouds, or even exactly above them, rare phenomena can occur in cruising flight. If ascending masses of warm and humid air reach high layers of the atmosphere, the TAT gets closer and closer to the freezing point of water. If you fly through ice clouds, the frozen ice crystals can turn back into water because of friction with the exterior skin of the plane. These are then known as supercooled water droplets, because they are greatly undercooled. They can adhere again and rapidly refreeze.

Sensor anomalies

The ice crystals don’t simply speed past the plane but can stick to exposed sections of the aircraft and cause icing. This happens not on the cold surfaces but there where it is relatively warm. This is particularly dangerous at the heated pressure sensors for the airspeed, angle of attack, and parts of the engines. If a block of ice forms at the temperature probe, the instrument will only display the temperature of frozen water: 0°C. An anomaly.

The weather radar system is capable of detecting large water drops or particles in the atmosphere. However, these ice crystals are much too small to be detected and displayed to the pilot. On his display he can only see weather phenomena below the jet.

Ice formation

Ice forms at the heated engine air intake, the fan blades, or the “spinner” (the component at the centre of a fan). Here it is not such a problem because, owing to the rotation and movement of the engine, it breaks off and melts again. However, if the ice forms in core regions of the compressor, the consequences can range from a loss of power to shutdown of the engine.

What happens next?

The pilot can take countermeasures if he notices early signs of icing by reducing thrust (both the engine temperature and engine speed sink). Turning on permanent ignition can also help. Automatic thrust can likewise malfunction, since the on-board computer can no longer classify the parameters. The failure of the airspeed displays is an event that is regularly practised in the simulator and should no longer be an issue for any pilot.

Early recognition of ice crystals

Most ice-crystal icing events occur in the region between 20 and 40 degrees north and south of the equator, during ascent and descent as well as during cruising flight. For the pilot there are therefore several factors that allow him to infer the occurrence of ice-crystal icing:

The pilot should then try to fly around weather phenomena at a large distance, at best on the weather side of the cloud (the side facing into the wind). The wind blows the ice crystals to the lee side (downwind) at the upper side of the cloud.

Nowadays there are standardised checklists that have to be followed if ice-crystal icing is suspected. Pilots are required to document incidents of this kind, with the aim of making it possible to get to the bottom of the cause in future.

Ice crystals on the plane
© WingMag

by Tim Takeoff

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