Even small amounts of ice accretion can cause major problems.

By Karsten Shein
Climate Scientist

Ground crew prepare to deice aircraft during a snowstorm. Deicing is needed anytime there is potential for ice buildup, including during snow events.

Draining his coffee, the copilot had grabbed his coat and went to do a hurried walk around of the bizjet in the clear, subfreezing early morning air. He kicked the tires and ran his gloved hand across the leading edges. Nothing looked out of place. Removing and stowing the intake and pitot covers, he dropped the stairs and quickly clambered inside.

The CEO's secretary had called them yesterday evening to inform them the boss needed to make a last minute meeting at a new client's office 2 states over. With that info, they'd called ahead to have the plane fueled, which required moving it out of the hangar.

Normally, the FBO would tow the aircraft back inside afterward, but a miscommunication with the line crew about the departure time caused them to leave the aircraft out.
The pilots were not too happy about this when they arrived at 5 am, but they figured they'd have some time to warm the cabin before the boss arrived, so they left things as is.

The chief pilot finished filing the flight plan, greeted his boss and walked him out to the aircraft where the copilot had already lit the fires and gotten some heat flowing. Settling in, and with no other aircraft moving about at this early hour, they were quickly making their way to runway 3. This was a much shorter runway than they liked, and there was a row of trees just past the airport boundary, but a maintenance crew was on the longer runway, and the cold dense air would favor a quick rotation and climb out.

During the short taxi, the crew focused on setting the FMS and checking the gauges. Everything still seemed fine as they accelerated down the runway. The chief pilot felt they weren't accelerating fast enough and thought about aborting, but quickly decided that was just an illusion created by using the shorter runway. The mains finally left the ground with around 700 ft of runway remaining. With no time for the crew to respond to the abrupt stall warning, the aircraft, which had only managed to climb about 70 ft, clipped the tree line and twisted out of control to crash into the field opposite the runway.

Investigators later concluded that a thin film of ice had coated the wings in an almost imperceptible glaze. A light snow had been falling as the line crew fueled the jet and the combination of a hangar-warmed aircraft skin and full fuel tanks slowly releasing heat kept the wings above freezing long enough for the snow to melt as it landed, only to refreeze as the aircraft skin temperature dropped below 0° C (32° F) after about 20 minutes.

Sadly, in the cold and dark, the copilot had hurried his preflight walk around, never taking off his gloves to physically inspect the wing surfaces. At that time of morning, the ice would have been invisible to the copilot's eye, but not to his finger. As a result, he never felt the need to deice the aircraft, or even turn on the anti-ice air bleed. Unfortunately, the chief pilot neglected to ask him about the walk around.

The investigator's calculations suggested that the aircraft was carrying at least 170 kg (375 lbs) of ice on the wing and elevator surfaces. Given the higher than normal climb angle used in an attempt to clear the trees, the ice also reduced lift by around 30% and increased the drag by 40%. There was simply no way for the aircraft to escape its fate.

Because of its ability to critically impact an aircraft's capability to stay in the air, aircraft ice accretion is one of the most dangerous meteorological threats to aviation. Aircraft icing can occur any time the aircraft is in air that is below freezing and there is water in liquid form (generally where the air is saturated). While it is well known that the freezing point of water is 0° C (32° F), this is conditional on the purity of the water. That value should be more correctly considered the melting point of ice, since pure water can remain liquid to temperatures of around -50° C (-56° F).

Water in the air

Clear ice coats the upper wing surface of a Bombardier CRJ-200. Freezing rain will often coat upper aircraft surfaces where onboard deicing systems cannot reach. Only chemical or radiant heat methods will remove such accretion prior to flight.

To understand icing, we 1st need to understand the behavior of water in the atmosphere–especially as clouds and precipitation. Most of the lower atmosphere contains water vapor. These water molecules are constantly evaporating and condensing as they absorb and release energy to maintain balance with the surrounding environment.

In unsaturated air, the rate of evaporation exceeds the rate of condensation, and water molecules tend not to remain in a liquid (or solid) state long enough to grow to cloud droplet sizes (around 0.02 mm diameter). But, if enough water is added to the air, the condensing molecules find it more and more difficult to acquire the energy to re-evaporate. Simultaneously, the more molecules exist in liquid state, the more likely they are to find an aerosol to which they can attach.

These so called condensation nuclei are microscopic dusts or salts that attract water. They may dissolve in the water, creating a solution, or simply be encapsulated. Suspended water droplets assume a spherical shape, and the high curvature of the droplet's surface makes it easier for water to condense onto the droplet than evaporating from it. This is what allows the droplet to resist re-evaporation and grow larger. However, to grow to raindrop proportions (around 1–5 mm diameter) air currents help the cloud droplets collide and coalesce.

If there is a sufficient source of moisture, and as long as drier air doesn't flow into the cloud to desaturate the air, the droplets will likely gain enough mass to counter any air currents that are keeping them suspended and they will begin to fall as precipitation. This is a big if, because as we can see in the atmosphere, most clouds never produce a drop of precipitation, and of those that do, some of that precipitation evaporates as it falls through drier air below, never reaching the ground.

Of course, while droplets may start to fall at a fairly uniform size, the air resistance flattens them out, and recent high-speed video of falling droplets reveals that this often inflates them like a parachute to the point where they simply disintegrate into smaller droplets. This is why a rain shower might produce a variety of droplet sizes.

Explanation of icing formation

If the cloud exists or extends above the freezing level, many of these cloud droplets will begin to freeze into ice crystals. However, unless the air temperature is very cold, a lot of the cloud droplets will remain liquid. In fact, spontaneous crystallization–a droplet of pure water instantaneously transforming into ice–requires temperatures of around -50° C (-56° F). But, with most droplets containing some condensation nucleus which will also act as an ice nucleus, the crystallization process will begin in earnest around 0° C and become widespread between around -15° C to -40° C (-5° F to -40° F).

Below -40° C, nearly all cloud and precipitation droplets have become ice crystals.
Although some liquid water will still exist below about -15° C, the danger zone is the cloud levels between 0° C and -15° C OAT. In this region, most of the liquid droplets have yet to freeze, and are instead supercooled. Supercooled droplets hold water that is in a suspended transitional state. As water freezes, it releases some of the heat energy it has stored.

The release of this latent heat actually serves to warm the water around the forming crystal, preventing additional growth. Only small disturbances are needed to disrupt that balance and complete the droplet's freezing process.

One of the biggest disturbances that the supercooled droplet can encounter is contact with a surface that has a temperature below freezing – such as striking the subfreezing skin of an aircraft. The reason that aircraft don't often accrete ice in a cloud that has no larger droplets is that the flow separation ahead of the aircraft is sufficient to deflect the droplets before they make contact. However, some parts of the aircraft, such as the pitot and engine inlets are designed and positioned to minimize the deflection of air, and so they may accrete small amounts of rime ice simply from contact with millions of small, supercooled cloud droplets.


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