With the weather deteriorating, Julio turned the aircraft toward the initial approach fix for a run at the ILS. The airport reported a 400-foot ceiling in the rain with a temperature of 6°C. He was flying strictly on instruments as the rain splattered across his windscreen. It wasn’t until the tower crackled in his headset to maintain 4,000 feet that he realized something was wrong. His altimeter read 4,000 on the nose and his airspeed looked good, yet the controller told him he was descending, passing through 3,200 feet. As Julio looked up briefly, he saw a milky white crust of ice at the edge of his windscreen, just above the OAT indicator that read -2°C. He scanned his panel and realized his pitot heat was off. Giving the throttle full power, Julio attempted to arrest the descent, but it was useless, there was too much ice already on his wings, and the engine seemed to struggle to find the additional power. Julio declared an emergency and asked for vectors direct to the runway – if only he could stay aloft long enough.
As the skies grow cooler across the Northern Hemisphere with the arrival of fall, aircraft icing is more and more likely. Icing is a leading contributor to aircraft accidents, and its avoidance is something all pilots should be familiar with. Ice accretion can occur anywhere the air temperature is below freezing, and there is visible moisture in the air. This includes most clouds and fog, particularly clouds producing precipitation. That said, there are a variety of other conditions under which an aircraft can accrete ice.
Freezing Rain
A common way that ice accretes is via freezing rain. Although the freezing point of water is 0°C, the small size and spherical nature of cloud and rain droplets and the limited quantity of freezing nuclei (aerosols that promote freezing) mean that the mostly pure water droplets will not freeze until either the temperature drops far below freezing or the supercooled droplet collides with a subfreezing surface, such as the skin of an aircraft.
The greatest danger of inflight icing is when OAT is between 0°C and -10°C. In this range, most droplets will be supercooled liquid instead of ice crystals. According to NASA, most reported icing comes from droplets 10-50 microns in diameter. This is the typical range of cloud droplets and is the size typically used to certify deicing systems. It is far smaller than rain droplets, which have a typical diameter of 1,000-2,000 microns (1-2 mm). These larger droplets are called supercooled large droplets (SLD). However, any liquid precipitation should be treated as SLD in subfreezing regions.
The problem with SLD is that, unlike smaller droplets that freeze instantly on collision with the aircraft skin, SLD take a few moments to freeze, often flowing aft of deicing measures. This can produce a sharp ice ridge that only melting can remove, and that can cause a loss of lift and aircraft control. Around thunderstorms and other convective systems, SLD ice may accrete faster than some deicing systems can remove it. If temperatures are close to freezing, the ice may retain some plasticity that air bleed heat or boots only push away without dislodging.
Additionally, ice can accrete around air intakes and on propellers. Both situations can reduce thrust, and if ice breaks off and clogs air inlet pipes or is ingested into the combustion areas, it can cause surges or even engine failure. Propeller icing can also be dangerous if ice sheds from a propeller blade unevenly, it may cause rough operation and stress engine mounts.
Avoiding Trouble
The best way to avoid ice accretion in flight is to avoid encountering supercooled droplets. If flying above the freezing level, give a wide berth to any areas of rainfall at your altitude. Most SLD will not be identifiable as supercooled on surface-based radar images. However, if radar suggests precipitation at your altitude and the OAT is below freezing, any liquid droplets will be supercooled. Fortunately, most SLD occurs in altitude bands less than 3,000 feet thick and mostly below 12,000 feet MSL – though thicker bands and higher altitude SLD do occur.
AIRMET chart for Feb. 14, 2014, showing numerous deep regions of moderate icing and freezing levels at or near the surface over large parts of the U.S. Maps such as this can help pilots avoid areas of potential ice accretion. (Source: NWS AWC)
If flight into icing is unexpected or unavoidable, ensure the pitot heat is on, and any anti-ice and deicing tools are activated per manufacturer instructions. Although many aircraft are certified for flight into known icing (FIKI) conditions, that should not be viewed as a license to fly into icing conditions, it’s better to avoid them. If ice accretes in flight, even if it can be removed by deicing systems, pilots should make their best route to warmer air. Only temperatures above freezing will ensure all ice melts from the aircraft. In the lower atmosphere, warmer air will almost always be found at a lower altitude.
The threat of icing will prompt issuance of AIRMETs (Zulu) for regions of moderate icing or low freezing levels. If icing across all or part of the region is expected to be severe, a SIGMET will be issued instead.
In some forecast products, such as the NWS Aviation Weather Center’s CIP/FIP (Current and Forecast Icing Product), areas of SLD will also be identified.
Midlevel prog chart for the North Atlantic identifying areas of convection that may include icing. Icing levels and intensity are shown with boxed arrows displaying base and upper altitudes and a hatched U. (Source: NWS AWC)
Grounded
Ice is frequently encountered on the ground as freezing rain falls to the surface, coating aircraft from above. Freezing rain typically produces a transparent shell of ice over any exposed surface and can be removed with deicing fluids or by moving an aircraft into a warmed hangar. In the fall, cold clear nights may produce frost that leaves a thin coat of ice on aircraft. Running bare hands over a wing can often reveal icing that may not be visible to the naked eye. Importantly, any ice present on an aircraft must be removed before attempting takeoff. Even a millimeter of rough ice on the wing can reduce lift by around 30% and increase drag at least as much. It can also add several hundred pounds to the aircraft’s weight. Special attention should be given to moving aircraft from warm hangars into snow, or aircraft that have been warmed by the sun prior to the onset of snow. The snow will melt onto the warm aircraft surface and refreeze in place as the aircraft skin cools.
Further, freezing rain also coats taxiways and tarmacs with ice, often making traction impossible. Hundreds of aircraft accidents have occurred when a landing or taxiing aircraft lost control on icy pavement. In these cases, pilots should exercise caution to avoid injury walking to and from their aircraft and check with ground controllers or maintenance crews to determine whether taxiways and runways have been deiced. Unfortunately, road salt and brine are of limited effectiveness in freezing rain. Sometimes, it is better to just wait things out with a hot cup of coffee.