For those of you who have flown during the cold winter months amidst thick, white blankets of snow and sub-zero ambient temperatures, the question of how airplanes manage to avoid freezing might have flickered through your mind. I know it has mine!
Airplanes do not freeze because designers ensure sensitive components are heated or kept away from snow accumulation, low freezing points of jet fuels, thorough and robust de-icing protocols both on the ground and in the air, and during flight, the air is drier even at higher, sub-zero altitudes.
You may have been on an airplane when it pulls up to a team of ground service agents that seem to pressure wash it but is really happening, and what do aircraft designers do to ensure airplanes do not freeze.
What Prevents Airplanes From Freezing?
For an airplane to freeze its needs to have several conditions surrounding it:
- Moisture – Rain, snow, drizzle, mist or fog
- Cold Temperature – Either air temperature or fusleage temperature
When moisture is surrounding the aircraft it can begin to stick to the fuselage if the fuselage surface is below freezing. Depending on how the moisture adheres to the aircraft will depend on how hazardous the ice buildup becomes.
When the fuselage is cold and flown through suspended moisture droplets like those in drizzle or in clouds it will create frost on the exposed surfaces, similar to your car on a cool spring morning. This is because the moisture in the air is just above freezing but when it touches the fuselage which is at or below 0°C/-32°F, it instantly sticks creating a rough, white frost layer.
If the aircraft is sitting or flying in warmer air, just above the freezing point but there is a warm air layer above a layer of cold air (known as a Temperature Inversion) it can allow rain to fall from the warm air, through the cold air layer and not freeze, then fall into the warm air layer containing the aircraft that rain droplet will freeze upon impact with the aircraft.
These are known as supercooled water droplets and cause clear, solid ice to form on any exposed surface and at a rapid rate.
Any ice buildup on lifting or control surfaces will affect their performance which will reduce how well the aircraft flies, especially as more ice builds up.
The main places that are of concern for ice buildups to pilots are:
- Leading edges of wings and rotorblades on a helicopter
- Control surfaces – Elevator, rudder, aelierons, flaps, airbrakes
Because these areas are critical, aircraft designers craft ingenious ways to prevent them from freezing or allow for any ice buildup to be automatically removed.
Here is a selection of design ideas that engineers have come up with to fight against freezing of the aircraft:
- Routing fuel lines alongside the engines and back to the fuel tank for the double advantage of simultaneously cooling the engines while warming the fuel
- Keeping the fuel inside the fuel tanks in constant motion to reduce the risk of developing frozen sections that impair the performance of the entire tank.
- Placing hydraulic lines that dissipate radiative heat alongside the fuel tanks and keep the fuel warm enough to prevent freezing
- Heating antennas and probes to ensure they do not freeze
- Heaing air intakes using hot air from the engines
- Electically heated leading edges of wings and control surfaces
- Electrically heated propellers
- Pneumatic boots that inflate along the leading edges to break off ice accumulation
- Electrically heated windows to prevent fogging
By clever design, some of these features tie back to a concept of dual-thermoregulation, which aims to keep hot parts cooler and cold parts warmer. It’s a win-win!
The larger the aircraft, the more systems it can carry to deal with icing and freezing. However, every aircraft has a limitations section within its flight manual that pilots must adhere to if they encounter such conditions that would surpass the aircraft’s allowable limits.
Minimum operating temperatures, flight into known icing conditions, or flight into snow for prolonged periods are just some of the limitations stipulated, especially for smaller aircraft.
If at any point the pilot suspects they have entered or are about to enter any weather that either would cause the aircraft to be accumulating ice beyond its design or limitations they must make the decision to remove the aircraft from that current weather situation.
This can be done by turning around, descending or climbing to warmer temperatures, or diverting around the weather cell/system.
Large commercial airliners are designed to fly through these conditions with ease because their size and power of them allow for far more additional systems to be installed compared to a small Cessna and they are designed to operate at much higher altitudes where moisture does not pose a threat.
Ground Handling Procedures
By law, no pilot can take off in an aircraft when there is ice or frost adhered to critical flight and control surfaces. This is known as the ‘Clean Aircraft’ concept. To combat this with larger aircraft most airports will have a ground de-icing area that all aircraft must taxi to before departing if the aircraft is thought to have ice/frost accretion or will be susceptible during takeoff and climb to altitude.
When the aircraft arrives at the de-icing station, one or several specialized application trucks will begin to cover critical areas with hot fluid that will help to de-ice any accumulated ice and coat those surfaces to prevent ice accretion.
There are four types of de-icing/anti-icing fluids used today:
This is orange in color and is of low viscosity. It is sprayed onto the aircraft at around 130-180°F and under high pressure. The job of this type of fluid is for ice removal and short-term protection. Because of its low viscosity, it soon drips off the aircraft and is removed as the aircraft takes off.
This is usually applied before a second type of fluid is applied to provide better anti-ice protection.
This is clear in color and has a thicker concentration. It is used on larger aircraft and allows for a longer time between application and takeoff. This fluid type will start to be removed from the aircraft once it reaches around 100knots on takeoff.
This is yellow in color and also has a thicker concentration. It is used on slower aircraft whose takeoff speed is below 100knots. It performs very similar to Type II but is designed to be shed at a much slower airspeed so as not to accumulate and create problems for aircraft maintenance personnel.
This is green in color and provides the longest protection. This is very similar to Type II but adheres for much longer and provides protection for aircraft at large airports that have to sit in line to take off. This is the most common fluid you will see dispensed as the time it takes to exit the de-ice station, taxi, and then take off at some airports can be 10-20 minutes.
Types II, III & IV are known as anti-icing fluids as their main job is to prevent ice accumulating on the critical surfaces and components of an aircraft, whereas Type 1 is purely for ice and frost removal.
Pay attention next time your aircraft enters a de-icing station and see what fluids they use on your aircraft!
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Why Does Airplane Fuel Not Freeze?
When cruising up at high altitudes the ambient air temperature will drop to around -55°C/-67°. For aircraft that operate in very cold climates where the typical air temperature can drop to much lower temperatures, the fuel can become an issue.
Aviation Fuel is designed to be able to withstand cold temperatures, but airliners need to go even further to prevent fuel from freezing.
Here is a list of the most common aviation fuels used around the world:
- United States (Jet A fuel): -40° F (-40° C)
- Mainly in Northern Canada, but very rare now. (Jet B fuel): -76 °F (-60° C)
- Mainly in Russia (TS-1 fuel): -58° F (-50° C)
- Rest of the world (Jet A-1 fuel): -53° F (-47.22° C)
As you can see, the fuel is designed not to freeze until very low temperatures but when the air temperature is colder than the fuel the aircraft needs to employ additional features to prevent it from freezing.
On most large commercial airliners fuel tanks have heaters installed and fuel lines are run alongside hot oil and hydraulic lines. All of these systems use electrotonic monitoring and control systems to ensure the fuel stays at the optimal temperature before being fed into the engine fuel control system.
Why Do Airplanes Not Freeze In Flight?
Flying at low altitudes below the main weather systems provides the greatest risk for an aircraft freezing up. Light aircraft, general aviation, and helicopters are the most vulnerable as they are confined to these lower altitudes.
The conditions mentioned at the start of this article can be present for weeks or months in many areas of the world and keen pilot attention is required to decide whether to cancel the flight, turn around or divert if any of the problematic weather conditions are encountered.
The flying conditions in the upper atmosphere where most airliners, private jets, and corporate jets operate are much better than on the ground or at lower altitudes. It is the clouds that contain moisture and below the clouds that falling or suspended moisture is found, even at temperatures below freezing.
Once aircraft reach higher altitudes the moisture begins to turn into ice crystals. Ice crystals cannot bond to aircraft as they are already in a solid form. These ice crystals are dry particles, and they don’t cause any icing on the wings and body of an airplane.
Once flying above the clouds there is no moisture present in the air. That is one of the main reasons why most airplanes today fly at high cruising altitudes for the majority of the flight.
The biggest areas of concern for pilots are when climbing out from an airport and ascending up through these layers of moisture until they climb above them. This is where the anti-icing fluids applied to the aircraft before takeoff provide protection.
When the airplanes are descending down through the clouds the amount of time they spend in the icing layers is minimal. If ice does begin to form it will soon melt once they descend into warmer temperatures during the approach to land.