ATSC 113 Weather for Sailing, Flying & Snow Sports

Aircraft Icing

Learning Goal 3g. Explain how and where supercooled water forms, and explain how ice on aircraft affects flight

Supercooled Water in the Atmosphere

Hard to believe, but liquid-water cloud and rain droplets can remain unfrozen at temperatures well below freezing in the atmosphere. In particular, these supercooled droplets can be liquid between temperatures of –40°C and 0°C. But supercooled liquid droplets freeze almost instantly when they touch something solid and cold such as an airplane, a snowflake, tree branches, power lines, runways, and road surfaces.

 

First diagram: Standard atmosphere temperature sounding. Higher in the graph corresponds to higher altitude above sea level.

Second image: Clear ice on leading edge of the wing, as viewed from inside the cockpit of a high-wing aircraft during flight.

At air temperatures between 0°C and -40°C, it is possible for the cloud and precipitation to be all ice crystals, all liquid water, or a mixture of liquid water and ice.  But at these temperatures, any liquid water is supercooled, and can freeze almost instantly when they are hit by an airplane. Recall from Learning Goal 3a that on average, the air temperature in the troposphere decreases with altitude (see first diagram above). So as you climb away from the earth's surface, you will reach a range of altitudes where the air temperatures are between 0°C and -40°C.  Namely, that range of altitude is the danger zone for ice formation on aircraft.  But the danger exists only if the aircraft is flying through liquid-water clouds or rain at those altitudes.  Namely, if you are flying in clear air at those altitude, or if you are flying through clouds that are already 100% ice crystals, then there is no hazard for ice to form on the aircraft (except for a few exceptions described below).

As you climb to near the top of the troposphere, the temperature becomes colder than –40°C. At this altitude and above (i.e., in colder air), all liquid water is already frozen into ice crystals, so there is not normally an icing hazard at these altitudes. At any altitude, if the precipitation particles are already frozen (i.e., are snowflakes or ice crystals or sleet), then they are NOT normally a hazard to aircraft (see exceptions later), because the ice crystals simply blow around the aircraft and don't stick to it.

The actual atmospheric temperature sounding varies from day to day, location to location, and season to season.  Thus, the actual sounding is always somewhat different from the idealized sounding shown above in the first figure.  In summer, the icing hazard to aircraft might mainly be in the top half of thunderstorms and other deep convective clouds. In winter, freezing rain can reach all the way to the ground, forming ice on parked aircraft. In fall and spring, the supercooled water hazard is often associated with weather fronts such as warm fronts. Examples of frontal conditions that cause significant icing hazards are in Learning Goal 3h.

Ice on Aircraft - - Hazards

Why do we care about supercooled water? Because many bad things happen when a flying aircraft starts to accumulate ice on the aircraft:

Courtesy of US FAA AMT Airframe Ch. 15.  "Stall speed" is the slowest speed an aircraft can fly without falling out of the sky.

Some tips:


Aside:  Optional Training Videos about Aircraft Icing
I am putting these videos in an "Aside" box instead of an EXperts box, because they might be interesting to non-pilots, even though the videos were designed for pilots. 
  • The first video is a VERY VERY old (1961) US Navy training video.  Nonetheless, it gives a nice overview of aircraft ice.  (You might want to view this at x1.5 speed, if you can still hear the audio while viewing.  Else, view at normal speed.)  https://www.youtube.com/watch?v=epwKST6CimI  
  • The second video is much newer, and is designed for general aviation (small airplane) pilots.  Nonpilots might enjoy the first portion of this video.  https://www.youtube.com/watch?v=j_D8tcnL2uQ (It is also a long video, so you might try to view it at 1.5x or 2x speed, if you can still hear the audio.)


Exceptions

Consider an aircraft that is very cold from flying at high altitudes. If the aircraft descends into warmer humid air, then frost can form on the aircraft even in non-cloudy air. This is similar to frost that forms on the inside of your house windows on a cold winter night. While the frost is not very heavy, it does increase drag and decrease lift, and if on the windscreen it can block your view.

Some of the newer high-bypass turbofan (jet) engines on modern fuel efficient airlines can develop ice on the turbines even when the aircraft is flying at high altitude where the temperature is colder than –40°C. Namely, there is no supercooled liquid water at these high altitudes — only ice crystals in the clouds. However, sometimes when these aircraft fly through high ice crystal clouds, the tiny ice crystals melt while flowing into the turbine and then refreeze on the turbine blades. This causes the engine to develop less power, or in extreme cases for the engine to stop running.

Types of Ice

For an aircraft that is flying, most of the ice forms on parts of the aircraft that the droplet-laden air hits straight on. These include leading edges of wings, horizontal and vertical stabilizer (in the tail), the nose (front) of the fusalege (aircraft body), propellor, tires, antennas, and other strutures that stick out into the air (pitot tube, thermometer, etc.).

— Figures of cross sections of wings with ice, courtesy of US FAA AC 91-74B.

There are two main types of ice that form when supercooled water freezes: clear and rime (white).

All images by Stull

Clear ice

Rime ice
On wing leading edge
On other items that stick out from the aircraft.

Left: thermometer

Right: antenna

Also, layers or mixtures of clear and rime ice can form on an aircraft. Mixed ice is more frequent when the aircraft is flying in clouds where air temperature is –5 to –15°C.

Frost forms when water vapor (not liquid water) deposits on the aircraft and forms a "fuzz" of small ice crystals on the aircraft.

How Do PIlots Deal with Ice?

First, IFR pilots (see Learning Goal 1g) do their "homework", checking weather forecasts and pilot reports to pick a route (or to cancel or delay their flight) to avoid likely areas of ice, unless their aircraft is certified to handle ice. VFR pilots stay out of clouds anyway, so no problem there. But VFR pilots (who normally are allowed to fly through rain showers if the visibility is good enough) should avoid flying through rain showers if the air temperature is below freezing at their flight altitude.

Some of the more expensive general aviation aircraft and almost all commercial aircraft and airlines have de-icing equipment or anti-icing equipment on the aircraft. De-icing removes ice that has formed. Anti-icing prevents the ice from forming.

Wing heated by jet exhaust hot wing leading edge in jet aircraft
Pneumatic de-icing boot
Chemical spray (the spray tubes look like windshield wipers in this photo)
 All images courtesy of the US FAA. AMT Airframe Handbook, Chapter 15 (see extra info below).

For aircraft that don't have de-icing equipment and accidently find themselves in a cloud with ice accumulating on the aircraft, three actions are recommended (assuming the pilot asks and receives the appropriate clearance from air traffic control):

But even after you fly out of the cloud, your aircraft still has a load of ice on it. This will evaporate (sublimate*) very slowly in clear air, even if you are still flying in air below freezing. However, it could take an hour or more of flight to evaporate all the ice. Usually it is easier to change altitude to where the air is warmer so the ice melts quickly.

Note: * sublimation = direct evaporation without melting first.

Pilot Reports (PIREPS) of Ice

If a pilot experiences ice accumulation on her/his aircraft, the pilot can help other pilots arriving later by radioing a report of the weather conditions (called a PIREP) to air traffic controllers or flight service station personnel on the ground. Both the US and Canada use the same terminology for aircraft icing intensity. Here are the quotes from the US AIM:

Ice on the Ground

Sadly, even when the aircraft is parked on the ground, freezing rain can cause ice to form on the aircraft. This ice forms on all the top surfaces of the aircraft, not on the leading edge. It adds tremendous weight to the aircraft, and must be removed before flight. Ways to remove this include spraying with a de-icer (hot antifreeze), move the aircraft in to a warm hangar, or wait until a sunny day to melt the ice.

First photo by Stull. Second photo courtesy of US FAA AMT Airframe handbook, Chapter 15.

However, these ice storms also coat the runways, taxiways, and parking ramps/aprons with a layer of ice, making it difficult or impossible to taxi (drive the aircraft on the ground) and takeoff or land. Also, the roads to the airport might be ice covered and slippery, so you might not be able to drive to the airport in your car.

Often on clear nights in air that is humid, frost can form on the top surfaces of the aircraft wings, fuselage, and empenage (tail). This can be avoided by putting a cover over the aircraft, or moving the aircraft into a hangar or under a tree. Once the frost has formed, the pilot can wait a couple hours for the sun to melt and evaporate the frost, or can try to brush off and rub smooth the frost. Also, the de-icing fluid (antifreeze) can be sponged, mopped, or sprayed onto the aircraft.

Ice Crystal Icing (ICI)

Normally, ice forms on aircraft when the aircraft flys through LIQUID water that is below freezing.  Also normally, if an aircraft flys through cloud particles that are already frozen (SOLID snowflakes or ice crystals), these just blow around the aircraft and don't stick, so they are not a hazard.

Often the intake nacelle of jet engines is intentionally heated by jet exhaust to prevent LIQUID supercooled water from freezing on it.  This works great to prevent the normal formation of ice, but there is an exception.

When an aircraft flys through an ice-crystal cloud, if conditions are right then the ice instantly melts when it enters the heated nacelle at the jet-engine inlet, but then re-freezes on turbine blades in the first part of the engine.  Ice can build up on those turbine blades.  This degrades performance of the engine, and can damage the engine if chunks of ice break off and shatter the turbine blades further back in the engine.  This phenomenon is called Ice Crystal Icing (ICI).

Although this is a very rare event, it has caused over 150 incidents of power-loss or engine damage to jet aircraft over the past 20 years.  Most reports were when jet aircraft were flying at high altitude through the ice-crystal tops of tropical thunderstorms.  This is NOT a hazard that most general aviation (light airplane) pilots would encounter, but it could affect passengers on commercial flights.  Luckily, in most cases when a jet engine is damaged due to ICI, the pilot can continue flying the aircraft on the other engines to reach the nearest airport for an emergency (safe) landing. 


Key words: supercooled droplets, clear ice, rime ice, mixed ice, frost, de-icing, anti-icing, ice-crystal icing.

Extra info for experts; not needed for this course.

Additional books that can be purchased:


Videos for experts:

All the photos were taken by Roland Stull, and the drawings were made by Roland Stull, and all are copyright by him and used with his permission, except as listed near the figures.

UBC ATSC 113 - Weather for Sailing, Flying & Snow Sports Copyright © 2016 - 2020 by Roland Stull.
Last updated May 2020.