Here’s What You Need to Know About Aircraft Deicing
by Crista Worthy
As most pilots know, aircraft icing is a serious, even deadly, problem. Minute amounts of ice (equivalent to medium-grit sandpaper) covering the leading edges or upper surfaces of wings can increase the stall speed up to 15 knots. Tail stalls and uncommanded rolls are also possible.
You always need to pre-flight your aircraft. If conditions are at or near freezing, pay particular attention to these items:
- Empennage and airfoil surfaces are clear of ice, snow or frost.
- Control surfaces can move freely and completely.
- Verify visually that all flap and control surface gaps are free from contamination.
- Pitot tubes, static ports and angle of attack vanes are clear.
- Pneumatic boot surfaces (if equipped) are functional, and free of holes and tears.
- Prop heat blanket (if equipped) is in good condition.
- Other ice protection system components are in good condition. TKS system (if equipped) is functional. Electric heating systems (if equipped) are functional.
- Propulsion system components are free of ice or slush.
- Landing gear and doors are clear of contamination that would inhibit their operation.
- OAT/TAT probe is functional.
- Windshield deice systems (if equipped) and wipers are functional.
All frozen contaminants on the aircraft's critical surfaces must be deiced and anti-iced (as appropriate) on the ground prior to takeoff. You can remove contaminants mechanically with a broom, rubber scraper or forced air. Or, treat the aircraft with deicing/anti-icing fluids, either in a heated hangar or outdoors. Even if FBO personnel deice your aircraft, you — as PIC — are ultimately responsible for ensuring that it was done properly.
The purpose of deicing fluid is to remove any frozen contaminants and then prevent any new contamination from forming before the aircraft has departed the ground. De-icing fluids are designed to be stripped from the aircraft by fast-moving air as the aircraft accelerates to rotation speed. If the fluid remains on the aircraft, it can disrupt flight surfaces and cause aerodynamic problems. Once flight begins, the pilot is supposed to try to avoid in-flight icing conditions as much as possible. If necessary, the pilot can activate anti-ice equipment to buy time as the pilot attempts to exit the icing conditions.
Types of Deicing Fluids
There are four types of deicing fluids. The more viscous (thicker) the fluid, the longer the holdover time during which the aircraft can sit on the ground without accumulating new contamination, but the faster the aircraft must be moving down the runway to strip the fluid off the aircraft before rotation. Choose the fluid that is right for your aircraft, based on rotation speed and needed holdover time. Type II and Type IV are designed for jets with rotation speeds in excess of 100 knots. Type III fluid works for small commuter aircraft but can be used on larger aircraft as well. Small general aviation aircraft should generally use Type I; only use the others with the approval of the aircraft manufacturer.
The best practices for deicing small general aviation aircraft include:
- Place the aircraft in a heated hangar to melt frozen contaminants, followed by anti-icing to prevent refreezing of the melted contaminants when the aircraft is removed from the hangar. Do not delay in pulling the aircraft out of the hangar or the fluid may drip off or become too thick.
- Use mechanical means, such as brooms, rubber scrapers or forced air.
- Spray the aircraft with Type I fluid that is diluted and heated to nominally 60°C (140°F) at the nozzle.
- Spray the aircraft with heated water if the temperature is -3°C (27°F) or warmer, immediately followed by a second anti-icing treatment of heated Type I fluid heated to nominally 60°C (140°F) at the nozzle.
For de-icing, it is the heat and hydraulic force that accomplish the task. For anti-icing, it is primarily the heat imparted to the airframe that accomplishes the task.
The glycol-to-water ratio determines the freezing point of the fluid. Use a glycol tester, refractometer or Brix refractometer to make this determination. Many FBOs have these testers or you can purchase one.
Just remember that Type I fluids have the shortest holdover time. When a Type I fluid fails, it fails suddenly, with the result that it will no longer be able to absorb and melt the moisture. On the ground, freezing precipitation and active frost continually dilute the fluid. You can recognize failed fluid when it transitions from being glossy to a dull, opaque appearance. However, in freezing rain or freezing drizzle, the surface will still appear glossy following fluid failure. You will need to touch the surface to tell the difference. On the other hand, if the ambient temperature is too cold, the fluid will be too thick to shear off before rotation. The lowest operational use temperature (LOUT) is the coldest air or aircraft skin temperature at which a deicing/anti-icing fluid can be used.
Fly Your Plan
Prior to flight, in addition to your regular weather briefing, study the NOAA’s icing forecast information. You must avoid extended exposure to icing conditions. Don't accept a clearance that unnecessarily increases the level of risk. If you believe that icing will be a problem on departure, ask ATC for a modified clearance.
Icing is most frequent when the static air temperature (SAT) is between 2°C and –20°C, although ice can accrete outside this range. Unstable air increases the risk further. Generally speaking, the more water is suspended in the air, and the larger the droplets, the greater the ice buildup. Icing can be most intense near the cloud tops, where the amount of liquid water is often greatest. Most of the time, the vertical extent of an icing layer in a stratus cloud is 3,000 feet or less. Supercooled large droplets (SLD) can run far behind the leading edge. Since ice protection systems usually remove ice from the leading-edge area, you may end up with a ridge of ice behind it, which will disrupt air flow over the wing. If you encounter SLD, you need to exit the conditions immediately, because even a Flight-Into-Known-Icing (FIKI)-certified aircraft will be adversely affected. Always try to navigate laterally around cumulus clouds, which often cause significant turbulence in addition to the possibility of severe icing in the right conditions.