De-icing aircraft in winter — fluids, methods and holdover time

Even a thin layer of rime frost on a wing can change the airflow enough that a plane cannot take off safely. That is why de-icing is not a service you skip when the calendar hits winter — it is a task that can stop the entire departure until it is performed correctly. As an airport operator you are often close to the process whether you are sitting in the de-icing vehicle yourself or coordinating around it on the apron.

§Why ice on an aircraft is so dangerous

An aircraft's wings are shaped so that air flows smoothly over them and creates lift. Ice, snow or rime frost destroys that smooth surface and can mean that the wing no longer creates enough lift to take off — or that the aircraft reacts unpredictably in the air. That is why the so-called 'clean aircraft' rule applies: an aircraft must not take off if there are critical amounts of ice, snow or frost on wings, tail plane or control surfaces. The rule is absolute and applies regardless of how busy it is or how small the deposit looks.

§Type I, II, III and IV — the difference between the fluids

There are several types of de-icing fluid and they are used for different purposes. Type I is a thin heated mixture of glycol and water. It is used for actual de-icing — that is to remove ice snow and frost already on the plane. Because the fluid is thin it is quickly washed or blown away and therefore provides only short-term protection against new precipitation. Types II III and IV are thicker gel-like fluids developed to remain on the surface after application. They are used for anti-icing — that is to prevent new ice or snow forming while the plane waits to take off. The thick non-Newtonian fluid is designed to slide off the wing by itself once the plane reaches a certain speed during takeoff.

§Hold-over time — a race against the weather

Hold-over time is the time from when the fluid is sprayed on until it can no longer protect the aircraft against new ice or snow. The time depends on fluid type, temperature and how hard it is snowing or raining — and it is found in international hold-over time tables which crew and de-icing teams look up for the current weather situation, every single time. According to the international SAE standards for anti-icing fluids, a Type II fluid must at minimum pass the so-called WSET test (Water Spray Endurance Test) with a hold-over time of at least 30 minutes, while a Type IV fluid must at minimum be able to hold for 80 minutes under similar test conditions. These are minimum requirements for the fluid's properties — the actual hold-over time on the day can be significantly shorter if the snow falls heavy or if it is very cold.

§How the building painter's education is structured

• 01The plane is moved to a de-icing position — either at the gate or in a dedicated de-icing area close to the runway
• 02The de-icing team inspects the wings, tail plane and other critical surfaces for ice, snow and frost
• 03If there is already ice on the plane, spray with heated Type I fluid first to remove it
• 04If protection from continued precipitation is needed, it is often followed by an anti-icing fluid (Type II, III or IV)
• 05The time of application, fluid type and concentration are carefully logged so holdover time can be tracked
• 06The herd is informed when de-icing is complete and what hold-over time applies

§Your responsibility as an airport operator

Although it is often specially trained de-icing teams who spray the fluid de-icing is a task the entire ramp crew should know about. You should know when a plane must hold back from pushback because de-icing is ongoing why a plane suddenly needs extra inspection even though it looks clean and why the pace on a snow-covered shift looks completely different from a dry summer day. Winter preparedness is ultimately about accepting that safety sets the pace — not the other way around.