Aircraft de-icing: fluids, holdover time and de-icing pads

It is just after seven, three degrees below zero, light snowfall. An Airbus does not taxi to the runway but onto a pad at the edge of the apron. Two vehicles extend their boom arms, and seconds later the aircraft disappears in an orange cloud of steam. Then a green film goes onto the wings, and the clock starts running. Aircraft de-icing is one of the most visible procedures in winter operations and one of the least understood. This guide explains which fluids there are, what holdover time means, why airports build dedicated de-icing pads and how to capture it all as a spotter.

Aircraft de-icing: the short answer

Aircraft are de-iced because nothing may adhere to the critical surfaces at takeoff: no ice, no snow, no slush, no frost. To do this, special vehicles first spray on a heated mixture of glycol and water that melts the deposits and washes them off. If precipitation keeps falling, a thickened protective fluid follows that keeps new ice off for a while. How long is given by the holdover time. If it runs out before takeoff, the aircraft is checked or treated again.

The principle is known in technical language as the clean aircraft concept. Transport Canada sums it up concisely in its guidelines: there is no such thing as an insignificant amount of ice. Even a roughness of around 0.4 millimeters noticeably disturbs the airflow over the wings. Lift drops, drag rises, the stall speed increases – and that in exactly the phase in which an aircraft has the smallest margins.

De-icing and anti-icing: two different jobs

In everyday speech people simply say “de-icing.” Technically there are two steps behind it, with different goals.

• De-icing means: whatever is already on the aircraft has to come off. For this, a fluid is heated strongly and sprayed on under pressure. The heat melts ice and snow, the jet washes them off. In the cold air part of it evaporates immediately – hence the clouds in which the aircraft almost disappears for a few seconds.
• Anti-icing means: nothing new should settle on the clean surface until takeoff. For this, a thickened fluid is used that stays on the surface as a film. Falling snowflakes dissolve in it instead of freezing on. During the takeoff roll the airflow shears the film off, so the wings are clean again in the air.

Whether only one step is needed or both depends on the weather. Frost on a clear, cold morning calls for something different than persistent snowfall at zero degrees.

One-step and two-step procedures

In the one-step procedure, a single treatment does both: the heated fluid cleans the surfaces and then offers a certain, usually short, protection. According to the Canadian guidelines, this procedure is used mainly when no precipitation is falling during the treatment – typical for frost after a clear night.

The two-step procedure consists of two separate passes. First hot fluid removes the deposits, then the protective film is applied. Timing is important: the second step should follow before the first layer freezes again. Transport Canada gives as a typical guideline that anti-icing should take place within three minutes after de-icing. This procedure is the standard when the aircraft is contaminated and it keeps snowing or freezing.

The fluids: Type I to Type IV

The fluids are divided into four types under standards of the SAE (Society of Automotive Engineers). All of them consist at their core of glycol and water. Usually propylene glycol is used, which is less toxic than ethylene glycol. On top of that come additives such as wetting agents, corrosion inhibitors, dyes and, in Types II to IV, thickeners.

Type	Typical color	Consistency	Main job	Protection time
Type I	orange	thin, sprayed on heated	remove ice, snow and frost	short, runs off quickly
Type II	mostly colorless to pale yellow	thickened, clings until the takeoff roll	anti-icing for larger aircraft	considerably longer than Type I
Type III	yellow	thickened, shears off earlier	anti-icing for aircraft with a low liftoff speed	between Type I and Type II
Type IV	green	thickened, clings until the takeoff roll	anti-icing in precipitation	longest protection time

Type I: the orange workhorse

Type I is the fluid you see most often. It is thin, diluted with water depending on the outside temperature and heated so that, according to the Canadian guidelines, it typically arrives at the nozzle at 60 to 82 degrees Celsius. Its strength is cleaning. Its protection against new ice, on the other hand, is short, because it quickly runs off the surfaces. Type I is therefore the first step in the two-step procedure or the sole treatment when no precipitation is falling.

Type II and Type IV: the protective film

Types II and IV are pseudoplastic: at rest they are viscous and stay on the wings. As soon as shear forces act during the takeoff roll, they become thin and flow off. This behavior is tuned for aircraft that lift off at higher speeds. Type IV is the further development with the longest protection time and is increasingly displacing Type II in anti-icing. The typical picture: a green, slightly glossy film on the wings and horizontal stabilizer that tears off in shreds at rotation.

Type III: for slower aircraft

Type III is intended for aircraft with a low rotation speed, so mainly turboprops and smaller regional aircraft. A Type IV fluid might not shear off completely on them before they lift off. Type III comes off earlier. It is rarer in Europe and North America; at airports with a lot of regional traffic such as Helsinki, with aircraft like the ATR 72-600 or the Dash 8-400, the topic of liftoff speed becomes especially tangible, though.

The freezing point buffer

The fluid is not applied neat but mixed with water. How much depends on the outside temperature. The freezing point of the mixture has to keep a safety margin from the outside air: the Canadian guidelines give a buffer of 10 degrees Celsius for Type I and 7 degrees Celsius for Types II to IV. At very low temperatures the procedure therefore reaches its limits – the colder it is, the more concentrated the mixture has to be.

Holdover time: the clock that sets the pace of winter

Holdover time (HOT) is the estimated period during which a protective layer, under certain weather conditions, prevents new ice from settling on the treated surfaces. It starts when the protective fluid is applied to the cleaned surfaces – not when the first snow falls. It ends when the fluid can no longer absorb the precipitation and ice crystals form that create a rough surface.

How long this window is depends on several factors:

• Fluid and manufacturer: Type IV usually protects longer than Type II, Type I for the shortest time. For the thickened types there are also product-specific values.
• Mixing ratio: Undiluted fluid lasts longer than a 75:25 or 50:50 mixture.
• Outside temperature: The colder it is, the shorter the protection usually is.
• Type and intensity of precipitation: Light snow, heavy snow, freezing drizzle, freezing fog or frost each give their own values. In heavy snowfall or freezing rain, the window can shrink to a few minutes.

The values are listed in holdover tables, which the US aviation authority FAA and Transport Canada publish jointly and update every year before the winter season. For this, the available thickened fluids are evaluated annually. The ranges are sometimes wide: for Type I, the FAA tables give values in the single-digit to low double-digit minute range depending on conditions, and for Type IV they reach, under favorable conditions, more than two hours.

After the treatment, the crew is told which fluid was applied, in which mixture and at what time. With this information and the current weather, they read off the appropriate holdover time. The clock then keeps running even if the precipitation pauses briefly.

What happens when the time runs out

If the holdover time runs out before the aircraft lifts off, it cannot simply take off. In Canada, for example, an inspection of the surfaces for contamination immediately before takeoff is then required, as a rule within five minutes before the start of the takeoff roll; special rules apply to Type I treatments. If that is not possible or if something is on the wings, it goes back to de-icing. That is exactly why departures back up on snowy days: every aircraft in the queue in front of the runway uses up its holdover time.

History shows how important this window is. In the crash of Air Florida Flight 90 in January 1982 in Washington, among other things a long wait between de-icing and takeoff and the engine anti-ice being switched off played a role. In the accident of SAS Flight 751 in December 1991 near Stockholm, clear ice that had gone unnoticed on the ground came off the wings and was ingested into the rear engines. Both cases still shape the training of crews and de-icing teams today.

The procedure on the de-icing pad

From the outside, de-icing looks like a short, chaotic operation. In fact it follows a fixed procedure.

The vehicles themselves are small marvels: they carry tanks for the fluids, heaters and high-pressure pumps and have a boom with a basket or closed cab from which an operator guides the nozzle. For large types such as the Boeing 747-8 or the Airbus A350-900, the booms have to reach up over the vertical stabilizer, and often two or more vehicles work on one aircraft at the same time.

De-icing pads: why airports build dedicated sites

De-icing can take place at the parking position, in the hangar or on a dedicated de-icing pad. More and more airports are relying on central pads, and there are three reasons:

• Short routes to the runway: If the pad is near the runway threshold, little holdover time passes before takeoff. The risk of having to de-ice a second time drops.
• Collecting the fluid: A purpose-built pad can collect the runoff fluid and meltwater. That is hardly possible to the same extent at the parking position.
• Keeping gates free: If de-icing does not take place at the parking position, the next flight can dock earlier. That helps the whole turnaround.

Toronto Pearson shows the scale: the airport operates a central de-icing facility with several bays in which several aircraft are treated at the same time, and processes more than 10,000 aircraft there per season. According to available figures, it is considered the largest facility of its kind in the world. In Europe, Zurich is a good example of an airport with its own de-icing pad, which you can even follow via an airport webcam. Spotting tips for it are in the guide Plane spotting in Zurich.

What happens to the glycol

Glycol is a problem for bodies of water: when it breaks down it consumes oxygen, which fish and other aquatic life then lack. That is why the fluid is collected on de-icing pads. Special suction vehicles pick up residues and slush. What happens next depends on the airport: in some cases the glycol is treated and reused, in others it is biodegraded or sent to wastewater treatment plants. Munich Airport, for example, captures glycol with the contaminated meltwater and either treats it or breaks it down into water and carbon dioxide with soil bacteria on a dedicated site.

De-icing, winter service and delays

Aircraft de-icing is only one part of winter operations. In parallel, crews clear runways, taxiways and the apron, spread de-icing agents for surfaces and measure braking action. How this is organized is described in the article on winter service at the airport. The two mesh together: if a runway is closed for clearing, aircraft wait longer – and use up their holdover time.

On snowy days you therefore see typical patterns:

• Queues in front of the de-icing pads, especially in the morning departure waves.
• Second de-icings when an aircraft stands in front of the runway for too long.
• Reduced takeoff rates, because runways are cleared in between.
• Delays that stretch through the day, because rotations no longer fit.

Aviation weather tells you in advance whether a day is heading in this direction. How to read METAR and TAF for snow, freezing rain or fog is shown in the guide How to read METAR and TAF.

De-icing as a spotter’s subject

For photos, de-icing is a gift: color in an otherwise gray season, steam clouds in backlight and work vehicles that look tiny next to a widebody. A bit of planning helps you catch the scene.

A few practical tips:

• Choose your spot by runway direction in use. If the de-icing pads are near the runway ends, the active takeoff direction decides where you should stand. Background on this is in the guide Understanding runway direction in use.
• Use decks with a view of the apron. From Zurich’s observation deck B you are more likely to see work at the positions and vehicle movements. In Scandinavia, places like the Arlanda spotters’ hill offer winter operations on a grand scale on snowy days.
• Keep an eye on exposure. Steam and snow fool the light meter. White fuselages blow out quickly; a slight correction and the histogram help. You will find more in the camera settings for plane spotting.
• Use backlight. A steam cloud looks especially dramatic with the low winter sun behind it.
• Protect yourself. Warm clothing, spare batteries close to your body and a bag for the camera when you go back inside are mandatory. The complete winter gear is described in Plane spotting in winter.

If you want to experience it all up close, guided apron tours in winter sometimes give you the chance to see a de-icing pad from close by.

Conclusion

Aircraft de-icing follows a clear logic: critical surfaces have to be clean at takeoff. Hot, orange Type I fluid clears them off, and thickened fluids such as green Type IV protect them afterward. Holdover time sets the window in which this protection works, and it explains why snowy days bring queues and delays. Central de-icing pads shorten the routes to the runway and capture the glycol. For you as a spotter, the whole system is an invitation: check the weather, know where de-icing takes place, get there early – and keep the camera warm.