Snow is falling on a dark winter morning, and your airliner has not moved from the de-icing pad. A truck with a cherry-picker basket rolls alongside and a worker blasts the wings with a steaming orange liquid. Minutes later comes a second pass, this time a thick, syrupy green. Somewhere up front, the pilots have started a clock.

That ritual exists because of a simple, uncomfortable truth: even a thin, rough layer of ice on a wing’s leading edge can make it stall at a markedly higher speed. Aircraft de-icing and anti-icing is the business of making sure that never happens, on the ground before take-off and in the clouds afterwards.

Here is how it works, from the four colour-coded fluids and the holdover clock to the rubber boots, hot air and electric heaters that keep ice off wings in flight.

Why a little ice is a big problem

A wing works because of its carefully shaped, smooth leading edge. Ice changes that shape and roughens the surface. The FAA’s pilot guide to flight in icing conditions is blunt about the numbers: a 30 percent cut in a wing’s maximum lift from a relatively small accretion is not unusual, and large “horn” shapes of ice can cut it by 40 to 50 percent. Drag commonly doubles, and can triple or worse.

Worse still, ice makes the wing stall at a lower angle of attack than a clean one. A pilot slowing down for landing, raising the nose as usual, can reach the stall well before the stall warning expects it. Ice also adds weight, can block engine intakes, and on propellers and rotor blades creates imbalance.

In flight, the culprit is usually supercooled water: cloud droplets that stay liquid below 0°C and freeze the instant they hit something. On the ground, it is snow, frost and freezing rain. The FAA even warns that clear ice can form on the upper surface of a wing above fuel tanks still cold-soaked from high altitude, in conditions that are above freezing and do not look like icing weather at all.

The clean aircraft rule

Because of all this, the rule for taking off is absolute. US airline regulations forbid it outright with contaminated wings, and airlines must run an approved ground de-icing and anti-icing programme whenever frost, ice or snow can reasonably be expected to stick to the aircraft.

That is the job of the de-icing crews. The work is often done in two steps. First comes de-icing: removing what is already there. Then comes anti-icing: coating the clean surfaces to stop new snow or ice from sticking until the aircraft is airborne.

Four fluids, four jobs

The fluids are standardised by SAE International in two specifications, AMS 1424 for Type I and AMS 1428 for Types II, III and IV. Most are based on propylene glycol or ethylene glycol mixed with water, with thickeners, wetting agents, corrosion inhibitors and dye. The colours are there so crews can see exactly where the fluid has gone.

Type I is the thin one, typically dyed orange. According to NASA’s pilot guide to ground icing, it is always applied heated and diluted, and for de-icing it is the heat and the force of the spray that do the work. Because it is unthickened it blows off at low speeds, around 60 knots, so it suits any aircraft. The catch is that it gives the shortest protection of all.

Type II and Type IV are thickened fluids that cling to the aircraft and keep absorbing and melting snow for much longer. The price is that they need speed to shear off: these fluids are meant for aircraft rotating at 100 knots or more. Type IV, typically dyed green, meets the same specification as Type II but protects for significantly longer. In North America it is usually applied cold, purely for anti-icing.

Type III, typically dyed bright yellow, sits in between. It is thicker than Type I but thin enough to leave the wing of a smaller commuter aircraft rotating at 60 knots or more.

The Wall Street Journal explains the logistics of de-icing airliners in winter.

Racing the holdover clock

That clock the pilots started is the holdover time: the estimated time the fluid will keep frost, ice and snow from building up on the critical surfaces. It starts when the final application begins, and when it runs out, the aircraft must be treated again.

Holdover times come from tables approved by the SAE G-12 committee and issued each winter by the FAA and Transport Canada. They are published as ranges, a shorter time for heavier precipitation and a longer one for lighter. There are no holdover times at all for heavy snow, hail, ice pellets or moderate and heavy freezing rain.

There is one more catch. The fluids are designed to flow off by the time the aircraft reaches rotation speed, so once it is airborne they protect nothing. From that moment, the aircraft’s own ice protection takes over.

In the air: boots, hot air and heaters

The oldest airborne solution is the pneumatic boot, a strip of rubber tubes on the leading edge of wings and tail. In icing conditions the tubes inflate and deflate on a timer, cracking the ice so the airflow can blow it away. BF Goodrich developed the boot in Akron, Ohio, around 1930, and it is still common on turboprops such as the Dash 8 Q400 pictured below.

Boots are de-icing systems: they let some ice build up, then shed it. The traditional advice was to let a set thickness build up before cycling them, partly for fear of “ice bridging”. FAA-sponsored icing tunnel tests since 1999 found no bridging on modern boot designs, and the FAA now recommends activating the boots at the first sign of icing.

Most jets use anti-icing instead, keeping the surface entirely free of ice. Hot bleed air from the engines’ compressors is ducted into perforated “piccolo” tubes behind the leading edges of the wings and engine nacelles. It is very effective, but it costs engine performance, which is why it is rare on small turbine aircraft. Some smaller jets, turboprops and piston aircraft use a “weeping wing” instead, which seeps a freezing-point-lowering fluid such as ethylene glycol or isopropyl alcohol through tiny holes in the leading edge.

Airline captain Captain Joe explains how wing and engine anti-ice systems keep an airliner clean in icing conditions.

The newest approach is electric. Electrothermal heaters have long protected propellers, helicopter rotors and windscreens, and the Boeing 787 took the idea to the wing: its GKN and Ultra-built system uses electrothermal heater mats embedded in the leading edges, removing the need to bleed hot air from the engines to protect the wings.

Where ice protection was born

Much of this technology traces back to one building in Cleveland, Ohio. NASA’s Icing Research Tunnel began operating in June 1944, built by NACA engineers to share the massive refrigeration system of the neighbouring Altitude Wind Tunnel. Within months, a Bell P-39 Airacobra was sitting in its test section for a propeller de-icing study.

More than eighty years on, the tunnel is still freezing wings on purpose, now including models of future ultra-efficient airliner designs such as the Transonic Truss-Braced Wing. Each layer of ice grown in the tunnel helps engineers predict what real clouds will do.

AOPA’s look at NASA’s icing wind tunnel in Cleveland, where ice is grown on aircraft parts on purpose.

So the next time you sit on the de-icing pad watching orange and green fluid stream off the wing, you are watching the end of a long chain: decades of tunnel research, a colour-coded chemical standard, and a clock that leaves no room for guesswork. The wing that leaves the runway has to be clean. Everything else follows from that.

Sources: FAA Advisory Circular 91-74B (Pilot Guide: Flight in Icing Conditions), 14 CFR 121.629 (eCFR), NASA Glenn Research Center (A Pilot’s Guide to Ground Icing; Icing Research Tunnel), SKYbrary (Holdover Time), FlightGlobal, Aviation Pros, Wikipedia (Ice protection system; Aircraft deicing fluid; Deicing boot)