How Flying Works
How De-Icing and Anti-Icing Actually Work
On 13 January 1982, Air Florida Flight 90 waited at Washington National Airport for 49 minutes between de-icing and takeoff while a blizzard kept dumping snow. By the time it left, fresh ice and snow had settled back on the wings. The Boeing 737 struggled to climb after lifting off, stalled, and crashed into the 14th Street Bridge and the frozen Potomac River. It killed 74 of the 79 people on board and four more on the bridge. The NTSB’s finding was direct: the crew didn’t use engine anti-ice on the ground and took off despite known ice and snow on the aircraft. It remains one of the clearest lessons that a wing that looks “mostly clean” isn’t clean. It’s why de-icing is a formal, timed, two-stage procedure rather than a quick look.
Two different jobs, two different fluids
Captain Joe’s video on wing and engine anti-ice systems sets out the same split airlines work to. De-icing removes contamination that’s already there. Anti-icing stops new contamination forming afterwards. Type I fluid, a heated mix of glycol and water, goes on first to melt off existing ice, snow or frost. It’s thin and runs off quickly, so on its own it protects for only a few minutes in falling snow or freezing rain. If snow or rain is still falling, a thicker fluid goes on next: Type II or Type IV, made gel-like with a long-chain polymer. It clings to the surface and holds off new ice, then thins out and slides cleanly off the wings as the aircraft speeds up for takeoff.
Try it
Let ice build up on the wing
Ice on the front of the wing spoils the smooth airflow. Even a thin, rough layer can cut the wing’s maximum lift by as much as 30% and make it stall at a lower angle, so the aircraft could stall at a speed that would normally be safe. That’s why aircraft are sprayed with de-icing fluid before take-off and use heated or inflatable leading edges in flight. Curves are illustrative; the 30% figure is from FAA guidance.
.jpg?width=1600)
The real numbers
How long protection actually lasts
1-22 min
Type I holdover, freezing rain
De-icing fluid only — thin, flows off fast, minimal duration
9-160 min
Type IV holdover range
The longest-lasting anti-icing fluid, varies by precipitation/temperature
49 min
Air Florida 90's ground wait
Time between de-icing and takeoff — long enough for new snow to resettle
74 of 79
Fatalities, Air Florida Flight 90
13 January 1982 — plus 4 more on the 14th Street Bridge
Why the clock matters as much as the fluid
Every application of anti-icing fluid comes with a published holdover time. It’s a window, based on real tests against specific types and rates of precipitation, after which the fluid can’t be trusted to have kept the surface clean. This isn’t a rough guide. Airlines and ground crews work from official tables that give exact ranges by fluid type, temperature and weather. When the window closes, the aircraft doesn’t just leave and hope. It’s either de-iced again or checked for contamination immediately before takeoff. Treating the clock as strictly as the spray is the most direct legacy of Air Florida 90. A 49-minute gap with no fresh check is exactly what the modern system is built to prevent.
Why the fluids are different colours
The colours mean something. Ground crews and pilots identify the fluid at a glance.
Type I is the de-icing fluid. It’s usually orange or pink and sprayed hot. It strips off ice and snow but gives little lasting protection. Type II is clear, and Type III is yellow. Type IV is green. It’s thick and sprayed cold, undiluted, as a protective coat.
The thickeners in Type IV are what make it last. It sits on the wing like a gel, absorbing falling snow before it can stick. In light snow it can give a holdover time of tens of minutes, and the official tables give the exact figure for each fluid and temperature. Holdover time is how long the fluid keeps protecting the wing before new ice can form, so the crew must take off within it or be sprayed again. At takeoff speed the airflow shears it away, so it is gone by the time the wing needs to be clean.
The main ingredient is glycol, usually propylene glycol because it is less toxic than ethylene glycol. Spent fluid is harmful to rivers, so airports collect it from drained de-icing pads. Where the glycol is concentrated enough, they recycle it instead of letting it run off.
Sources & Further Reading
Common Questions
Frequently Asked Questions
Why does a small amount of ice on a wing matter so much?
Even a thin, rough layer of ice or frost disturbs the smooth airflow a wing needs. It cuts lift and adds drag at takeoff, the slow moment when the aircraft has the least margin to spare. It doesn’t take a thick coating: a layer as fine as sandpaper can measurably change how a wing performs.
What's the actual difference between de-icing and anti-icing?
De-icing removes contamination that’s already there: heated Type I fluid is sprayed on to melt existing ice, snow or frost. Anti-icing stops new contamination forming afterwards: a thicker Type II or IV fluid clings to the surface and delays refreezing for a limited window, the “holdover time”. Once the aircraft is airborne and moving fast, the fluid blows off.
Why is holdover time treated so strictly?
Because it really does run out. Type I fluid might protect for only a few minutes in freezing rain. Even Type IV, the longest-lasting fluid, still has a finite holdover time: anything from a few minutes in heavy precipitation to a few hours in light conditions, depending on the precipitation and temperature. When the holdover time is up, the aircraft is either de-iced again or checked by eye and by touch to confirm the wings are still clean before it may depart.
Did Air Florida 90 change how de-icing is done today?
A great deal. The NTSB found the crew didn’t use engine anti-ice and took off with known ice and snow on the wings after a long wait on the ground. That finding shaped today’s much stricter holdover-time tables, compulsory pre-takeoff contamination checks and formal industry-wide de-icing procedures. The accident is still a standard reference for why those rules are so tight.
Last reviewed: October 2026. Facts that change, such as fleets, airspace and airline ownership, are re-checked on a schedule. Spotted an error? Email contact@skyplus.news and we’ll fix it. See how we check facts.