Just like salt lowers the freezing point of water on our roadways, there is a substance that is used to lower the freezing point of water on the outside of an airplane: glycol. You may already be familiar with the waiting and the maple syrup smell associated with de-icing a plane, but there may be more to it than you think.
Before takeoff, planes go through two processes: de-icing and anti-icing. De-icing is what washes away all the snow and ice that accumulated on the plane from its previous journey, while anti-icing prevents snow and ice buildup before takeoff for its next journey. In the air, temperatures are usually even lower than the freezing point that glycol can help water reach, so this substance is not much help up there. While flying, planes have their own systems in place to prevent frosty build-up. The hot air from the engine is actually circumvented around the outside of the aircraft and the plane essentially uses itself as a heater.
However, takeoff is the process that benefits mostly from de/anti-icing, not landing. So it is important to make sure the plane is as free from frost, snow, and ice as possible before taking off. De-icing and anti-icing are actually used 9 months out of the year and cost about $5.00 per gallon. Yikes!
De-icing liquid is made up of glycol and water and is heated before being applied to the exterior of the plane – like taking a nice hot shower after playing in the snow. After all of the frozen bits are cleared away, it is time for anti-icing. Anti-icing liquid is still glycol and water, but there is much more glycol in this mixture and it is not necessary to heat before application. This liquid is applied within three minutes of de-icing being complete, and planes are then expected to take off within a predetermined “holdover time.” Holdover time is determined on a case-by-case basis and takes factors into account such as the external temperature, the amount of snow falling, etc. This can sometimes be just a few minutes. Basically, the holdover time is the amount of time that the anti-icing will last before snow and ice begin building up again. In the case of a delay, some planes need go through the de/anti-icing process again before they are able to take off.
There are two big reasons that these processes are important for takeoff: lift and drag. Airplanes are designed to “fly clean,” meaning there should be no additional materials or debris on the outside of the aircraft. Air should flow smoothly along the sides. When there is bumpy, rough ice instead of a smooth surface, the plane will not lift for takeoff as expected and will be held down by this extra material.
If you find yourself frustrated the next time you are stuck between pushback and takeoff due to a de-icing delay, try and find peace in knowing that this process, while slightly time-consuming, is truly necessary for the safest and smoothest flight possible. Hopefully you are headed somewhere tropical!
For more travel articles please subscribe to our weekly newsletter, The Van Zile Traveler.
Written by Carol Gabbert, Van Zile Travel
Airplane deicing: The how and why. (n.d.). Retrieved from https://www.cnn.com/2010/TRAVEL/12/22/airplane.deicing/index.html
How Aircraft De-Icing Works. (2018, December 4). Retrieved from https://thepointsguy.com/news/how-aircraft-de-icing-works/
How do they deice airplanes? (2015, July 17). Retrieved from https://science.howstuffworks.com/deice-airplanes.htm