Ground deicing and anti-icing

Immediately after liftoff the aircraft started rolling to the left, and despite the crew’s full right aileron and rudder deflections the roll rate was undiminished. Within six seconds after liftoff the aircraft struck the ground in an inverted attitude and all aboard perished.

What caused such a dramatic loss of control was determined to be no more than frost. The crew of a regional jet parked near the Challenger found one or two millimeters of frost on their jet’s wings, and the accident pilots were recorded on the cockpit voice recorder (CVR) discussing the frost they both saw present. Despite this knowledge they did not request deicing of the aircraft, and accident investigators determined the hot exhaust of the auxiliary power unit (APU) was likely pushed by a light tailwind over the right wing, partially or fully melting that wing’s frost. The left wing’s undiminished frost reduced lift enough that the wing stalled after liftoff, with the asymmetry of frost distribution between the wings causing the leftward roll.

Entering another icing season, pilots would do well to heed the accident record of their predecessors who attempted takeoff with contaminated wings. There is no other point during flight when an aircraft is airborne so close to its stalling speed, and thus so sensitive to the reduction in lift that even the smallest amount of frost or ice can cause. Beyond being a good idea, proper deicing is required by FAR 91.527, which prohibits takeoff with “frost, ice, or snow adhering to any propeller, windshield, stabilizing or control surface; to a powerplant installation; or to an airspeed, altimeter, rate of climb, or flight attitude instrument system or wing”.

Deicing in the United States is typically conducted in one of two ways: by putting the airplane in a heated hangar until all contamination has melted (and been removed lest it re-freeze), or through chemical deicing. With the latter method, a mixture of glycol and water is heated to at least 140 degrees F and sprayed onto the aircraft components to be deiced. A common misconception is that the fluid dissolves the contamination, when in fact it is the heat of the mixture that removes the contaminants. The glycol simply serves to ensure the slurry of melting contamination and fluid doesn’t refreeze on the aircraft before it can drip off.

Deicing is relatively simple if the ambient conditions are not conducive to further ice formation. The contamination is removed and the aircraft can safely depart. In active icing conditions, however, a second anti-icing step is needed to keep any precipitation from adhering to the aircraft in the time between its being deiced and taking off. For optimal protection, the anti-icing step will use a different fluid than the “Type I” fluid applied for deicing—one that is much thicker and is applied cold, not heated. While Type I fluid can also be used for anti-icing, these thicker fluids, especially the Type IV most commonly used in the United States for anti-icing, allow for significantly longer protection.

The amount of time an anti-icing fluid is expected to remain protective is called the holdover time (HOT), and is influenced by a laundry list of variables including fluid type (which can get as specific as company of manufacture and exact product version); precipitation intensity; outside temperature; dilution ratio with water; aircraft surface material; and even if flaps are set for takeoff or left up during taxi. Each year the FAA publishes updated HOT tables encompassing the above variables for pilots to use during ground deicing.

For example, the table shown displays the HOT for a generic Type IV fluid when the pilot doesn’t know what brand fluid is being used. We can see that if using undiluted fluid at an OAT of minus-5 degrees Celsius during snowfall, the HOT ranges from 30 minutes to over two hours, depending mostly on precipitation intensity. An important “gotcha” applies here: When selecting the snowfall intensity, the pilot must use the proper chart in the HOT publication entitled “Snowfall Intensities as a Function of Prevailing Visibility,” and not simply enter the chart with the intensity reported on the ATIS or ASOS.

Particularly at night, the appropriate intensity can vary from that reported on the ATIS by an entire level, with significant implications for the resultant HOT. For example, if the airport is reporting light snow with one-and-a-half-mile visibility at night, it is considered moderate snow for the purposes of calculating HOT, with a resultant drop of protection time by nearly half.

Conducting deicing and/or anti-icing isn’t particularly complex, but there are some nuances to keep in mind. Turbine aircraft manufacturers publish supplemental information on specific procedures and aircraft configurations to be used when deicing, and the pilot must be familiar with them. Most will enumerate the areas of the aircraft that must not have fluid sprayed directly onto them (e.g., pitot/ static probes and ports, brakes, and engine nacelles), and spell out what, if any, restrictions or prohibitions exist on the types of anti-icing fluid allowed. As thickened fluids require high speeds before they shed off the wing, some light jets restrict their use to certain flap settings and/or apply a takeoff distance penalty when using these fluids.

Further, before starting a deicing or anti-icing session, the pilot should ensure the personnel performing the spraying have been briefed, and are on the same page as the flight crew with regard to the myriad details that go into a proper deicing. Beyond the basics of what fluids will be used and if they will be diluted with water or not, the pilot in command should 1) ensure the crew knows where to spray and where not to, 2) determine whether the deicing crew will inspect the aircraft after spraying, and 3) find out how the flight crew will know when all equipment and personnel are clear of the aircraft so it can taxi.

NASA has created an excellent, free, online deicing course, which should be considered mandatory viewing as initial or recurrent deicing season training. Go to aircrafticing.grc.nasa.gov and select the ground deicing section.