November 1, 2012
Photograph by Paul L. Csizmadia
Every private pilot is exposed to the aerodynamic tidbit that ice, snow, or frost the thickness of sandpaper on the leading edge of a wing can reduce lift by 30 percent and increase drag by 40 percent. Because stall speed varies as a function of square root of max lift coefficient, a decrease in lift of 30 percent will cause an aircraft’s stall speed to increase by 20 percent. The V2 speed that manufacturers determine will be reached by 35 feet agl on takeoff is only required to be 20 percent over stall speed, so a contaminated aircraft may be exiting ground effect on the very edge of a stall.
While limping off the ground—teetering on the edge of stall—is frightening, the case of a takeoff with contamination not equally developed on both wings could be even worse. Consider a Challenger that experienced a rapid roll immediately after takeoff. Despite full opposite rudder and aileron, the aircraft struck the ground inverted, resulting in the deaths of all aboard. The cause of the accident was determined to be frost on the wings, which likely built up asymmetrically because the auxiliary power unit exhaust partially melted the frost on one wing ( “Rocky Mountain Low,” October 2006 AOPA Pilot).
Because of the harsh aerodynamic penalties of contamination, FAR 91.527 specifies that no pilot may take off an airplane that has frost, ice, or snow adhering to (among other things) any wing or stabilizing or control surface. Prior to 2010 pilots were allowed to depart with frost on the wings, provided it had been polished to make it smooth. At least 12 accidents occurred after taking off with frost that had been polished smooth, so the FAA revised this policy, and now frost must be removed.
An aircraft can be deiced in three ways—mechanically, by physically removing contaminants like dry snow that have not frozen to the aircraft; thermally, via forced hot air or a warm hangar; or with the use of deicing fluid. With a mixture of water and a freezing-point depressant, hot deice fluid melts and flushes the contamination on the aircraft, while the chemical additive ensures the solution won’t refreeze.
Anti-icing must also be considered if an aircraft must start up and taxi in conditions where contamination such as snow may accumulate. Rather than removing contamination, anti-icing coats the aircraft’s critical surfaces with cold fluid designed to absorb the precipitation, then cleanly flow off during the takeoff roll. While the same fluid is sometimes used for both the deicing and anti-icing tasks, different cocktails of chemicals, called types, are formulated to be better at specific tasks.
Type I, for example, is typically the only fluid used for deicing in the United States. Types II, III, and IV, in contrast, are nearly always used for anti-icing purpose. Thickeners are added to type II-IV to keep the fluid on the wing longer, thus increasing the protective ability of the fluid. Not all aircraft are allowed to use all types of fluid—the very thickness of type II and IV fluid, for example, makes them unsuitable for aircraft with a rotation speed below 100 knots. Further, not all fluid types are available at all airports. Many smaller airports stock only Type I fluid, which is very limited in its ability to protect against active precipitation.
Deicing is not as simple as spraying fluid on an airplane and heading out; an assortment of details must be considered by the PIC and coordinated with deicing personnel. These include areas not to spray on the airplane, such as engine inlets and windows; which way the aircraft should be pointed prior to application (into the wind); and where on the aircraft fluid application should begin (often different for the deicing and anti-icing sprays). Because of the complexity of proper deicing, it’s essential that a briefing be conducted between the pilot and deicer prior to beginning fluid application.
Once fluid application is complete, there is a finite amount of time for which the aircraft is considered protected against falling precipitation. Called the holdover time (HOT), this period varies primarily with type of fluid applied, type of precipitation falling, and outside temperature. Each autumn the FAA publishes a document of the HOT tables for all types of fluid in production, use of which is necessary in conjunction with anti-icing fluid application.
Finding the table that corresponds to the type, and sometimes brand, of fluid being applied, pilots can find the HOT for the current conditions. HOTs vary from less than 5 minutes for Type I protecting against moderate snow, to more than an hour for Type IV in light snow. The clock starts ticking on the HOT as soon as the anti-ice spraying begins, so for a large aircraft short HOTs can be eaten up just by the fluid application.
If an aircraft is ready to take off within the HOT, the pilot is not quite off the hook. A pretakeoff inspection still must be completed, usually performed from inside the aircraft cabin looking out at the wing. The fluid is examined to see if it still looks glossy, smooth, and uncontaminated. If so, it is deemed not to have “failed,” and the aircraft may safely depart. If the pilot cannot be certain the fluid has not failed via a visual inspection, or if the HOT time has elapsed, a tactile inspection must be completed. Just as it sounds, the tactile inspection requires getting out of the aircraft and feeling the upper surface of the wing to determine if the fluid is still absorbing contamination. Disposable gloves are highly recommended for this task.
If the aircraft fails a pretakeoff inspection, it must return to square one. As new anti-ice fluid cannot be applied over failed fluid, the failed fluid must first be flushed off with Type I. Only then can a new, clean layer of anti-icing fluid be applied, and the HOT clock starts ticking again.
Neil Singer is a Master CFI with more than 7,200 hours in 15 years of flying.
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